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Professional techniques for C# - Lecture Notes Part 4 of 4

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13 May 2016CPOL
The last part discusses attributes, iterators, and some more advanced topics.

These articles represent lecture notes, which have been given in form of tutorials originally available on Tech.Pro.

Image 1

  1. Introduction
  2. More control on events
  3. Overloading operators
  4. The yield statement
  5. Iterators
  6. Understanding co- and contra-variance
  7. Using attributes effectively
  8. Elegant binding
  9. Unsafe code
  10. Communication between native and managed code
  11. Effective C#
  12. Outlook
  13. Other Articles in this Series
  14. References
  15. History

This tutorial aims to give a brief and advanced introduction into programming with C#. The prerequisites for understanding this tutorial are a working knowledge of programming, the C programming language and a little bit of basic mathematics. Some basic knowledge of C++ or Java could be helpful, but should not be required.

Introduction

This is the fourth (and most probably last) part of a series of tutorials on C#. In this part we will look at some more advanced techniques and features of the language. We will see that we can actually control what the compiler does when event handlers are added or removed, how we can bring in our own operator definitions or get information about member names. We will also see that building iterators is in fact quite easy with C#.

Additionally we will strengthen our knowledge about how to write effective C#, the Garbage Collector and attributes. Finally we are also discussing how to talk with native programs and libraries using C# via unsafe contexts and native interoperability. This tutorial should provide some useful tips and tricks for experienced C# developers.

For further reading a list of references will be given in the end. The references provide a deeper look at some of the topics discussed in this tutorial.

More control on events

In the previous tutorial we discussed the .NET standard event pattern and the event keyword. We have noticed that in principle the event keyword protects a simple public delegate instance for being accessed directly. Instead we just have the possibility to add (+=) or remove (-=) handlers from outside.

It has already been explained that the reason for this is that the compiler extends our class definition with two more methods, which will be called when adding or removing an event handler. Those methods then perform operations in a thread-safe manner, like calling the static Combine method of the Delegate class.

C# also allows us to define what these methods do. This can be of great benefit, since this allows us to create weak event handlers and other more sophisticated patterns. This also allows us to control which handlers can be registered (or unregistered) for our event(s). The basic syntax is quite close to properties with their get and set blocks. Let's see this in action:

class MyClass
{
    public event EventHandler EventFired
    {
        add
        {
            Console.WriteLine("Tried to register a handler.");
            //value is the name of the delegate instance
        }
        remove
        {
            Console.WriteLine("Tried to unregister a handler.");
            //value is the name of the delegate instance
        }
    }
}

We could now either use another (private and not marked as an event) instance of the delegate (in this case EventHandler) or we could do some other (perhaps more special) action. One thing we could do is to have a single handler instead of multiple handlers:

class MyClass
{
    EventHandler singleHandler;

    public event EventHandler EventFired
    {
        add
        {
            singleHandler = value;
        }
        remove
        {
            if (singleHandler == value)
                singleHandler = null;
        }
    }
}

However, this pattern is just useful in some exclusive scenarios as well. So let's go on and have a look at the probably most common usage of writing our own add and remove blocks.

Hashtable events = new Hashtable();

public event EventHandler Completed 
{
    add 
    {
        events["Completed"] = (EventHandler)events["Completed"] + value;
    }
    remove
    {
        events["Completed"] = (EventHandler)events["Completed"] - value; 
    }
}

Here we use the plus and minus operators, which are overloaded by delegate to call Combine or Remove.

Overloading operators

What we did not discuss in this series of tutorials yet, is the possibility of implementing operators for our own objects. C# allows us to overload implicit cast operators, explicit cast operators, and a whole range of arithmetic / comparison operators (+, -, !, ~, ++, --, true, false, +, -, *, /, %, &, |, ^, <<, >>, ==, !=, <, >, <=, >=). Other operators either cannot be overloaded (like the assignment operator) or are implicitly overloaded (like += is overloaded if we overload +).

It is not possible to overload the index operator [], but we can create our own indexers:

class MyClass
{
    public int this[int index]
    {
        get { return index + 1; }
    }
}

//Using it:
var myc = new MyClass();
int value = myc[3]; //Will be 4

Indexers are like properties, however, the differences are that they have a special name (this) and require one or more parameters to be specified (in case of the example we define an integer parameter with name index). This way we can easily create our own multi-dimensional indexers:

class Matrix
{
    double[][] elements;

    /* some code */

    public double this[int row, int column]
    {
        get 
        {
            if (row < 0 || column < 0)
                throw new IndexOutOfRangeException();
            else if (elements.Length >= row)
                return 0.0;
            else if (elements[row].Length >= column)
                return 0.0;

            return elements[row][column];
        }
        set
        {
            if (row < 0 || column < 0 || elements.Length >= row || elements[row].Length >= column)
                throw new IndexOutOfRangeException();
            
            elements[row][column] = value;
        }
    }
}

We can have an arbitrary amount of such indexers, as long as each one has a unique signature (different number or types of parameters).

To demonstrate how operator overloading works with the previously named operators, we will work with an example that creates a simple structure called Complex (this struct will represent a complex number in double precision):

public struct Complex
{
    double re;
    double im;

    public Complex(double real, double imaginary)
    {
        re = real;
        im = imaginary;
    }

    public double Re
    {
        get { return re; }
        set { re = value; }
    }

    public double Im
    {
        get { return im; }
        set { im = value; }
    }
}

The first thing we might want to implement is an implicit conversion from double to Complex. A double is like a real value, i.e., this conversion should give us a complex value that has imaginary part 0:

public static implicit operator Complex(double real)
{
    return new Complex(real, 0.0);
}

We will see that all operator definitions require the operator keyword. For overloading implicit conversions the implicit keyword is additionally required. Needless to say that every operator overload has to be static.

There should be also an explicit conversion from Complex to double. This explicit conversion is then used to use the absolute value of the complex number:

public static explicit operator double(Complex c)
{
    return Math.Sqrt(c.re * c.re + c.im * c.im);
}

Maybe we want to use our complex object for defining true (to be used in if (instance) ...) or false (which will be used in combination with overriding &). Let's see how this could be implemented:

public static bool operator true(Complex c) 
{
    return Math.Abs(c.re) > double.Epsilon || Math.Abs(c.im) > double.Epsilon;
}

public static bool operator false(Complex c) 
{
    return Math.Abs(c.re) <= double.Epsilon && Math.Abs(c.im) <= double.Epsilon;
}

On the other side, if we implement representations for true or false, we should also implement an (at least explicit) cast to bool:

public static explicit operator bool(Complex c)
{
    return new Boolean(Math.Abs(c.re) > double.Epsilon || Math.Abs(c.im) > double.Epsilon);
}

Finally we should think about implementing the (required) arithmetic operators:

public static Complex operator +(Complex c1, Complex c2) 
{
    return new Complex(c1.re + c2.re, c1.im + c2.im);
}

public static Complex operator -(Complex c1, Complex c2) 
{
    return new Complex(c1.re - c2.re, c1.im - c2.im);
}

public static Complex operator *(Complex c1, Complex c2) 
{
    return new Complex(c1.re * c2.re - c1.im * c2.im, c1.re * c2.im + c1.im * c2.re);
}

public static Complex operator /(Complex c1, Complex c2) 
{
    double nrm = Math.Sqrt(c2.re * c2.re + c2.im * c2.im);
    return new Complex((c1.re * c2.re + c1.im * c2.im) / nrm, (c2.re * c1.im - c2.im * c1.re) / nrm);
}

And finally we also might want to implement some comparison operators. This follows the same pattern as above, however, as return type we now specify a Boolean to be used:

public static bool operator ==(Complex c1, Complex c2)
{
    return Math.Abs(c1.re - c2.re) <= double.Epsilon && Math.Abs(c1.im - c2.im) <= double.Epsilon;
}

public static bool operator !=(Complex c1, Complex c2)
{
    return !(c1 == c2);
}

If we implement these comparison operators the C# compiler encourages us (with a warning) to also override the methods GetHashCode and Equals:

public override int GetHashCode()
{
    return base.GetHashCode();
}

public override bool Equals(object o)
{
    if (o is Complex)
        return this == (Complex)o;
    
    return false;
}

One question that could arise at this point is: What about operators involving, e.g., double and Complex? Don't we need something like the following as well?

public static Complex operator +(Complex c, Double x) 
{
    return new Complex(c.re + x, c.im);
}

The specific answer is: yes and no. In general we might need something like this, but then we would also need the following operator overload to be defined:

public static Complex operator +(Double x, Complex c) 
{
    return new Complex(c.re + x, c.im);
}

However, in our case we specified an implicit (!) conversion from Double to Complex. This means that if a Complex type would be needed where an object of type Double is given, a conversion is performed automatically (the compiler inserts the required instructions). Of course sometimes it could be beneficial (performance-wise or logical-wise) to include such overloads of an operator explicitly.

Finally we can use our own type as follows:

Complex c1 = 2.0;//Implicit cast in action
Complex c2 = new Complex(0.0, 4.0);
Complex c3 = c1 * c2;
Complex c4 = c1 + c2;
Complex c5 = c1 / c2;
double x = (double)c5; //Explicit cast in action

So what should be the key lessons in this section?

  • C# allows us to overload a broad range of operators, however, not as many (and not such critical ones like the assignment operator) as C++.
  • Overloading (standard) operators requires us to implement static methods, which carry the operator keyword and are declared public.
  • We cannot overload the index operator, but create as many indexers as we like to. The indexers are differentiated by their signatures.
  • Explicit conversions are performed like (double)c5, implicit conversions are automatically triggered by the compiler. However, also implicit conversions can be used explicitly, like (Complex)2.0.

The yield statement

In C# it is quite common to use the foreach loop, even though there are some significant drawbacks compared to the classical for loop. One big drawback is the immutability of the loop iterator, i.e., we cannot change the variable of the loop. So the following code is not possible:

var integers = new [] { 1, 2, 3, 4, 5, 6 };

foreach(var integer in integers)
    integer = integer * integer;

This is because the foreach loop works only with elements that implement the IEnumerable<T> interface, like every array or collections naturally does. Once some class implements this interface the method GetEnumerator can be called. This returns a special kind of class, which implements the IEnumerator<T> interface. Here we have an object that has a property called Current and the methods MoveNext and Reset. This property is read-only.

The first thing to look right now is to implement the IEnumerable<T> interface in our own class. The following example should demonstrate this:

class MyClass : IEnumerable<int>
{
    public IEnumerator<int> GetEnumerator()
    {
        return null;
    }
    
    IEnumerator IEnumerable.GetEnumerator()
    {
        return null;
    }
}

We could already use this in a foreach loop. The compiler would let this pass, however, at runtime we would face serious problems, since there is no check for null implied in the generated instructions. Now we have two possibilities:

  1. Create a class that implements IEnumerator<T> (with T being int in this case).
  2. Use a nice C# language feature.

Here we will actually do both, however, only to see why the second way is much nicer (shorter) than the first. Let's have a look at the code that is required for the first approach.

class MyClass : IEnumerable<int>
{
    public IEnumerator<int> GetEnumerator()
    {
        return new Squares();
    }
    
    IEnumerator IEnumerable.GetEnumerator()
    {
        return new Squares();
    }

    class Squares : IEnumerator<int>
    {
        private int number;
        private int current;

        public int Current
        {
            get { return current; }
        }
        
        object IEnumerator.Current
        {
            get { return current; }
        }

        public bool MoveNext()
        {
            number++;
            current = number * number;
            return true;
        }

        public void Reset()
        {
            number = 0;
            current = 0;
        }
        
        public void Dispose()
        { }
    }
}

Puuuh! Now this is quite long and required us to write a whole class! Before we go into details of the second way, we have a short look at using this code:

var squares = new MyClass();

foreach (var square in squares)
{
    Console.WriteLine(square);

    if (square == 100)
        break;
}

Now that we are quite excited about a much shorter way, let's have a look at it, which involves the (not yet discussed) yield keyword.

class MyClass : IEnumerable<int>
{
    public IEnumerator<int> GetEnumerator()
    {
        for (int i = 1; ; i++)
            yield return i * i;
    }
    
    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

Now this is quite a reduction in lines of code! Of course this example will usually end up in an infinite loop. Maybe this is not wanted. The key question is now - can we break the loop at a specific point? There are multiple ways of doing this. One way would be to just limit the for loop in the example like this:

public IEnumerator<int> GetEnumerator()
{
    for (int i = 1; i < 10; i++)
        yield return i * i;
}

However, sometimes code is a little bit longer and quite complicated. In such scenarios we would like to return something, which stops the iteration. Again the yield keyword is key, however, this time in combination with the break keyword:

public IEnumerator<int> GetEnumerator()
{
    for (int i = 1; ; i++)
    {
        var square = i * i;

        if (square > 100)
            yield break;    
            
        yield return square;
    }
}

This let's us use the code like in the following snippet:

var squares = new MyClass();

foreach(var square in squares)
    Console.WriteLine(square);

Finally we have a nice way to create iterators in C#! But what are key benefits of using iterators and what has to be known before using them?

Iterators

An iterator is an object that enables us to traverse a container. We've just seen how easily we can create an iterator. The previous example should have also demonstrated us, that traversing a container could be something like traversing a list of items in a collection (or an array), but could be also much different. In the previous example our container did not contain any elements, but was able to generate elements that have been subject for an iterator.

The yield keyword and the iterator in C# have been inspired by the programming language CLU.

The most important characteristic if any object definition is that it implements the IEnumerable (or even better: the IEnumerable<T>) interface. This requires a method GetEnumerator to be implemented and enables performing foreach loops. There are even other benefits: Since (P)LINQ is built upon the foreach loop (all entry extension methods target IEnumerable<T>) we can write queries agains such iterators.

What is this section about? As we've seen in the last section, it is quite easy to built our own iterator in C#. We have also seen that foreach uses this iterator concept to traverse the given container. However, even without using foreach we can benefit from this concept.

Let's consider the following lines of code as an example:

static void Main()
{
    var myc = new MyClass();
    var it = myc.GetEnumerator();
    Examine(it);
}

static void Examine(IEnumerator<int> iterator)
{
    while (iterator.MoveNext())
    {
        if ((iterator.Current & 1) == 0)
            Even(iterator);
        else
            Odd(iterator);
    }
    
    Console.WriteLine("No more elements left!");
}

static void Even(IEnumerator<int> iterator)
{
    Console.WriteLine("The element is even ...");
}

void Odd(IEnumerator<int> iterator)
{
    Console.WriteLine("The element is odd ...");
}

Of course this simple code could be also written using the foreach loop, however, the example should show that we can actually build up a whole collection of methods, which in the end just take an iterator and work with it. Therefore this code could be extended to the following version:

void Examine(IEnumerator<int> iterator)
{
    List<int> entries = new List<int>();

    while (iterator.MoveNext())
    {
        entries.Add(iterator.Current * iterator.Current);
    
        if ((iterator.Current & 1) == 0)
            Even(iterator);
        else
            Odd(iterator);
    }
    
    var next = entries.GetEnumerator();
    Console.WriteLine("No more elements left!");
    
    if (entries[entries.Count - 1] < 1000000000)
        Examine(next);
}

Here we are using our method again - this time with an iterator from a list that has been generated from the squares of the given elements. Calling Examine again will result in numbers to the power of 4, then to the power of 8 and so on. This means that we can expect some output like the following:

The element 1 is odd ...
The element 4 is even ...
// ...
The element 100 is even ...
No more elements left!
The element 1 is odd ...
The element 16 is even ...
// ...
The element 10000 is even ...
No more elements left!
The element 1 is odd ...
The element 256 is even ...
// ...
The element 100000000 is even ...
No more elements left!

Iterators are really useful when have to do a (forward-) examination of a container. The following diagram illustrates the iterator pattern again. It should be noted that nothing stops us from creating a loop here, such that the iterator never reaches an end (MoveNext will always return true in such a case).

The iterator pattern in C#

The only thing one needs to be aware of is that IEnumerable is an interface. Interfaces can be implemented in classes an structures, and if we receive a structure, then passing the iterator won't work as probably expected (since with out own iterators we always received classes, which are passed as references).

Let's see an example of such a problem:

static void Main()
{
    List<int> squares = new List<int>(new int[] { 1, 4, 9, 16, 25, 36, 49, 64, 81, 100 });
    var it = squares.GetEnumerator();//The var is problematic here...
    ConsumeFirstFive(it);//This should move the iterator 5 times ...
    Console.WriteLine("The 6th entry is ... " + it.Current); //This should print 36
}

static void ConsumeFirstFive(IEnumerator<int> iterator)
{
    int consumed = 0;

    while (iterator.MoveNext() && consumed < 5)
        consumed++;
}

The expected outcome is 36, however, the real outcome is 0. This means the iterator has not been moved / touched yet. How can that be? We used var to hide the real type of the iterator, which happens to be List<int>.Enumerator or the nested struct Enumerator of List<int>.

Apparently somebody believed that implementing an iterator in a struct might have some advantages. Indeed this is true, however, since the usually generated IEnumerator<T> implementation is a class, the confusion might be a problem at this point.

Understanding co- and contra-variance

Generics in C# are really great, but without co- and contra-variance they did not shine so bright all the time. The following code should illustrate the issue:

abstract class Vehicle
{
    public bool IsParked { get; set; }
}
class Car : Vehicle { }
class Truck : Vehicle { }

void ParkVehicles(IEnumerable<Vehicle> vehicles) 
{ 
    /* some code */ 
}

static void Main()
{
    List<Car> carpool = new List<Car>();
    carpool.Add(new Car());
    carpool.Add(new Car());
    ParkVehicles(carpool);
}

Now from our point of view it is obvious that a list of cars is a list of (more specialized) vehicles. So an IEnumerable<Car> could be used as an IEnumerable<Vehicle>. However the same argument could then be said about List<Car>. Now we see that this might lead to a problem, since if we could actually use the List<Car> as a List<Vehicle>, then we could also store Truck instances in it. Therefore we can say that what we actually want is:

  • We want to treat List<Car> as List<Vehicle> for reading.
  • We do not want to treat List<Car> as List<Vehicle> for writing.

So what does that mean? In our case what we want is a co-variant treatment of the T in IEnumerable<T>. Something is called co-variant if it preserves the ordering of types, i.e., from more specific to more generic. We have already seen covariance from the first moment on. Remember the following:

object str = "I am a string stored in a variable of type object!";

In contrast we talk about contra-variant if the ordering is reversed, i.e., types are ordered from more generic to more specific ones. This does not happen in the type system, where we would need to write:

string o = new object(); //ouch!

However, there are some cases where the latter is quite useful. Remember delegates? Usually we want to put the least specific type on the input parameters (i.e., if a Vehicle contains all properties we need why should we constraint the parameter to Car?) and the most specific type on the return parameter (why should we return Object when the type is actually a Car?).

Therefore input parameters are a perfect match for contra-variant constraints (specific to general), output parameters are a perfect match for co-variant constraints (general to specific). If the type has to stay the same (not moving up or down in the hierarchy), then it is called invariant.

The solution in case of the IEnumerable was to do the following:

public interface IEnumerable<out T>
{
    /* ... */
}

This was not done with List<T>, since we also have write-operations inside. Of course the given delegates are also co- and contra-variant (since C# 4):

public TReturn Func<in T1, in T2, ..., out TReturn>(T1 arg1, T2 arg2, ...);

The key questions now are:

  1. Where is this built-in? Which interfaces have been updated since .NET 3.5?
  2. When to use co- and contra-variance?

The first question has a simple (and yet deep) answer: All interfaces where either co- or contra-variance is useful have been updated. The IEnumerable<T> has been updated (co-variant, i.e., IEnumerable<out T>), since data is only received. On the other hand interfaces like List<T> have been kept invariant, since data is about to be changed inside. We also have some contra-variant interfaces, as for instance the IComparer<T> has been changed to IComparer<in T>. Here data is only sent, making it a perfect fit for input-parameters-like-behavior.

This already answers part of the second question: We should use this concept for generics (of course!) when we have an interface (we should not use it on full classes due to maintainability and possible problems) that consists only of functions that either only return (co-variance) parameters of a certain type, or which only receive parameters of a certain type.

Comparing co-and contra-variance

This statement can be extended to generic functions and delegates as well. Once we construct a type or signature of a function that uses these types only as input or (this or is exclusive!) output, we benefit (in the long run) by decorating the type argument as being co- or contra-variant.

Using attributes effectively

Another novel features of C#, which is actually part of the Common Language Runtime, is the notion of Attributes. Attributes are meta-data about code, communications from the programmer to the compiler and run-time system. In C, such things are often done with pragmas and non-standard keywords; in Java, marker interfaces are used. Attributes are much richer than any similar capability in comparable languages. They also add a very complex facet to the language, one which many developers will not need. Fortunately, it is possible to write many C# programs without uses attributes at all.

Let's recap what attribute we have already seen: When we introduced enumerations in C# we had a look at the [Flags] attribute, which marks an enumeration as being a bit-flag. Consequently this enables a more optimized string generation and a gives other programmers a hint that combinations of values are supported or even expected.

Another very useful attribute is [Serializable]. This attribute tells the compiler that instances of the given class or struct are allowed to be serialized into bytes. We will not go into details here.

Instead we will focus on a declarative set of attributes. In Windows Forms development one will (sooner or later) start building custom controls. We already touched this topic shortly. A question that has not been answered in the section, which introduced the concept of creating custom controls, was: "How does the integrated designer know the category or default value of a property?". When we have a close look at the property dialog we will see that by default it is ordered by the ascending names of the properties. However, the usual preference is to order the entries in this dialog not only by name, but also by the different categories given by the properties of the control.

The answer of this riddle seems to be in strong relation with attributes (or why is it being answered in this section?). Let's see what kind of attributes are available there by looking at some properties:

public class MyControl : Control
{
    [Category("Universe")]
    [Description("The answer to life, universe and everything.")]
    [DefaultValue(42)]
    public int AnswerToEverything
    {
        get;
        set;
    }
}

The given attributes are defined in the namespace System.ComponentModel. This set of attributes allows us to do most of the specification work when defining properties in our own controls. Additionally DefaultProperty (if the given property should be selected by default), DefaultEvent (if the given event should be selected by default), DisplayName (the name that is displayed by the designer) and Localizable (this property should be localized) are very useful attributes in some scenarios. The following image shows the result as it is shown by the integrated designer.

A look at our own property with description in the designer

Before we can go on with more complicated example we need to see what attributes actually are and how when can use them. Attributes are a type of meta-data that will be accessed by the compiler (special attributes) or can be accessed during runtime over reflection (there it is again!). These attributes are quite tricky. They are instance independent and therefore only type dependent.

Every attribute has to inherit from Attribute. This is very similar to Exception. However, compared to Exception objects, Attribute objects have some restrictions. In their prime usage (as attributes) they can only have independent constructor parameters. Therefore we can not pass in delegates or instance dependent references. The reason is that the attribute instance is created at compile-time, without a running application. So any reference to an application / instance dependent variable would not be resolvable.

Having that said let's create a very simple attribute:

[AttributeUsage(AttributeTargets.Class)]
public class LinkAttribute : Attribute
{
    public LinkAttribute(string url)
    {
        Url = url;
    }

    public string Url { get; private set; }
}

The interesting thing here is that we use an attribute to mark this attribute. Here we use the AttributeUsage attribute to mark our Link attribute to be only applicable for classes. Such a statement is not required, however, quite useful from time to time.

Using this attribute works as expected:

[Link("http://www.google.com")]
public class Google : Webpage
{
    /* ... */
}

As we can see, the constructor is now implicitly called. We also see that the word Attribute has been removed. This is a convention and not required. We could also call the attribute by using LinkAttribute as the name instead of just Link. Since our own attributes do not represent meaningful content for the compiler we need to read them out manually. It has already been said that attributes and reflection belong together, so we know that the way to read those attributes is by using reflection.

public string GetLink(Webpage page)
{
    var attrs = page.GetType().GetCustomAttributes(typeof(LinkAttribute), false);

    if (attrs.Length == 0)
        return string.Empty;

    return ((LinkAttribute)attrs[0]).Url;
}

Here we should note that we explicitly set our attributes to be allowed multiple times or not. By default an attribute can occur multiple times. Also attributes are never required, so there is always the possibility of having no attribute. These cases need to be covered.

Elegant binding

Modern UI applications make heavy use of binding possibilities. Binding is a way of coupling one value to some other. In case of a graphical user interface we want to make a connection between the value that is displayed on the screen and the real value in the right spot. In general this problem is not solvable. There is no CPU instruction that is triggered on access of a certain address.

Therefore this problem is solved in the programming language. In C/C++ getter and setter methods have been introduced (by the programmer). In C# the concept of properties is offered to the programmer. We already discussed the advantages of this approach. Now we still have 2 problems:

  • While reading the value could also go directly over the field (in the corresponding class), writing accesses should always go over the property. Otherwise logic or update routines will not be executed.
  • Just having a property does not solve the problem. The property does also have to trigger some update logic or call another method.

While the first problem is just in our hands (always remember to change UI dependent values only over the property / in a way that the UI knows that the value has been changed) the second problem is not our problem any more. Why that?

There are a lot of (very good) frameworks or libraries out there that manage this binding quite nicely. We will now look at the in-built binding capabilities of the WPF UI framework, which is (in a way) the successor of the Windows Forms UI framework. WPF includes an interface called INotifyPropertyChanged. If we implement this interface in our class we need to implement an event called PropertyChanged. Usually, it looks similar to this:

class MyData : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    void RaisePropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

If we now want to use this in combination with a property we would write code like this:

int answer;

public int Answer
{
    get { return answer; }
    set 
    {
        answer = value;
        RaisePropertyChanged("Answer");
    }
}

The WPF framework takes care of the rest. It is possible to bind instances of our own data classes to the UI. WPF will display the values and allow the user to change them. Each change will result in the property being called. Finally the UI is updated when (bound) properties are raising their changed events.

The problem with this approach is that we always have to write the name of the property twice. Once as an identifier and a second time as a string. The problem is not in the additional typing, but in the (huge) possibility of mistyping the name. Since this is just a string we do not have a compiler check if the given string is a valid property or the property that we actually are referring to. While the first kind of problem could be checked with automatic tests, the second one is much harder to detect.

A very good solution to this problem is given by the following code snippet:

protected void RaisePropertyChanged([CallerMemberName] string propertyName = null)
{
    if (PropertyChanged != null)
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}

Here we are using a special kind of attribute, which tells the compiler to replace the name of the argument with the (member) name of the caller (i.e., the name of the property) if no argument has given. So our property from before could be changed to:

public int Answer
{
    get { return answer; }
    set 
    {
        answer = value;
        RaisePropertyChanged();
    }
}

This version is shorter and less error-prone. There is, however, also a second version (which does not require attributes). While this first version is short, fast and robust, it is also not so flexible and quite limited. For instance we could not pass in the name of other attributes in a robust way. Here we still would have to fall back to the string version, which is not very robust. Another limitation is that a huge load of work is required to read out the actual value. Both of these problems are attacked by using the Expression class.

Let's see an example first:

public static string GetPropertyName<TClass, TProperty>(Expression<Func<TClass, TProperty>> expression)
{
    MemberExpression memberExp;

    if (TryFindMemberExpression(expression.Body, out memberExp))
    {
        var memberNames = new Stack<string>();

        do
        {
            memberNames.Push(memberExp.Member.Name);
        } while (TryFindMemberExpression(memberExp.Expression, out memberExp));

        return string.Join(".", memberNames.ToArray());
    }
    
    return string.Empty;
}

static bool TryFindMemberExpression(Expression exp, out MemberExpression memberExp)
{
    memberExp = exp as MemberExpression;

    if (memberExp != null)
        return true;

    if (IsConversion(exp) && exp is UnaryExpression)
    {
        memberExp = ((UnaryExpression)exp).Operand as MemberExpression;

        if (memberExp != null)
            return true;
    }

    return false;
}

static bool IsConversion(Expression exp)
{
    return (exp.NodeType == ExpressionType.Convert || exp.NodeType == ExpressionType.ConvertChecked);
}

This snippet can now work on codes like

public int Answer
{
    get { return answer; }
    set 
    {
        answer = value;
        RaisePropertyChanged(this => Answer);
    }
}

with RaisePropertyChanged being changed to

void RaisePropertyChanged<TClass, TProperty>(Expression<Func<TClass, TProperty>> expression)
{
    if (PropertyChanged != null)
    {
        var propertyName = GetPropertyName(expression);
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

The big advantage of this construct is that we can still get the value with no problem. All we have to do is evaluate the given method:

static void RaisePropertyChanged<TClass, TProperty>(TClass source, Func<TClass, TProperty> expression) where TClass : MyData
{
    if (PropertyChanged != null)
    {
        string propertyName = GetPropertyName(expression);
        var f = expression.Compile();
        TProperty propertyValue = f(source);
        PropertyChanged(source, new PropertyChangedEventArgs(propertyName));
    }
}

Here we combine several advantages and obtain an extensive solution. To conclude this section: If we only need to know the name of the calling property we can use the compiler attribute, otherwise we might want to use the Expression based one.

Of course, if we are not interested in the property value then the previously described pattern is not interesting at all. More interesting would be an improved version of the originally presented RaisePropertyChanged implementation. Here the following works quite good as an intermediate layer:

protected bool SetProperty<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
{
    if (!object.Equals(storage, field))
    {
        field = value;
        OnPropertyChanged(propertyName);
        return true;
    }

    return false;
}

This snippet can now work on codes like:

public int Answer
{
    get { return answer; }
    set { SetProperty(ref answer, value); }
}

As a general remark with C# 6 we should use the `nameof` operator once we need to supply strings corresponding to type, property, or method (among others) names.

Unsafe code

C# also has the possibility to access the memory directly in a native-like manner. Consider the following C code, where byte is a typedef for char (1 byte):

int main() {
    int a = 35181;
    byte* b = (byte*)&a;
    byte b1 = *b;
    byte b2 = *(b + 1);
    byte b3 = *(b + 2);
    byte b4 = *(b + 3);
    return 0;
}

This code would allow us to split the 4 byte integer type into its components. Nicely though, this code also is perfectly valid C# code!

unsafe static void Main()
{
    int a = 35181;
    byte* b = (byte*)&a;
    byte b1 = *b;
    byte b2 = *(b + 1);
    byte b3 = *(b + 2);
    byte b4 = *(b + 3);
}

Unfortunately (or fortunately as we will see later) this does not work out of the box, since the default compilation option excludes unsafe contexts. We have to enable the "Allow unsafe code" option in the corresponding project's properties. The corresponding dialog is shown in the following image:

Allowing unsafe code by checking the box in the project's properties

There are some reasons for requiring this option:

  • Some platforms run C# only in managed mode, forbidding any unsafe compilation. Hence code written with unsafe blocks will not run there.
  • The general kind of objects is managed, which means they can be repositioned in memory. If we have (fixed) pointers to their addresses we might end up in invalid memory (segmentation fault), since these objects might have been repositioned without us knowing about it.
  • Unsafe code cannot be optimized like managed code. This is not as bad as assembly code in C (from an optimization point of view), but can have negative performance implications.

However, unsafe code might also bring the cure to some performance problems. In unsafe blocks we could iterate over an array without using the C# index operator. This is good, since the C# index operator is causing some overhead by checking if it is within the bounds. In some cases this might be a huge issue, like in analyzing a bitmap.

Before we will discuss this example for an effective usage of the unsafe keyword, let's discuss some other properties (or features) of unsafe scopes. We can create full scopes, methods or definitions (like a class) using the unsafe keyword:

unsafe class MyClass
{
    /* pointers can be used here */
}

class MyOtherClass
{
    unsafe void PerformanceCriticalMethod()
    {
        /* pointers can be used here */
    }
}

class MyThirdClass
{
    void ImportantMethod()
    {
        /* some stuff */

        unsafe
        {
            /* pointers can be used here */
        }

        /* more stuff */
    }
}

One pitfall of unsafe code that has already been mentioned is fixed (pun intended) by using the fixed keyword.

static int x;

unsafe static void F(int* p)
{
  *p = 1;
}

static void Main() 
{
    int[] a = new int[10];
    unsafe 
    {
        fixed (int* p = &x) F(p);
        fixed (int* p = &a[0]) F(p);
        fixed (int* p = a) F(p);
    }
}

A fixed statement is used to fix an array so its address can be passed to a method that takes a pointer. A very convenient use of this (or unsafe blocks in general) is iterating over an array by using pointer arithmetics. Therefore we could use this to fill all entries in multi-dimensional arrays like in the following example:

static void Main()
{
    int[,,] a = new int[2,3,4];
    unsafe 
    {
        fixed (int* p = a)
        {
            for (int i = 0; i < a.Length; ++i)
                p[i] = i;
        }
    }
}

One last thing (before we discuss the example of accessing bitmap data efficiently) that is interesting about such unsafe code is the possibility of replacing new with stackalloc. The problem is the following: What if we want to allocate memory from the call stack to be used by an array of an elementary (or unmanaged) type (like char, int, double, ...)? Right now every array is naturally placed on the heap, where it is managed by the garbage collector (there are ways around it, which will be discussed in the next section).

char* buffer = stackalloc char[16];

The stackalloc keyword can only be used within unsafe contexts. The allocated memory is automatically discarded when the scope is left. Basically the keyword is equivalent to new (from our perspective), with the difference that a clean-up is performed instantly after the scope, maybe resulting in less garbage collector pressure and better performance.

This corresponds to the alloca function, an extension commonly found in C and C++ implementations.

Coming back to the promised example of getting performance boost while manipulating images using an unsafe block. The problem is that the only access in C# to bitmap data (in the Bitmap class) is given by iterating over a 2D array. Each value is then a structure with multiple values for the colors. The whole situation depends naturally on the bitmap (e.g., how many bytes per color) itself.

var path = @"1680x1050_1mb.jpg";
var bmp = (Bitmap)Bitmap.FromFile(path);
var sw = Stopwatch.StartNew();

for (var i = 0; i < bmp.Width; i++)
{
    for (var j = 0; j < bmp.Height; j++)
    {
        bmp.GetPixel(i, j);
    }
}
        
sw.Stop();
Console.WriteLine(sw);

This (pointless) code takes about 2550 ms on my machine. Let's use some unsafe code to speed it up!

var path = @"1680x1050_1mb.jpg";
var bmp = (Bitmap)Bitmap.FromFile(path);
//This is basically a (managed) call to fix the bitmap (no memory movement)
var data = bmp.LockBits(new Rectangle(Point.Empty, bmp.Size), ImageLockMode.ReadOnly, bmp.PixelFormat);
//This gets the number of bytes per pixel, usually the same as the number of colors ...
var bpp = data.Stride / data.Width;
var sw = Stopwatch.StartNew();

unsafe
{
    //This gets the base pointer address - where we can start to scan
    byte* scan0 = (byte*)data.Scan0.ToPointer();
    //This is just a duplicate, however, the one which will be moved
    byte* scan = scan0;
    
    for (var i = 0; i < bmp.Width; i++)
    {
        for (var j = 0; j < bmp.Height; j++)
            scan += bpp;
    }
}
        
sw.Stop();
//Here we free the bitmap, so that it can be moved again
bmp.UnlockBits(data);
Console.WriteLine(sw);

On my machine this code runs in 335ms, or about 8 times faster. One of the biggest drawbacks is that we still have 2 loops. Of course this could be condensed into one loop, however, we now want to be even faster without going into unmanaged code.

How can we achieve this? The magic word is: Interoperability (or short interop). COM interop is a technology included in the .NET CLR that enables COM objects to interact with .NET objects, and vice versa. We can use this to let native code do some array copying. Afterwards we access the whole byte data of the image in a linear array.

var path = @"1680x1050_1mb.jpg";
var bmp = (Bitmap)Bitmap.FromFile(path);
//Same as before - this time required for the interop
var data = bmp.LockBits(new Rectangle(Point.Empty, bmp.Size), ImageLockMode.ReadOnly, bmp.PixelFormat);
//We still need to know how many bytes to skip per pixel
var bpp = data.Stride / data.Width;
//Now we need the number of bytes - data.Stride is a whole line
var bytes = data.Stride * bmp.Height;
//Just to do something in the loop
byte value;
var sw = Stopwatch.StartNew();

//Get the start address - this time it is enough to stay with the IntPtr type
var ptr = data.Scan0;
//Create an empty byte array - this will be the destination
var values = new byte[bytes];
//Marshal is a static class with interesting interop calls
Marshal.Copy(ptr, values, 0, bytes);
    
for (var i = 0; i < bytes; i += bpp)
    value = values[i];
        
sw.Stop();
//Again unlocking is important
bmp.UnlockBits(data);
Console.WriteLine(sw);

Here we start copying at the given address specified by the IntPtr instance. As destination we hand over the byte array called values, with an offset of 0. In total we want to receive bytes bytes.

This code runs in just 14 ms, which is about 20 times as fast as before and 160 times as fast as the first version. Regardless of this performance gain we should still note that no actual work has been done in any of those three scenarios, so the second or the third code have both their good and bad sides.

The problem with the third one is that changes on the image have to be transferred back to the original data source. So here we have a lot of memory transfers going on, with duplicated memory along the way.

The fastest solution is to use the unsafe variant with caching of the properties. The little tweak is to use the following code inside the unsafe block:

byte* scan0 = (byte*)data.Scan0.ToPointer();
byte* scan = scan0;
var width = bmp.Width;
var height = bmp.Height;

for (var i = 0; i < width; i++)
{
    for (var j = 0; j < height; j++)
        scan += bpp;
}

Now we are just above 2 ms, which is close to another magnitude in speed and nearly 3 magnitudes faster than the original version. It turns out that the real performance optimization was something always lying in front of us - caching C# property access to avoid (redundant) expensive computations.

Communication between native and managed code

We already touched the topic of COM interop at the end of the last section. In this section we want to see how we can actually use it and when COM interop becomes quite useful. We will not create own COM interop components or interfaces.

COM interop is one possible way to access functions of native applications from C#. Therefore it is also the way to access the Win32 API from C#. Since the Windows kernel is mostly written in pure C the API is also mostly C like. This means we have a big number of parameters, with return values sometimes also given in form of (to speak some C# here: out) parameters.

Now before we turn to code we have to realize some truths:

  • Many API calls are based on constant values (from enumerations). The problem is that C does not support encapsulation of enumeration values as C# does, resulting in us having to know the exact values of the constants. This means we need to rewrite the enumeration (at least partially) again in C# (sometimes just knowing a single constant is sufficient, however, most of the time we are interested in multiple values).
  • C also does not support a delegate type. Nevertheless sometimes we have to hand in callback functions. These functions require some signature that would not be checked as explicitly as with a delegate in C#. Needless to say we should pass in correct signatures, which means we have also to build required delegates (additionally to enumerations).
  • Some API calls make heavy usage of structures. In C the only way of encapsulating data is given by forming structures. I think from the two points above it is quite obvious that we have to rebuild these struct types in C#.

The last point is actually quite interesting. While a delegate is just a way of ensuring a certain signature and a constant is just a number, a struct is an actual object with a memory address. This object might be moved by the garbage collector or (more likely and worse) structured differently than in C. If we construct the following struct in C (code given in C#)

struct Example
{
    public int a;//4 bytes
    public double b;//8 bytes
    public byte c;//1 byte
}

we know that it will be 13 bytes. Even more we also know that the first 4 bytes are representing an integer, the next 8 a double and the last one a single byte. This order is guaranteed in C, however, in C# it is not. Needless to say we require the guaranteed order for communicating with an API that has been written in C.

There is a neat trick to achieve this in C#. Actually it is not a trick, it is just an attribute to tell the compiler to preserve the ordering.

[StructLayout(LayoutKind.Sequential)]
struct Example
{
    public int a;//4 bytes
    public double b;//8 bytes
    public byte c;//1 byte
}

According to the official specification this attribute is applied automatically to structures, at least when they are used for interop. Nevertheless the recommendation is to use the attribute explicitly just to ensure that everything works as expected. There are other ways to use the StructLayoutAttribute attribute. Another popular way is to specify each field by hand:

[StructLayout(LayoutKind.Explicit, Pack=1, CharSet=CharSet.Unicode)]
struct Example
{
    [FieldOffset(0)]
    public int a;
    [FieldOffset(4)]
    public double b;
    [FieldOffset(12)]
    public byte c;
}

This could also be used to produce union-like structures in C#:

[StructLayout(LayoutKind.Explicit, Pack=1)]
struct Number
{
    [FieldOffset(0)]
    public int Value;
    [FieldOffset(0)]
    public byte First;
    [FieldOffset(1)]
    public byte Second;
    [FieldOffset(2)]
    public byte Third;
    [FieldOffset(3)]
    public byte Fourth;
}

/* Code to use */
var num = new Number();
num.Value = 482912781;
/* In this case we have First = 13, Second = 170, Third = 200, Fourth = 28 */

A union is no new concept. In fact unions are pretty common in C. The following picture demonstrates what's going on in the memory:

Unions have various representations for the same data

Now that we have an impression on how to construct parameters for interop calls, we should look at an actual interop call: We want to access the native GDI API for reading out a certain value (color) of a pixel on the screen.

[DllImport("gdi32.dll")] //GDI is defined in gdi32.dll
 static extern uint GetPixel(IntPtr handle, int x, int y);//uint GetPixel(int*, int, int) is the signature

That's all! If we call the GetPixel method now, the system will look for the file gdi32.dll. It starts in the current directory and continuous in all set paths. Finally it will be found in the System32 directory of Windows itself. After the DLL has been loaded, it does not have to be loaded again.

Everything here seems clear, but wait - what is the address of the handle where we want to get the pixel from? If it is the screen, how do we get the address? Every int* is packed in a managed IntPtr instance. We know that getting the handle of any Control is possible by using the Handle property, however, we do not have a "screen" control.

Two more methods have to be imported to get this handle:

[DllImport("gdi32.dll")]
static extern IntPtr CreateDC(string driver, string device, string output, IntPtr data);

[DllImport("gdi32.dll")]
static extern bool DeleteDC(IntPtr handle);

Now we can write the desired function to get the color of an arbitrary pixel on the screen.

public Color GetScreenPixel(int x, int y)
{
    var screen = CreateDC("Display", null, null, IntPtr.Zero);
    var value = GetPixel(screen, x, y);
    var pixel = Color.FromArgb(value & 0xFF, (value & 0xFF00) >> 8, (value & 0xFF0000) >> 16);
    DeleteDC(screen);
    return pixel;
}

The conversion of the integer is required since the API returns a color in the format ABGR (red and blue swapped). If we want to make a screenshot using this method we will see that every API call requires some time resulting in a really bad performance. Even grabbing 32x32 pixels (1024 API calls) requires about 20 seconds. This means that every P/Invoke call costs us about 20 milliseconds.

The DLL import attribute constructor has some other interesting options like CallingConvention, which uses the CallingConvention enumeration. Here we have possibilities like StdCall (default value) or Winapi (takes the default value from the OS) or Cdecl. The last one supports a variable number of arguments, i.e., it is the best fit for C functions with a variable number of arguments:

[DllImport("msvcrt.dll", CharSet=CharSet.Unicode, CallingConvention=CallingConvention.Cdecl)]
public static extern int printf(String format, int i, double d); 

Additionally there is ThisCall, which could be used to communicate with C++ methods. Here the pointer to the class itself (this) is the first argument - it will then be placed in a special register by convention.

There are a lot more things to mention (as usual), however, in a nutshell everything builds up on the basics that have been discussed in this section. To conclude:

  • In communication with native code always think about only using objects and data types that are the closest to the original. If we pass in instances of classes and other CLR objects then the possibility of having exceptions or false return values is quite big.
  • We should think twice before making a native call. If we do that we should do it only a few times. Therefore searching for already managed code alternatives is always a good start.
  • Always think about the possibility that the required DLL is not on the target system or that we have a typo in the name of the library. Better check twice here.

The references include a link to the page PInvoke.net. Here the most common Windows API calls are described with interop code snippets. This is a great source for getting API access and more without having to dig through the MSDN for hours.

Effective C#

The most important rule in writing efficient C# code is to remember that we are actually bound to CLR types with (some) overhead and managed memory. Now that we think about actual performance optimizations we should always respect code maintainability and readability before optimizations. After everything works just fine we can have a look at the actual performance. If the result is satisfying there is no reason to keep working. Otherwise we can use a tool like PerfView to investigate further.

PerfView lets us investigate CPU usage, managed memory, blocked time and hotspot investigation. Here we see the so-called hot path in our code directly. This is very important, since we do not want to waste time on optimizing unimportant methods or algorithms. Once we go on to do some optimization we should do think about the following items of the following (not complete) list:

  • Optimizing parameters
  • Using static fields and static methods
  • Reducing function calls and using switch
  • Flatten arrays
  • Specifying the capacity for collections
  • Using struct or stop using it
  • Reducing string instantiations and reduce ToString calls
  • Knowing when to use StringBuilder and reusing it

The first item, optimizing parameters, is meant to reduce the number of parameters to the least required number of parameters. Even though this sounds trivial it is often not done correctly. Consider the following code:

int Multiply(int x, int y)
{
    return Multiply(x, y, 1);
}

int Multiply(int x, int y, int z)
{
    return x * y * z;
}

Of course here we make multiple performance mistakes at once. First of all we are not using static methods even though these methods do not rely on any global variables. Also we are invoking another methods call Multiply in Multiply. Additionally the last one places even more elements on the stack (another copy of x and y) with the (in this case) not required variable z. Let's improve this:

static int Multiply(int x, int y)
{
    return x * y;
}

static int Multiply(int x, int y, int z)
{
    return x * y * z;
}

Of course this makes the code less maintainable (if we change something in the method with 3 parameters we will have (most likely) make this change also on the version with 2 parameters), however, the performance is optimized. We could also think about using ref parameters to prevent the local copy, but then (instead of the value) we would just receive a copy of the pointer (which will be even bigger since an int is 4 bytes while a pointer is 8 bytes on a (de-facto standard) 64-bit system).

When we call any method that was not inlined by the JIT, a copy of the variable (which is the value in case of struct variables and the pointer in case of class variables) will be used. This causes stack memory operations. Therefore: It is faster to minimize arguments, and even use constants in the called methods instead of passing them arguments.

Another point we have seen in the example above is the usage of static methods. We should always mark methods as static when they are independent of the instance (no global variables or instance dependent methods are used). Static fields are also faster than instance fields, for the same reason that static methods are faster than instance methods. When we load a static field into memory, the runtime does not have to resolve the instance expression. Static methods do not require the this pointer to be set by the runtime.

Inlining methods is something very common to C/C++ developers. In C# no inline directive or keyword exists. Nevertheless some inlining will be done under certain circumstances by the JIT, which is often conservative. It will not inline medium-sized or large methods and it strongly prefers static methods.

A very important performance optimization is the heavy usage of switch. switch is like a less flexible and more assembly like statement than if. Let's compare the following two code snippets:

if (a == 3)
{
    /* Block 1*/
} 
else if (a == 4)
{
    /* Block 2 */
}
else
{
    /* Block 3 */
}

switch(a)
{
    case 3:
        /* Block 1 */
        break;
    case 4:
        /* Block 2 */
        break;
    default:
        /* Block 3 */
        break;
}

The first question would be about the readability: This is really a matter of taste, however, I think that the switch version has some benefits. The real advantage now lies in its performance. While the if version will do several compares and jumps internally, the switch will just do a massive compare and jump. Using jump tables makes switches much faster than some if-statements. Also, using a char switch on a string is very fast. In general we might say that using char arrays is sometimes the fastest way to build up and examine a string.

The topic of arrays brings us to the potential performance optimization by flattening arrays. In a short micro-benchmark we could see that multi-dimensional arrays are relatively slow. Therefore creating a one-dimensional array and accessing it through arithmetic can boost the performance significantly. This dimensional reduction is called flattening an array.

Converting a 2D array to a 1D array

Let's see an example of computing the sum of a 2D array of doubles:

double ComputeSum(double[,] oldMatrix)
{
    int n = oldMatrix.GetLength(0);
    int m = oldMatrix.GetLength(1);
    double sum = 0.0;

    for (int i = 0; i < n; i++)
        for (int j = 0; j < m; j++)
            sum += oldMatrix[i, j];

    return sum;
}

double ComputeSum(double[] matrix)
{
    int n = matrix.Length;
    double sum = 0.0;

    for (int i = 0; i < n; i++)
        sum += matrix[i];

    return sum;
}

For collections, we should try to find the optimum value for the optional capacity argument. This argument is able to influence the initial buffer sizes. A good choice helps to avoid many allocations when appending elements. In general collections are slower than fixed arrays. A List<double> is 50% to 300% slower than a double[] depending on the compilation optimizations.

A critical and difficult performance optimization is the topic of using structures. While they can improve the performance of the GC by reducing the number of distinct objects, they are also always passed to methods by value. Therefore Microsoft applies the rule that struct types should not contain more than 16 bytes in data. They also should only be used for creating really elementary data types.

It is quite inefficient to use the ToString method too often. First of all its another method call (there is some price tag on this), additionally we are creating a string instance! This allocation requires some memory and puts some pressure on the GC. A really inefficient example is shown below:

static bool StringStartsWithA(string s)
{
    return s[0].ToString() == "a" || s[0].ToString() == "A";
}

A much more improved version would look like this:

static bool StringStartsWithA(string s)
{
    return s.Length > 0 && char.ToLower(s[0]) == 'a';
}

This does not look like much improvement, but actually we did a lot of things:

  • We reduced 2 ToString calls
  • We omitted 2 string creations
  • We made the code more stable (what if the string is empty?) (Note: checking for null could also be good)
  • We are comparing characters instead of strings

Those string conversions are quite nasty. We should try to avoid them as often as possible. For example, we may need to ensure that a string is lowercased. If the string is already lowercase, we should avoid allocating a new string entirely. This is quite the same scenario as above, only that instead of a single character we would compare against a single string (lower case). This should be done character-wise.

We already discussed that a StringBuilder object is much faster at appending strings together than using Concat with strings. However, StringBuilder is only the far better choice with big or many strings. If we have only a few concatenations or very small strings (2 or characters), then creating a whole StringBuilder instance is too much. If we need to use the StringBuilder quite often we should think about creating a StringBuilder spooler, where just a few instances (2 to 3 are usually more than enough) are used. Those instances can then be recycled.

StringBuilder pooling is the most efficient method

Last but not least we could also think about replacing divisions in our code or creating string tables for many used int values. Those are quite effective changes, however, they are quite time-consuming to implement. C# is relatively slow when it comes to division operations. One alternative is to replace divisions with a multiplication-shift operation to optimize the performance. Therefore we have something like the following:

static int Divide(int num)
{
    return num / div;//div has to be a constant value - we have to know it
}

static int MulShift(int num)
{
    return (num * mul) >> shift; //if div above is known then mul and shift can be inferred by us
}

Now the hard part is to compute mul and shift for the specific value of div. In general it is there even better to avoid divisions as often as possible:

static Point3 NormalizePointInefficient(Point3 p)
{
    double norm = Math.Sqrt(p.X * p.X + p.Y * p.Y + p.Z * p.Z);
    return new Point(p.X / norm, p.Y / norm, p.Z / norm);
}

static Point3 NormalizePointImproved(Point3 p)
{
    double norm = 1.0 / Math.Sqrt(p.X * p.X + p.Y * p.Y + p.Z * p.Z);
    return new Point(p.X * norm, p.Y * norm, p.Z * norm);
}

In conclusion we can say that performance optimization is highly dependent on the application one is writing. Usually (in C#) we want to preserve readability where possible, however, if we experience performance drawbacks, we still have some powerful weapons to attack those issues. The most important concept is finding out what methods eat the performance and optimizing those in a priority list.

Outlook

This time there is no outlook! Unless I somehow change my mind in the future, this will be the last part in this series of tutorials on programming with C#. I hope I could show you that C# is a modern, effective (measured in time spent per project) and elegant programming language, which provides a clear structure and robust code-basis.

Even though C# (usually) produces managed code, we are able to bring in some optimizations and use (external) native code for performance critical parts. Nowadays people are playing around with C# to native code compilers and it seems that even whole operating systems could be developed purely in C# without huge performance drawbacks.

Other Articles in this Series

  1. Lecture Notes Part 1 of 4 - An advanced introduction to C#
  2. Lecture Notes Part 2 of 4 - Mastering C#
  3. Lecture Notes Part 3 of 4 - Advanced programming with C#
  4. Lecture Notes Part 4 of 4 - Professional techniques for C#

References

History

  • v1.0.0 | Initial Release | 21.04.2016
  • v1.0.1 | Added article list | 22.04.2016
  • v1.0.2 | Updated some typos | 23.04.2016
  • v1.1.0 | Updated structure w. anchors | 25.04.2016
  • v1.1.1 | Added table of contents | 29.04.2016
  • v1.2.0 | Thanks to Christian Andritzky for pointing out the bitmap perf. problem | 10.05.2016
  • v1.3.0 | Thanks to Alexey KK for mentioning the INotifyPropertyChanged issue | 13.05.2016

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Florian Rappl
Chief Technology Officer
Germany Germany
Florian lives in Munich, Germany. He started his programming career with Perl. After programming C/C++ for some years he discovered his favorite programming language C#. He did work at Siemens as a programmer until he decided to study Physics.

During his studies he worked as an IT consultant for various companies. After graduating with a PhD in theoretical particle Physics he is working as a senior technical consultant in the field of home automation and IoT.

Florian has been giving lectures in C#, HTML5 with CSS3 and JavaScript, software design, and other topics. He is regularly giving talks at user groups, conferences, and companies. He is actively contributing to open-source projects. Florian is the maintainer of AngleSharp, a completely managed browser engine.

Comments and Discussions

 
SuggestionYou missed very important thing on unmanaged code Pin
Vander Wunderbar27-Aug-16 10:18
memberVander Wunderbar27-Aug-16 10:18 
GeneralRe: You missed very important thing on unmanaged code Pin
Florian Rappl27-Aug-16 11:17
professionalFlorian Rappl27-Aug-16 11:17 
GeneralRe: You missed very important thing on unmanaged code Pin
Vander Wunderbar27-Aug-16 11:30
memberVander Wunderbar27-Aug-16 11:30 
PraiseWell done Pin
Espen Harlinn11-Jun-16 2:42
mentorEspen Harlinn11-Jun-16 2:42 
GeneralRe: Well done Pin
Florian Rappl11-Jun-16 2:53
professionalFlorian Rappl11-Jun-16 2:53 
PraiseGreat Pin
OS196031-May-16 5:19
professionalOS196031-May-16 5:19 
GeneralRe: Great Pin
Florian Rappl31-May-16 6:17
professionalFlorian Rappl31-May-16 6:17 
PraiseCo-variance vs. contra-variance Pin
ronlease24-May-16 4:00
professionalronlease24-May-16 4:00 
GeneralRe: Co-variance vs. contra-variance Pin
Florian Rappl24-May-16 4:47
professionalFlorian Rappl24-May-16 4:47 
Question5 Pin
Karthik_Mahalingam15-May-16 5:05
mveKarthik_Mahalingam15-May-16 5:05 
AnswerRe: 5 Pin
Florian Rappl15-May-16 8:49
professionalFlorian Rappl15-May-16 8:49 
QuestionSorry but after Richter, John Skeet and Charles Petzold you should verify this word "professional" Pin
Alexey KK11-May-16 8:17
professionalAlexey KK11-May-16 8:17 
AnswerRe: Sorry but after Richter, John Skeet and Charles Petzold you should verify this word "professional" Pin
Florian Rappl11-May-16 8:34
professionalFlorian Rappl11-May-16 8:34 
Suggestion2nd version of accessing bitmap pixels is flawed Pin
Christian Andritzky10-May-16 7:07
memberChristian Andritzky10-May-16 7:07 
GeneralRe: 2nd version of accessing bitmap pixels is flawed Pin
Florian Rappl10-May-16 7:17
professionalFlorian Rappl10-May-16 7:17 
GeneralRe: 2nd version of accessing bitmap pixels is flawed Pin
Florian Rappl10-May-16 7:44
professionalFlorian Rappl10-May-16 7:44 
Generalmy vote of 5 Pin
Southmountain26-Apr-16 17:36
memberSouthmountain26-Apr-16 17:36 
GeneralRe: my vote of 5 Pin
Florian Rappl26-Apr-16 20:36
professionalFlorian Rappl26-Apr-16 20:36 
GeneralRe: my vote of 5 Pin
annaaaa9129-Apr-16 4:36
memberannaaaa9129-Apr-16 4:36 
GeneralMy vote of 5 Pin
Dmitriy Gakh25-Apr-16 20:00
professionalDmitriy Gakh25-Apr-16 20:00 
GeneralRe: My vote of 5 Pin
Florian Rappl25-Apr-16 21:58
professionalFlorian Rappl25-Apr-16 21:58 
GeneralMy vote of 5 Pin
SMerrill8825-Apr-16 14:52
memberSMerrill8825-Apr-16 14:52 
GeneralRe: My vote of 5 Pin
Florian Rappl25-Apr-16 21:57
professionalFlorian Rappl25-Apr-16 21:57 
GeneralWOW Pin
AnnaTillett7525-Apr-16 12:47
memberAnnaTillett7525-Apr-16 12:47 
GeneralRe: WOW Pin
Florian Rappl25-Apr-16 21:57
professionalFlorian Rappl25-Apr-16 21:57 

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Posted 20 Apr 2016

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