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In this article we will try to understand two terminologies: Reflection and the
dynamic keyword. Many developers get confused between them because both of them help us to do dynamic invocation.
In this article we will try to unleash the differences and see in what scenarios we will use which. But before we start comparing let's understand each separately and then at the end of the article we will do a full comparison.
What is Reflection and why we need it?
Reflection is needed when you want to determine / inspect contents of an assembly. For example look at your Visual Studio editor intellisense, when you type “.” (dot) before any object, it gives you all the members of the object. This is possible because of Reflection.
Reflection also goes one step further; it can also invoke a member which is inspected. For instance if Reflection detects that there is a method called
GetChanges in an object, we can get a reference to that method instance and invoke it on runtime.
In simple words Reflection passes through two steps: “Inspect” and “Invoke” (optional). The "Invoke" process is optional.
Implementing reflection in c# is a two step process , 1st get the “type” of the object and then use the type to browse members like “methods” , “properties” etc.
Step 1: The first step is to get the type of the object. So for example you have a DLL ClassLibrary1.dll which has a class called
Class1. We can use the
Assembly (belongs to the
System.Reflection namespace) class to get a reference to the type of the object. Later we can use
Activator.CreateInstance to create an instance of the class. The
GetType() function helps us to get a reference to the type of the object.
var myAssembly = Assembly.LoadFile(@"C:\ClassLibrary1.dll");
var myType = myAssembly.GetType("ClassLibrary1.Class1");
dynamic objMyClass = Activator.CreateInstance(myType);
Type parameterType = objMyClass.GetType();
Step 2: Once we have a reference of the type of the object we can then call
GetProperties to browse through the methods and properties of the class.
foreach (MemberInfo objMemberInfo in parameterType.GetMembers())
foreach (PropertyInfo objPropertyInfo in parameterType.GetProperties())
In case you want to invoke the member which you have inspected, you can use
InvokeMember to invoke the method. Below is the code:
BindingFlags.NonPublic | BindingFlags.InvokeMethod |
BindingFlags.Instance,null, objMyClass, null);
What are the practical uses of Reflection?
- If you are creating an application like a Visual Studio editor where you want to show the internals of an object by using intellisense.
- If you are creating a unit testing framework. In unit testing frameworks we need to invoke methods and properties dynamically for testing purpose.
- Sometimes we would like to dump properties, methods, and assembly references to a file or show it on screen.
What is the use of the dynamic keyword?
Programming languages can be divided into two categories: strongly typed and dynamically typed. Strongly typed languages are those where checks happen during compile time while dynamic languages are those where type checks are bypassed during compile time. In a dynamic language object types are known only during runtime and type checks are activated only at runtime.
We would like to take advantage of both worlds. Because many times we do not know the object type until the code is executed. In other words we are looking at something like a dynamically and statically typed kind of environment. That’s what the
dynamic keyword helps us with.
If you create a variable using the
dynamic keyword and if you try to see members of that object, you will get a message as shown below “will be resolved at runtime”.
Now try the below code out. In the code I have created a
dynamic variable which is initialized with string data. And in the second line I am trying to have fun by trying to execute a numeric incremental operation. So what will happen now? Think....
dynamic x = "c#";
Now this code will compile fine without any complaints. But during runtime it will throw an exception complaining that the mathematical operations cannot be executed on the variable as it's a string type. In other words during runtime the
dynamic object gets transformed from the general data type to a specific data type (e.g.:
string for the below code).
One of the biggest practical uses of the
dynamic keyword is when we operate on MS Office components via interop.
So for example if we are accessing Microsoft Excel components without the
dynamic keyword, you can see how complicated the code gets. Lots of casting happening in the below code, right?
Application excelApplication = new Application();
((Excel.Range)excelApp.Cells[1, 1]).Value2 = "Name";
Excel.Range range2008 = (Excel.Range)excelApp.Cells[1, 1];
Now look at how simple the code becomes by using the
dynamic keyword. No casting needed and during runtime type checking also happens.
dynamic excelApp = new Application();
excelApp.Cells[1, 1].Value = "Name";
Excel.Range range2010 = excelApp.Cells[1, 1];
What is the difference between Reflection and dynamic?
- Both Reflection and
dynamic are used when we want to operate on an object during runtime.
- Reflection is used to inspect the meta-data of an object. It also has the ability to invoke members of an object at runtime.
dynamic is a keyword which was introduced in .NET 4.0. It evaluates object calls during runtime. So until the method calls are made the compiler is least bothered if those methods / properties exist or not.
dynamic uses Reflection internally. It caches the method calls made thus improving performance to a certain extent.
- Reflection can invoke both public and private members of an object while
dynamic can only invoke public members.
dynamic is instance specific: you don't have access to static members; you have to use Reflection in those scenarios.
Below is the detailed comparison table which shows in which scenario they are suited:
|Inspect (meta-data) ||Yes ||No |
|Invoke public members||Yes||Yes|
|Invoke private members||Yes||No|
|Static class ||Yes ||No |
Below is a simple diagram which summarizes visually what Reflection can do and what the
dynamic keyword can do.
I am thankful to the www.questpond.com team without whom it would have been difficult to complete this article. You can visit Questpond for .NET interview questions with answer videos.
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