Dictionary using member as key

Question

1.00/5 (2 votes)

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how many times have you created a dictionary that looks like this:

C#

MyClass a;
var data = new Dictionary<string, MyClass>();
data.Add(a.Name,a);

Or maybe worse:

C#

var data = new Dictionary<string, string>();
data.Add(a,a);

what if we could create a Dictionary that bound the key to a member?
something like:

C#

MyClass a;
var data = new MemberBindingDictionary<MyClass>("Name");
data.Add(a);
if(data.ContainsKey(a.Name))
{
...
}

I couldn't find anything like it. Is that because it's a stupid idea? or is it too expensive?

Posted 4-Mar-13 6:31am

Yossi Yaari

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Comments

Sergey Alexandrovich Kryukov 4-Mar-13 16:06pm

Not clear what's the problem. It happens all the time and is very usual. How can it possibly compromise performance?
This question is not needed at all, you should just speculate by definition, and you will understand what happens.
—SA

2 solutions

Solution 2

I would probably just use link with an array of MyClass like this.

C#

IEnumerable<myclass> data = new MyClass[] { new MyClass("Greg"), new MyClass("Peter"), new MyClass("Bobby") };

if (data.Count(x => x.Name == "Peter") > 0)
{
   // Do something.
}

Posted 4-Mar-13 7:28am

BC @ CV

Updated 4-Mar-13 7:33am

v3

Comments

Yossi Yaari 5-Mar-13 0:49am

that would have very slow performance.
O(n) instead of O(1)

BC @ CV 5-Mar-13 17:24pm

"Premature optimization is the root of all evil" -- DonaldKnuth
In my 10 years as a programmer I have rarely had a serious problem with code that performed too slow and have regularly had headache inducing problems with overly complicated and hard to maintain code. Unless the project requires super optimized performance I always go for the simplest most easy to read solution.

Yossi Yaari 10-Mar-13 11:42am

I do agree with you in principal.

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Thomas Daniels · Accepted Answer · 2013-03-04T07:25:00

Solution 1

Hi,

There's not a class like that in C#. But you can create a class like that yourself:

C#

public class MemberBindingDictionary<T>:Dictionary<object, T>
    {
        Func<T,object> _binding = null;
        public MemberBindingDictionary(Func<T,object> binding)
        {
            _binding = binding;
        }
        public void Add(T value)
        {
            this.Add(_binding(value), value);
        }
    }

I don't use a string as a parameter in the constructor, but a Func. Now, you can use this class, and bind the value to a key using a lambda expression:

C#

MyClass a;
var data = new MemberBindingDictionary<MyClass>(x => x.Name);
data.Add(a);
if(data.ContainsKey(a.Name))
{
...
}

So, don't write "Name", but write x => x.Name. This is a lambda expression[^]

Hope this helps.

[Edit]Formatting problems fixed[/Edit]

Posted 4-Mar-13 7:25am

Thomas Daniels

Updated 5-Mar-13 7:14am

v2

Comments

Maciej Los 4-Mar-13 13:29pm

BIG 5!

Thomas Daniels 4-Mar-13 13:34pm

Thank you!

Yossi Yaari 4-Mar-13 13:40pm

that's definitely the direction I was thinking.
just was wondering how bad the implementation would turn out. mostly in terms of performance.

and of course you could add a ContainsByKey using the same binding func. :-)

Thomas Daniels 4-Mar-13 13:42pm

You don't must declare a ContainsKey method yourself. Because I inherit from Dictionary, this method is inherited already.

Yossi Yaari 5-Mar-13 0:48am

what I mean is that you could add a method that accepts object of Type <t> and calculates the key by itself using the binding method. so that in theory, if the binding changed, you could leave the code the same.

MyClass a;
var data = new MemberBindingDictionary<myclass>(x => x.Name);
data.Add(a);
if(data.ContainsKey(a)) //<-- would stay true for any change of ^^^ binding method
{
...
}

Thomas Daniels 5-Mar-13 13:12pm

If you want to do that, have a look at this article:
Bind Better with INotifyPropertyChanged[^]