C# inheritance in generics question

Question

I have two interfaces:

public interface A { 
 void aMethod(); 
}


public interface B : A {
 void bMethod();
}

Later I'm basically using a dictionary like this:

Dictionary<int, A> dict = new Dictionary<int, B>();

C# is saying I can't convert from the right to the left, even if I cast it. Is there a way to use generics in C# so that this can work? If I make them abstract classes it seems to be ok but I need these as interfaces.

@Dinah: Why would the framework version make any difference in this case? — LukeH, Nov 10 '10 at 17:27
@LukeH: because .NET 4.0 has Covariance and Contra-variance which may address this. If dmurph is having this issue despite being on 4.0, the issue would be different than if the issue occurred pre-4.0 — Dinah, Nov 10 '10 at 17:40
@Dinah: C#4 introduced variance for generic interfaces and delegates, *not* classes, so would make no difference if trying to assign a `Dictionary` to a `Dictionary`. — LukeH, Nov 10 '10 at 22:06
It looks like I'm not even using 4.0. Everyone here looks correct... — Daniel Murphy, Nov 13 '10 at 00:46

score 11 · Accepted Answer · edited May 23 '17 at 12:09

11

The feature you are looking for is what's referred to as generics variance (covariance and contravariance). There is limited support for this starting in .net framework 4. Here's an interesting post: How is Generic Covariance & Contra-variance Implemented in C# 4.0?

And here's the MSDN entry on Covariance and Contravariance in Generics.

edited May 23 '17 at 12:09

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answered Nov 10 '10 at 16:45

Mike Dinescu

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thanks for the msdn link, that was helpful, looks like I'll have to tackle my problem in some other way – Daniel Murphy Nov 13 '10 at 00:50

score 9 · Answer 2 · edited Jul 21 '22 at 19:46

Nope it's a covariance issue. If you could do:

Dictionary<int, A> dict = new Dictionary<int, B>();

It would be possible without a compiler error to put an object of Type A in dict.

The problem is that dict looks like: Dictionary<int, A> but it is really type Dictionary<int, B>() (so placing an object of Type A would throw a runtime error because of an invalid cast), so you shouldn't be allowed to even try to place an object of Type A in dict, which is why you can't do :

Dictionary<int, A> dict = new Dictionary<int, B>();

It's protecting you from making a runtime mistake.
You want to check out Eric Lippert's blog on the subject.

It's one of his favorite topics to talk about, so it's quite thorough.

score 2 · Answer 3 · edited Jan 17 '22 at 09:05

I don't think you're going to find a way around your problem because the thinking behind it seems flawed. To illustrate, let's create another interface:

public interface C : A
{
    void cMethod();
}

Now, let's use your code:

Dictionary<int, A> dict = new Dictionary<int, B>();

What happens when I try the following?

C c = new ClassThatImplementsC(); 
dict.Add(1, c);

Take a look at Eric Lippert's Covariance and Contravarience FAQ for many, many more details.

C# inheritance in generics question

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