C# generics: using class generic in where clause of method generic

Question

Can anyone explain why isn't this possible (at least in .Net 2.0):

public class A<T> { public void Method<U>() where U : T { ... } } ... A<K> obj = new A<K>(); obj.Method<J>();

with K being the superclass of J

EDIT

I've tried to simplify the problem in order to make the question more legible, but I've clearly overdo that. Sorry!

My problem is a little more specific I guess. This is my code (based on this):

public class Container<T> { private static class PerType<U> where U : T { public static U item; } public U Get<U>() where U : T { return PerType<U>.item; } public void Set<U>(U newItem) where U : T { PerType<U>.item = newItem; } }

and I'm getting this error:

Container.cs(13,24): error CS0305: Using the generic type Container<T>.PerType<U>' requires2' type argument(s)

It is possible in 4.0. Not very useful, because you do not use the type parameter in the signature, but definitely possible. — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Jun 4, 2012 at 17:16
You are absolutely right, I have edited the question. Sorry. — andresp
– andresp, Commented Jun 4, 2012 at 17:40
Well, I definitely get the error above. This happens inside Unity which uses Mono compiler. I don't know if it might have something to do with it. — andresp
– andresp, Commented Jun 4, 2012 at 18:14

YavgenyP · Accepted Answer · 2012-06-04 17:19:30Z

8

Actually it is possible. This code compiles and runs just fine:

public class A<T> { public void Act<U>() where U : T { Console.Write("a"); } } static void Main(string[] args) { A<IEnumerable> a = new A<IEnumerable>(); a.Act<List<int>>(); }

What is not possible is using covariance / contravariance in generics, as explained here:

IEnumerable<Derived> d = new List<Derived>(); IEnumerable<Base> b = d;

answered Jun 4, 2012 at 17:19

YavgenyP

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6 Comments

YavgenyP Over a year ago

still, it compiles and works (of course U Item will remain a null at all times - you didnt provide a way to initialize your PertTyp class / its item field)

andresp Over a year ago

Please check my latest comment to my question. The initialization is supposed to be done by calling the Set method. obj.Set<J>(jObj) The PerType class should be static. I've removed it while debugging. I've edited the question to add the keyword again.

YavgenyP Over a year ago

it still works.. maybe you should provide ur entire complete code?

andresp Over a year ago

This is my entire code. There is really nothing more. I've started a new Unity project with just this Class and it gives the same error. Maybe it is a Mono or Unity specific error. I'll try to test it in other platforms. Thanks for your help.

andresp Over a year ago

It looks like it can eventually be related with the Mono compiler in Unity. answers.unity3d.com/questions/262475/… I'll try to further confirm this hypothesis tomorrow. Thank you for your help.

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Olivier Jacot-Descombes · Accepted Answer · 2012-06-04 19:50:21Z

It works for me (VS 2008).

Do you have a problem with a class visibility? (class not public, wrong namespace)

Which error message are you getting?

UPDATE

Given your implementation of Container<T> I can write

class A { } class B : A { } class Test { public void MethodName( ) { var obj = new Container<A>(); obj.Set(new B()); } }

This works perfectly. Are you sure that B derives from A? Note that for instance List<B> does NOT derive from List<A> (see YavgenyP's answer).

The error message could be a hint, telling you that there exists another Container<T> in another namespace requiring a second type argument for PerType<U, X??? >.

This is probably a bug with Mono compiler or Unity. I've asked this same question in their Q&A system. Currently I have no way to test this (I will check it tomorrow at the office), but it seems to be a problem with the compiler: answers.unity3d.com/questions/262475/… Thanks for your insight though.
I've just confirmed it. adding the "Container<T>." prefix solves the problem. it should be caused by some bug in Mono C# compiler.

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