run-time generics exception

Question

The following example compiles without errors, but causes an exception when executed. The exception message reads: 'tried to implicitly implement an interface method with weaker type parameter constraints'. I have two questions:
Why does this exception occur when the code seems to be well formed?
Why does this exception happen at run-time rather than compile time?

Code Snippet

namespace Test
{
 interface IBase<T>
 {
  void Method<T1>() where T1: T;
 }

 class Base<T>: IBase<T>
 {
  public void Method<T1>() where T1: T
  {}
 }

 interface IDerived: IBase<int>
 {}

 class Derived: Base<int>, IDerived
 {}

 class Program
 {
  static void Main(string[] args)
  {
   IDerived s = new Derived();
  }
 }
}

Thanks,
Tomas

=== Edited by tomc_bc @ 29 Mar 2007 9:32 PM UTC===
I did some further investigation and here are the results:

I rewrote the above code in managed C++ and surprisingly, the error did not occur. I disassembled the C# and C++ assemblies and found one relevant difference; below are snippets of the disassembled code:

C#:
.class private auto ansi beforefieldinit Test.Derived
       extends class Test.Base`1<int32>
       implements Test.IDerived,
                  class Test.IBase`1<int32>;
C++:
.class private auto ansi beforefieldinit Test.Derived
       extends class Test.Base`1<int32>;
       implements Test.IDerived

I removed the 'class Test.IBase`1<int32>' declaration from the C# code and reassembled it. Lo and behold, this fixed the problem. Unless there is some reason why the C# code emitter is putting in the inherited interface implementation, I would venture to guess that this is a bug in the C# code emitter. Is there any way to make the C# code emitter work like the C++ code emitter?

Your comments would be most appreciated.

 

Answer 1

Tomas,

you can avoid your error by implementing explicitly IBase in Base:

 

Code Snippet

class Base<T> : IBase<T> {

  void IBase<T>.Method<T1> () { }

}

 

HTH
--mc

Answer 2

Hi Thomas,

 

I compiled and dissassembled your code with my C# 3.5 (Orcas) compiler and this is what I got

 

.class private auto ansi beforefieldinit Test.Derived

extends class Test.Base`1<int32>

implements Test.IDerived,

class Test.IBase`1<int32>

 

It seems like this is a fixed bug .

 

Nice catch! and great explanation.

 

Thanks,

 

Marcelo.