c# - Why calling ISet<dynamic>.Contains() compiles, but throws an exception at runtime?

Question

Please, help me to explain the following behavior:

dynamic d = 1;
ISet<dynamic> s = new HashSet<dynamic>();
s.Contains(d);

The code compiles with no errors/warnings, but at the last line I get the following exception:

Unhandled Exception: Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: 'System.Collections.Generic.ISet<object>' does not contain a definition for 'Contains'
   at CallSite.Target(Closure , CallSite , ISet`1 , Object )
   at System.Dynamic.UpdateDelegates.UpdateAndExecuteVoid2[T0,T1](CallSite site, T0 arg0, T1 arg1)
   at FormulaToSimulation.Program.Main(String[] args) in 

As far as I can tell, this is related to dynamic overload resolution, but the strange things are

(1) If the type of s is HashSet<dynamic>, no exception occurs.

(2) If I use a non-generic interface with a method accepting a dynamic argument, no exception occurs.

Thus, it looks like this problem is related particularly with generic interfaces, but I could not find out what exactly causes the problem.

Is it a bug in the compiler/typesystem, or legitimate behavior?

Answer 1

The answers you have received so far do not explain the behaviour you are seeing. The DLR should find the method ICollection<object>.Contains(object) and call it with the boxed integer as a parameter, even if the static type of the variable is ISet<dynamic> instead of ICollection<dynamic> (because the former derives from the latter).

Therefore, I believe this is a bug and I have reported it to Microsoft Connect. If it turns out that the behaviour is somehow desirable, they will post a comment to that effect there.