How can I use interface as a C# generic type constraint?

Question

Is there a way to get the following function declaration?

public bool Foo<T>() where T : interface;

ie. where T is an interface type (similar to where T : class, and struct).

Currently I've settled for:

public bool Foo<T>() where T : IBase;

Where IBase is defined as an empty interface that is inherited by all my custom interfaces... Not ideal, but it should work... Why can't you define that a generic type must be an interface?

For what it's worth, I want this because Foo is doing reflection where it needs an interface type... I could pass it in as a normal parameter and do the necessary checking in the function itself, but this seemed alot more typesafe (and I suppose a little more performant, since all the checks are done at compiletime).

Answer 1

No, actually, if you are thinking class and struct mean classes and structs, you're wrong. class means any reference type (e.g. includes interfaces too) and struct means any value type (e.g. struct, enum).

Answer 2

You cannot do this in any released version of C#, nor in the upcoming C# 4.0. It's not a C# limitation, either - there's no "interface" constraint in the CLR itself.

Answer 3

What would be the benefit of having a constraint of this type? There is nothing you could do that you couldn't do by using the "class" restraint.

Answer 4

The closest you can do (except for your base-interface approach) is "where T : class", meaning reference-type. There is no syntax to mean "any interface".

This ("where T : class") is used, for example, in WCF to limit clients to service contracts (interfaces).

Answer 5

Use an abstract class instead. So, you would have something like:

public bool Foo() where T : CBase;