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() where T : interface;

ie. where T is an interface type

Answer 1

To follow up on Robert's answer, this is even later, but you can use a static helper class to make the runtime check once only per type:

public bool Foo() where T : class
{
    FooHelper.Foo();
}

private static class FooHelper where TInterface : class
{
    static FooHelper()
    {
        if (!typeof(TInterface).IsInterface)
            throw // ... some exception
    }
    public static void Foo() { /*...*/ }
}

I also note that your "should work" solution does not, in fact, work. Consider:

public bool Foo() where T : IBase;
public interface IBase { }
public interface IActual : IBase { string S { get; } }
public class Actual : IActual { public string S { get; set; } }

Now there's nothing stopping you from calling Foo thus:

Foo();

The Actual class, after all, satisfies the IBase constraint.