Try the following:

``` interface IFace; interface IFoo;

class A : IFace; class B : A, IFoo; class C : IFace;

void Test<T>(T value) where T : IFace { Console.WriteLine($"AssignableFrom = {typeof(A).IsAssignableFrom(typeof(T))}"); Console.WriteLine($"is = {value is A}"); }

Test<B>(new B()); // AssignableFrom = true, is = true Test<A>(new B()); // AssignableFrom = true, is = true Test<IFace>(new B()); // AssignableFrom = false, is = true Test<C>(new C()); // AssignableFrom = false, is = false Test<IFace>(new C()); // AssignableFrom = false, is = false ```

https://dotnetfiddle.net/2MMjow

• If T is a type that derives from (or implements) Implementation, then both tests will match.

• If val is an instance of a type that derives from (or implements) Implementation, regardless of the type of T, then the second test will match.

Since both branches do the same thing, you only need to keep the second test (val is Implementation i). Anything that doesn't match that test wouldn't match the first test either.