Question

I have a generic method with this signature:

private void MyGenericMethod<T>(T arg) where T : class
{}

If I pass, say, an integer to this method, I get an ArgumentException that the passed value does not match the restriction. This is great, but how can I predetermine that what I am passing will match the “class” constraint so that the exception is not thrown?

Answer 1

The compiler will already do that for you – you should actually see:

The type ‘int’ must be a reference type in order to use it as parameter ‘T’ in the generic type or method ‘blah.MyGenericMethod(T)’

at compile-time.

The tricky scenarios are:

• generics upon generics upon generics – all those type-constraints stack, so you end up with where T : class a lot. Sometimes it is better to use runtime validation against T
• reflection (MakeGenericMethod, etc) – again, just check at runtime

Also, note that where T : class doesn’t actually mean T is a class – it means it is a reference-type, which can include interfaces and delegates. Likewise, where T : struct doesn’t actually mean T is a struct – it means it is a struct that is not Nullable<>.