Well firstly, Nullable<T> is a struct, so there isn’t an object as such. You can’t call GetType(), as that will box the value (at which point you either get null and thus an exception, or a boxed non-nullable value and therefore not the type you want).
(Boxing is what’s messing up your assertion here – I would assume that IsType accepts object.)
You can use type inference though to get the type of the variable as a type parameter:
public bool IsNullable<T>(T value)
{
return Nullable.GetUnderlyingType(typeof(T)) != null;
}
That’s not a huge amount of use when you know the exact type at compile-time as in your example, but it’s useful for generics. (There are alternative ways of implementing it, of course.)
What’s your real life situation? I assume it’s not an assertion like this, given that you know the answer to this one at compile time.