Which is the preferred syntax for a Generic Class Derivation Constraint?

Question

The documented derivation constraint uses a where T : clause and the sample code that I'm tinkering with is

public class TwoThingsIPC<T> where T : IPassClass
{ ...
}

where IPassClass is an interface.

Code from a third-party that I am using has the format

public class TwoThingsIPC<IPassClass>
{ ...
}

Both result in the same behaviour in my code, but are they the same and if not what is the difference?

Answer 1

They are not the same. The second declaration is misleading:

public class TwoThingsIPC<IPassClass>
{ ...
}

does not constrain the type to the IPassClass interface. It uses a poor choice of names for a generic argument. There's nothing preventing you from creating an instance of TwoThingsIPC<int> - the IPassClass references in the class's code would just be "replaced" by int.¹

On the other hand, a variable of type TwoThingsIPC<IPassClass>, for example:

TwoThingsIPC<IPassClass> myVar = new TwoThingsIPC<IPassClass>();

does constrain the type to the IPassClass interface.

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^{1 That's not what really happens, but I don't have a better explanation yet.}