Generic method isn't choosing the most specific constructor signature?

Question

When I use a method with a generic parameter to create another object, the generic object isn't selecting the most specific constructor. That sounds confusing, so here's some sample code to demonstrate what I mean...

Can anyone explain why the output of this program is:

guid    <-- easy - no problem here
object  <-- WHY?  This should also be "guid"?!

...and how to make the generic Add<T> function call the correct constructor of C?? Here's the code:

void Main()
{
    B b = new B();

    C c = new C(Guid.Empty);
    b.Add<Guid>(Guid.Empty);
}

public class B
{
    List<C> cs = new List<C>();
    public void Add<T>(T v) { cs.Add(new C(v)); }
}

public class C
{
    public C(Guid c) { Console.WriteLine("guid"); }
    public C(object c) { Console.WriteLine("object"); }
}

Answer 1

Overload resolution is done at compile time, not at runtime. So when you call new C(v) from the Add<T> method, the compiler doesn't know that T will actually be Guid, so it uses the only overload that is guaranteed to be compatible, which is public C(object c)

Answer 2

Generics in C# do not work the same as C++ templates - they are not expanded at compile time based on the usage. A single method is created and the methods called from within it are resolved statically.

Therefore, within Add, v can be any type, so the only thing known about it is that inherits from object so the object constructor for C is the only candidate.

To get the behaviour you want you'll have to add another overload of Add e.g.

public void Add(Guid g) { cs.Add(new C(g)); }