C# generics - why is an explicit cast needed to from a concrete type back to T?

Question

This has been bothering me for a while. I'm having a little trouble understanding why explicit casts are needed in the following code:

public static class CastStrangeness
{
    public class A
    {
    }

    public class B
        : A
    {
    }

    public static void Foo<T>(T item)
        where T : A
    {
        // Works.
        A fromTypeParameter = item;

        // Does not compile without an explicit cast.
        T fromConcreteType = (T)fromTypeParameter;

        // Does not compile without an explicit cast.
        Foo<T>((T)fromTypeParameter);
    }

    public static void Bar(A item)
    {
        // Compiles.
        Foo<A>(item);
    }
}

It seems to me that T is guaranteed to be an A, so surely the compiler could infer that any instance of A is guaranteed to be assignable to T? Otherwise, I wouldn't be able to pass an A or a B into Foo(). So what am I missing?

PS. I've tried searching for endless permutations of keywords on this, but every result seems to wind up referring to covariance and contravariance WRT generic interfaces :)

Answer 1

Take the case of:

Foo(new B());

Your first assignment is ok:

A fromtypeParameter = item;

Since B : A.

But this assignment is not ok:

T fromConcreteType = fromTypeParameter;

Because you could very well have assigned fromTypeParameter as:

fromTypeParameter = new A();

Which you obviously cannot cast to T (which is B in this instance). T is more specific than A, it could be derived from A. So you can go one way but not the other, without an explicit cast (which may fail).

Answer 2

It seems to me that T is guaranteed to be an A, so surely the compiler could infer that any instance of A is guaranteed to be assignable to T?

Well, no.

string is an object. But

string s = new object();

is illegal.