Why do I have to cast to type parameter and can't use the constrained type?

Question

Can anyone explain why I have to cast to T and why Add2 does not accept Bar as a parameter?

class Foo<T> where T : class, IBar
{
    void Add1(IDo<T> @do) { @do.Stuff(new Bar() as T); }

    // Add2 does not compile:
    // Argument type Bar is not assignable to parameter Type T
    void Add2(IDo<T> @do) { @do.Stuff(new Bar()); } 
}

interface IBar {}

class Bar : IBar {}

interface IDo<in T> {
    void Stuff(T bar);
}

Answer 1

It may not be appropriate. For example, consider:

class Other : Bar {}

...

IDo<Other> do = new DoImpl<Other>();
Foo<Other> foo = new Foo<Other>();
foo.Add2(do);

With your current code, that would be calling do.Add2(new Bar())... which is clearly invalid as a Bar isn't an Other, required by IDo<Other>.Stuff.

Casting to T (or using as) isn't appropriate either - you can't cast a new Bar() to Other, and if you use as you'll just get a null reference.