Is it possible in C# to overload a generic cast operator in the following way?

Question

Just wondering if there is anyway to represent the following code in C# 3.5:

public struct Foo<T> {

    public Foo(T item) {
        this.Item = item;
    }

    public T Item { get; set; }

    public static explicit operator Foo<U> ( Foo<T> a )
        where U : T {

        return new Foo<U>((U)a.Item)
    }
}

Thanks

Answer 1

Conversion operators can't be generic. From the spec section 10.10, here's the format of a conversion-operator-declarator:

conversion-operator-declarator:
    implicit   operator   type   (   type   identifier   )
    explicit   operator   type   (   type   identifier   )

Compare this with, say, a method-header:

method-header: attributes_opt method-modifiers_opt partial_opt return-type member-name type-parameter-list_opt ( formal-parameter-list_opt ) type-parameter-constraints-clauses_opt

(Sorry about the formatting - not sure how to do it better.)

Note that the operator format doesn't include a type parameter list or type parameter constraints.

Answer 2

Your code boils down to the line: return new Foo<U>((U)a.Item)

Where you try to assign a baseclass to an inherited class, which is impossible.

Let's say T (base-class) is of type Stream and U is of type MemoryStream (inherited class), you cannot assign a Stream to a variable of type MemoryStream.

Answer 3

I think the short answer is "Not possible. Try using a method instead"

Also seems to be dupe of this question Solution for overloaded operator constraint in .NET generics