在C＃中是否可以通过以下方式重载通用强制转换操作符？

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 : \n    implicit operator type ( type identifier ) \n    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