Question

I have Parent class X, and Parent class H.

H has in its field data a reference to a type X.
H’s constructor requires an instance of type X, which is then stored in the reference.

I then derive from X a Child class x1, and from H a child class h1.

h1 will accept in its constructor an instance of X or of x1.

But the methods and properties of x1 which are not already defined in its parent class X will not be available to h1.

How can I permanently cast x1 to the type of x1 within my class h1?

Answer 1

There is no such thing as a “permanent” cast (or, rather, the idea of a “permanent cast” is nonsensical, as that implies that casting does something to the object, which it does not, so there is nothing to either persist or undo). Your variable is of type H, and the type of the variable will never change. Your instance is of type H1, and the instance’s type will never change. Casting simply tells the compiler “Yes, even though I am only referencing this instance as type H, I actually know that it is of type H1, so it’s safe to store it in any variable that can reference an instance of H1.”

If your structure looks like this:

class H
{

}

class X
{
    public H HValue { get; private set; }

    public X(H h)
    {
        HValue = h;
    }
}

class H1 : H
{
    public void Foo() { }
}

class X1 : X
{
    public X1(H1 h1) : base(h1)
    {

    }
}

Then you’re going to have to store the value for the h1 variable somewhere else if you always want to use that value without downcasting.

Now, a somewhat “smelly” way of accomplishing what you want (not having to write the casting code every time), you could do this:

class X1 : X
{
    public X1(H1 h) : base(h) { }

    public new H1 HValue 
    { 
        get { return (H1)base.HValue; }
    }
}

This would allow you to refer to the same property named HValue anywhere you had a reference to X1 in a variable of type X1. So…

X x = new X1(new H1());  // x.HValue would be of type H, even though the 
                         // reference itself is H1
X1 x = new X1(new H1()); // x.HValue would be of type H1