Casting between two types derived from the (same) interface

Question

I have an interface and two types that derive from it.

However, I cannot do the following:

B objectB = (B) objectA

Where B derives from Interface1 (I am making up the name of classes but the point still stands), and likewise for objectA (which is of type A). I get the following error message:

Cannot cast expression of type A to B.

Both types are deriving from the interface, what am I missing?

Answer 1

1. Types do not derive from an interface. They implement an interface.
2. The fact that both an Elephant and a Spider are Animals doesn't mean that you can convert one to the other.

Answer 2

An object is assignable to an ancestor (direct or indirect base type) or to an interface it implements, but not to siblings (i.e. another type deriving from a common ancestor); however, you can declare your own explicit conversions:

class FooObject : IObject
{
    public string Name { get; set; }
    public int Value { get; set; }

    public static explicit operator FooObject(BarObject bar)
    {
        return new FooObject { Name = bar.Name, Value = bar.Value };
    }
}

class BarObject : IObject
{
    public string Name { get; set; }
    public int Value { get; set; }

    public static explicit operator BarObject(FooObject bar)
    {
        return new BarObject { Name = bar.Name, Value = bar.Value };
    }
}

Now you can write

var foo = new FooObject();
var bar = (BarObject)foo;

or

var bar = new BarObject();
var foo = (FooObject)bar;

without getting errors.

You can also create implicit conversions, if it feels natural. E.g. int is implicitly convertible to double: int i = 5; double x = i;.

(This is also an answer to the closed question How do I cast Class FooObject to class BarObject which both implement interface IObject?).