Casting between inherited classed

The following cast doent work. I think it should, can you explain to me why not? Both WebserviceErrorMessage and BTMacResponse inherit from the WebserviceMessage class

     开发者_如何学Go  DataLayer.WebserviceMessage msg = new Service.WebserviceErrorMessage(ex) ;
       DataLayer.BTMacResponse macrsp = (DataLayer.BTMacResponse)msg;

Unless WebserviceErrorMessage inherits from BTMacResponse, I don't see why this should work. You're trying to cast a value to BTMacResponse when the object simply isn't a BTMacResponse.

To put it another way, would you expect to be able to cast a FileStream to a MemoryStream just because they both inherit from Stream?

Would you expect to be able to cast a Button to a String just because they both inherit from Object?

You can't cast that way. The following is illegal:

class Base {

}

class A : Base {

}

class B: Base {
}

Base someBase = new A();
B someB = (B)someBase;

You cannot cast down the hierarchy because when you create an object of the parent, you do not create the child object with it, however when you create an object of the child an instance of the parent is created, thats why you can upcast and not downcast.

in your case you can cast DataLayer.WebserviceMessage to its children and not the opposite.

there is no contractor for DataLayer.BTMacResponse that get DataLayer.WebserviceMessage as a parameter you need to implement this:

public BTMacResponse(WebserviceMessage w)
        {
                // copy stuff to 'this'
        }

        public static implicit operator BTMacResponse(WebserviceMessage w)
        {
                BTMacResponse b = new BTMacResponse(w);
                return b;
        }