Casting classes

Question

I vave 2 class:

public class ClassA

public class ClassB (from another namespace) : ClassA

I use methdo at ClassA

public static ClassA Deserialize(string path)
{
ClassA classA;

//classA=code...

return classA;
}

I invoke this method at classB

public void DoSomething()
{
ClassB classB=(ClassB)ClassA.Deserialize("c:\directory\file.xml);
}

method deserialize works, but i get error that cannont cast ClassA to ClassB.

How to deal with this?

 public static ClassA DeserializeFromXml(string path)
        {

 XmlSerializer s = new XmlSerializer(typeof(ClassA));
            ClaasA h;

            TextReader r = new StreamReader(path);


                h = (ClassA)s.Deserialize(r);

                r.Close();

                return h;
}

Maybe something with deserialize(string path, Type objectType ) ??

I could can change method Deserialize if it would necessary

Answer 1

Without showing your Deserialize code it's pretty hard to say what's going on. This is likely to be the heart of the problem - you need to make Deserialize actually create an instance of ClassB (or a derived class) if you want to be able to cast the result to ClassB. If your Deserialize method creates an instance of ClassA and then sets a bunch of properties, you'll need to either change it or find another way of creating a ClassB instance later.

You can only an expression cast to ClassB if the value is a reference to an actual instance of ClassB. If the object is only an instance of ClassA then you won't be able to cast - what would you expect to happen to any extra fields etc in ClassB? Unless a user-defined conversion, casting for a reference type only performs a reference conversion - it doesn't create a new object. (See Eric Lippert's blog post on representation and identity for more details.)

Answer 2

Your method returns a void, therefore you trying to cast the return of void to be ClassB.

Answer 3

A isn't a B. B is an A

(ClassB) something_that_is_A cannot be done unless it is a B or a derivative of it.

Answer 4

Having ClassB inherit from ClassA does not allow a ClassA object to be cast into a ClassB object, as all ClassA objects are not inevitably ClassBobjects.

If you want to know if your deserialized object is a ClassB object, you can use this code :

ClassA a = ClassA.Deserialize("c:\directory\file.xml");

ClassB b = a as ClassB;

If the object isn't a ClassB instance, b will be null. Otherwise you will have your instance as a ClassB object.