Question

Writing an asp.net mvc app and i have something like this…

Public Class AA
'... has some variables...
End Class

Public Class BB
Inherits AA
Public ExtraVariable As Integer ' just adds another variable and thats it!
End Class

So, now in my program, i just want to copy object of type AA to an empty variable of type BB?

it makes sense to do this, as i expect all the fields in AA type object to be copied to the newly created BB type object, and the ExtraVariable in the BB type object i would (will) just assign a value to it later (after the copy) on my own time!!

I know copying BB type to AA type would not be appropriate as there would be lost data!

But im trying to copy AA to BB, I’ve used both DirectCast and CType to do this, and i keep getting “unable to cast” error!!!

Note: I’m using vb.net (but can read c#, no problems)

Answer 1

As anon says you probably want to pass AA into BB’s constructor, where you can copy all the elements:

public class AA
{
//some variables
}

public class BB : AA
{
    public BB(AA aa)
    {
    //Set BBs variables to those in AA
    someVariable= aa.someVariable
    }

    public int SomeExtraProperty{get;set;}
}

but obviously you’ll have any inherited constructors available to you which you could reuse if appropriate.

EDIT

based on some of the comments above, you could also do:

public class BB : AA
{
    private AA _aa;
    public BB(AA aa)
    {
    //Set BBs variables to those in AA
    _aa=aa;
    }

    public int SomeExtraProperty{get;set;}

    //override inherted members and just delegate to the internal object
    public override int SomeMethod()
    {
       return _aa.SomeMethod();
    }
}

or maybe go for the Decorator Pattern