Question

I have two classes. The base class is A. The inherited class is B. I would like copy a base class from one object into the base class of another object without affecting the original class. However, .NET seems to ignore the copying. Is this not possible in .NET. I know this is possible in C++. I have included C++ code to illustrate what I am trying to achieve.

I understand in that in this particular example I can directly assign the value bClass.ValueA = aClass.ValueA. But what if Class A had private members? This would not be possible.

Example Classes

Public Class A
    Public ValueA As String

End Class

Public Class B
    Inherits A
    Public ValueB As String

End Class

The Code

    Dim aClass As New A
    Dim bClass As New B

    aClass.ValueA = "AClass"

    bClass.ValueA = "BClass"
    bClass.ValueB = "BClass"

    Dim baseBClass As A
    baseBClass = CType(bClass, A)
    baseBClass = aClass

Results:

aClass.ValueA = “AClass”

bClass.ValueA = “BClass”

bClass.ValueB = “BClass”

baseBClass.ValueA = “AClass”

Intended Results:

aClass.ValueA = “AClass”

bClass.ValueA = “AClass”

bClass.ValueB = “BClass”

baseBClass.ValueA = “AClass”

C++ Explanation Comparison

#include <string>
#include <iostream>

using namespace std;

class A {
public:
    string ValueA;
};

class B : public A {
public:
    string ValueB;
};



int main(int argc, char *argv[]) {
    A aClass;
    B bClass;
    aClass.ValueA = "AClass";

    bClass.ValueA = "BClass";
    bClass.ValueB = "Blcass";

    cout << aClass.ValueA <<endl;
    cout << bClass.ValueA << endl;
    cout << bClass.ValueB << endl;

    A *baseBClass;
    baseBClass = (A*) &bClass;
    *baseBClass = aClass;

    cout << aClass.ValueA <<endl;
    cout << bClass.ValueA << endl;
    cout << bClass.ValueB << endl;
    cout << baseBClass->ValueA << endl;


    return 0;
}

Intended and Actual Results:

aClass.ValueA = “AClass”

bClass.ValueA = “AClass”

bClass.ValueB = “BClass”

baseBClass->ValueA = “AClass”

I do not think this is possible without pointers. I have tried

Ctype(bClass, A) = aClass
Directcast(bClass, A) = aClass

I get this Error:

Expression is a value and therefore
cannot be the target of an assignment.

Answer 1

One alternative is to have a C# class that does the pointer manipulation and adds extension method to the B Class. It would be very similar to the C++ solution. This seems the best solution as it would be almost identical to the C++ implementation.

Instead I used reflection. It is only a shallow copy but in this case it does not matter. C++ default copy constructor is only a shallow clone. With some embellishment you can check to see if IClonable exists and attempt to clone it, otherwise instantiate a new object. I have updated this to include private members.

---

Imports System.Runtime.InteropServices
Imports System.Reflection

Public Class A
    Private ValueA As String
    Public Sub New(ByVal ValueA As String)
        Me.ValueA = ValueA
    End Sub
    Public Overrides Function ToString() As String
        Return String.Format("{0}:{1}={2}", _
        Me.GetType.Name, "ValueA", ValueA)
    End Function
End Class

Public Class B
    Inherits A
    Private ValueB As String
    Public Sub New(ByVal ValueA As String, ByVal ValueB As String)
        MyBase.New(ValueA)
        Me.ValueB = ValueB
    End Sub
    Public Overrides Function ToString() As String
        Return String.Format("{0}:{1}={2},{3}", _
        Me.GetType.Name, "ValueB", ValueB, MyBase.ToString)
    End Function
End Class

Module Inheritence

    Sub Main()
        Dim aClass As New A("AClass")
        Dim bClass As New B("BClass", "BClass")

        Console.WriteLine(aClass)
        Console.WriteLine(bClass)

        For Each item As FieldInfo In aClass.GetType.GetFields( _
                                             BindingFlags.Public _
                                             Or BindingFlags.Instance _
                                             Or BindingFlags.NonPublic _
                                             Or BindingFlags.Static)
            item.SetValue(bClass, item.GetValue(aClass))
        Next
        Console.WriteLine(aClass)
        Console.WriteLine(bClass)
        Console.ReadKey()
    End Sub

End Module

Base Class Clone

This is the site I based my solution upon. It is a shallow clone but easy and quick to implement
http://www.codeproject.com/KB/cs/cloneimpl_class.aspx

This solution is better as it attempts to do a deep clone on objects that support IClonable otherwise it just sets a new instance.
http://whizzodev.blogspot.com/2008/03/object-cloning-using-il-in-c.html

Clone Routines:

This is an interesting site that explains various cloning techniques.
http://developerscon.blogspot.com/2008/06/c-object-clone-wars.html

Another interesting Resource.
Deep cloning objects

Discussion about Deep cloning
Create a Deep Copy in C#