Shallow Clone and Deep Clone

 Shallow Clone and Deep Clone

      First, let me explain the concepts of Shallow Clone and Deep Clone. Shallow Clone only copies the value type of a class. If a reference type is present in class, then the reference is copied, but the referred object is not copied. Deep Clone copies everything of class, includes the referred object pointed to the reference.

     If you want to add a clone method to a custom class, the ICloneable interface must be implemented. Here’s a sample:

 // a sample for adding a clone method to a custom class

class Class1 : ICloneable   

{

    public int x = 10;

    public virtual Object Clone()

    {

        return MemberwiseClone();

    }

}

     We use a “MemberwiseClone” method in Class1. the method is from Object class, it is protected, so it can only be invoked in a derived class of Object.  The method will perform a shallow clone operation.

If we have a class – Class3, the class inherits from Class1.Here’s sample:

 class Class2 : ICloneable 

{

    public int y = 10;

    public virtual Object Clone()

    {

        return MemberwiseClone();

    }

}

class Class3 : Class1   // shallow clone

{

    public int z = 10;

    public Class2 c2 = new Class2();

    public override Object Clone()

    {

        return base.Clone();

    }

}

Now we invoke the Clone method of Class3 to copy the Class3 object.

Class3 c3 = new Class3();

    Class3 c3_clone = (Class3)c3.Clone();

    Console.WriteLine("c3.z = " + c3.z.ToString());

    Console.WriteLine("c3_clone.z = " + c3_clone.z.ToString());

    Console.WriteLine("c3.c2.y = " + c3.c2.y.ToString());

    Console.WriteLine("c3_clone.c2.y = " + c3_clone.c2.y.ToString());

    c3.z = 20;

    c3.c2.y = 40;  // The both c2.y(c3 and c3_clone) are altered.

    Console.WriteLine("c3.z = " + c3.z.ToString());

    Console.WriteLine("c3_clone.z = " + c3_clone.z.ToString());

    Console.WriteLine("c3.c2.y = " + c3.c2.y.ToString());

Console.WriteLine("c3_clone.c2.y = " + c3_clone.c2.y.ToString());

 

 You can find a problem from above code. When 40 is assigned to the “c3.c2.y”, the value of the “c3_clone.c2.y” also is changed. This is because that the reference of c3.c2 is as same as the one of c3_clone.Namely their refered objects are same.

The running result is:

c3.z = 10

c3_clone.z = 10

c3.c2.y = 10

c3_clone.c2.y = 10

 

c3.z = 20

c3_clone.z = 10

c3.c2.y = 40

c3_clone.c2.y = 40

 

Then we should how to solve the problem. Ok, that is deep clone. Please look at the code of Class4.

 

class Class4 : Class1   // deep clone

{

    public int z = 10;

    public Class2 c2 = new Class2();

    public override Object Clone()

    {

        Object obj = base.Clone();

        ((Class4)obj).c2 = (Class2)c2.Clone();

        return obj;

}

}

 In deep clone, we must copy all the refered object pointed to reference besides cloning the value type by a bit-by-bit copy. In the sample, invoking the Clone method of Class2 to copy the c2 to a new object. To test the Class, please look at the follow code:

          

Class4 c4 = new Class4();

    Class4 c4_clone = (Class4)c4.Clone();

    Console.WriteLine("c4.z = " + c4.z.ToString());

    Console.WriteLine("c4_clone.z = " + c4_clone.z.ToString());

    Console.WriteLine("c4.c2.y = " + c4.c2.y.ToString());

    Console.WriteLine("c4_clone.c2.y = " + c4_clone.c2.y.ToString());

    c4.z = 20;

    c4.c2.y = 40;

    Console.WriteLine("c4.z = " + c4.z.ToString());

    Console.WriteLine("c4_clone.z = " + c4_clone.z.ToString());

    Console.WriteLine("c4.c2.y = " + c4.c2.y.ToString());

Console.WriteLine("c4_clone.c2.y = " + c4_clone.c2.y.ToString());

 

Seeing from above code, the c4_clone.c2.y still is 10.

The running result is

c4.z = 10

c4_clone.z = 10

c4.c2.y = 10

c4_clone.c2.y = 10

 

c4.z = 20

c4_clone.z = 10

c4.c2.y = 40

c4_clone.c2.y = 10

 

    The whole code is on https://files.cnblogs.com/bughole/clone.rar
    The sample must be run in visual studio 2005.