ObjectMemberwiseClone Method
 

Creates a shallow copy of the current Object.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)

Syntax
protected object MemberwiseClone()
protected:
Object^ MemberwiseClone()
member MemberwiseClone : unit -> Object
Protected Function MemberwiseClone As Object

Return Value

Type:

A shallow copy of the current Object.

Remarks

The MemberwiseClone method creates a shallow copy by creating a new object, and then copying the nonstatic fields of the current object to the new object. If a field is a value type, a bit-by-bit copy of the field is performed. If a field is a reference type, the reference is copied but the referred object is not; therefore, the original object and its clone refer to the same object.

For example, consider an object called X that references objects A and B. Object B, in turn, references object C. A shallow copy of X creates new object X2 that also references objects A and B. In contrast, a deep copy of X creates a new object X2 that references the new objects A2 and B2, which are copies of A and B. B2, in turn, references the new object C2, which is a copy of C. The example illustrates the difference between a shallow and a deep copy operation.

There are numerous ways to implement a deep copy operation if the shallow copy operation performed by the MemberwiseClone method does not meet your needs. These include the following:

  • Call a class constructor of the object to be copied to create a second object with property values taken from the first object. This assumes that the values of an object are entirely defined by its class constructor.

  • Call the MemberwiseClone method to create a shallow copy of an object, and then assign new objects whose values are the same as the original object to any properties or fields whose values are reference types. The DeepCopy method in the example illustrates this approach.

  • Serialize the object to be deep copied, and then restore the serialized data to a different object variable.

  • Use reflection with recursion to perform the deep copy operation.

Examples

The following example illustrates the MemberwiseClone method. It defines a ShallowCopy method that calls the MemberwiseClone method to perform a shallow copy operation on a Person object. It also defines a DeepCopy method that performs a deep copy operation on a Person object.

using System;

public class IdInfo
{
    public int IdNumber;

    public IdInfo(int IdNumber)
    {
        this.IdNumber = IdNumber;
    }
}

public class Person 
{
    public int Age;
    public string Name;
    public IdInfo IdInfo;

    public Person ShallowCopy()
    {
       return (Person) this.MemberwiseClone();
    }

    public Person DeepCopy()
    {
       Person other = (Person) this.MemberwiseClone();
       other.IdInfo = new IdInfo(IdInfo.IdNumber);
       other.Name = String.Copy(Name);
       return other;
    }
}

public class Example
{
    public static void Main()
    {
        // Create an instance of Person and assign values to its fields.
        Person p1 = new Person();
        p1.Age = 42;
        p1.Name = "Sam";
        p1.IdInfo = new IdInfo(6565);

        // Perform a shallow copy of p1 and assign it to p2.
        Person p2 = p1.ShallowCopy();

        // Display values of p1, p2
        Console.WriteLine("Original values of p1 and p2:");
        Console.WriteLine("   p1 instance values: ");
        DisplayValues(p1);
        Console.WriteLine("   p2 instance values:");
        DisplayValues(p2);

        // Change the value of p1 properties and display the values of p1 and p2.
        p1.Age = 32;
        p1.Name = "Frank";
        p1.IdInfo.IdNumber = 7878;
        Console.WriteLine("\nValues of p1 and p2 after changes to p1:");
        Console.WriteLine("   p1 instance values: ");
        DisplayValues(p1);
        Console.WriteLine("   p2 instance values:");
        DisplayValues(p2);

        // Make a deep copy of p1 and assign it to p3.
        Person p3 = p1.DeepCopy();
        // Change the members of the p1 class to new values to show the deep copy.
        p1.Name = "George";
        p1.Age = 39;
        p1.IdInfo.IdNumber = 8641;
        Console.WriteLine("\nValues of p1 and p3 after changes to p1:");
        Console.WriteLine("   p1 instance values: ");
        DisplayValues(p1);
        Console.WriteLine("   p3 instance values:");
        DisplayValues(p3);
    }

    public static void DisplayValues(Person p)
    {
        Console.WriteLine("      Name: {0:s}, Age: {1:d}", p.Name, p.Age);
        Console.WriteLine("      Value: {0:d}", p.IdInfo.IdNumber);
    }
}
// The example displays the following output:
//       Original values of p1 and p2:
//          p1 instance values:
//             Name: Sam, Age: 42
//             Value: 6565
//          p2 instance values:
//             Name: Sam, Age: 42
//             Value: 6565
//       
//       Values of p1 and p2 after changes to p1:
//          p1 instance values:
//             Name: Frank, Age: 32
//             Value: 7878
//          p2 instance values:
//             Name: Sam, Age: 42
//             Value: 7878
//       
//       Values of p1 and p3 after changes to p1:
//          p1 instance values:
//             Name: George, Age: 39
//             Value: 8641
//          p3 instance values:
//             Name: Frank, Age: 32
//             Value: 7878
Public Class IdInfo
    Public IdNumber As Integer

    Public Sub New(IdNumber As Integer)
        Me.IdNumber = IdNumber
    End Sub
End Class

Public Class Person 
    Public Age As Integer
    Public Name As String
    Public IdInfo As IdInfo

    Public Function ShallowCopy() As Person
       Return DirectCast(Me.MemberwiseClone(), Person)
    End Function

    Public Function DeepCopy() As Person
       Dim other As Person = DirectCast(Me.MemberwiseClone(), Person) 
       other.IdInfo = New IdInfo(IdInfo.IdNumber)
       other.Name = String.Copy(Name)
       Return other
    End Function
End Class

Module Example
   Public Sub Main()
        ' Create an instance of Person and assign values to its fields.
        Dim p1 As New Person()
        p1.Age = 42
        p1.Name = "Sam"
        p1.IdInfo = New IdInfo(6565)

        ' Perform a shallow copy of p1 and assign it to p2.
        Dim p2 As Person = p1.ShallowCopy()

        ' Display values of p1, p2
        Console.WriteLine("Original values of p1 and p2:")
        Console.WriteLine("   p1 instance values: ")
        DisplayValues(p1)
        Console.WriteLine("   p2 instance values:")
        DisplayValues(p2)
        Console.WriteLine()

        ' Change the value of p1 properties and display the values of p1 and p2.
        p1.Age = 32
        p1.Name = "Frank"
        p1.IdInfo.IdNumber = 7878
        Console.WriteLine("Values of p1 and p2 after changes to p1:")
        Console.WriteLine("   p1 instance values: ")
        DisplayValues(p1)
        Console.WriteLine("   p2 instance values:")
        DisplayValues(p2)
        Console.WriteLine()

        ' Make a deep copy of p1 and assign it to p3.
        Dim p3 As Person = p1.DeepCopy()
        ' Change the members of the p1 class to new values to show the deep copy.
        p1.Name = "George"
        p1.Age = 39
        p1.IdInfo.IdNumber = 8641
        Console.WriteLine("Values of p1 and p3 after changes to p1:")
        Console.WriteLine("   p1 instance values: ")
        DisplayValues(p1)
        Console.WriteLine("   p3 instance values:")
        DisplayValues(p3)
   End Sub

    Public Sub DisplayValues(p As Person)
        Console.WriteLine("      Name: {0:s}, Age: {1:d}", p.Name, p.Age)
        Console.WriteLine("      Value: {0:d}", p.IdInfo.IdNumber)
    End Sub
End Module
' The example displays the following output:
'       Original values of m1 and m2:
'          m1 instance values:
'             Name: Sam, Age: 42
'             Value: 6565
'          m2 instance values:
'             Name: Sam, Age: 42
'             Value: 6565
'       
'       Values of m1 and m2 after changes to m1:
'          m1 instance values:
'             Name: Frank, Age: 32
'             Value: 7878
'          m2 instance values:
'             Name: Sam, Age: 42
'             Value: 7878
'       
'       Values of m1 and m3 after changes to m1:
'          m1 instance values:
'             Name: George, Age: 39
'             Value: 8641
'          m3 instance values:
'             Name: Frank, Age: 32
'             Value: 7878

In this example, the Person.IdInfo property returns an IdInfo object. As the output from the example shows, when a Person object is cloned by calling the MemberwiseClone method, the cloned Person object is an independent copy of the original object, except that they share the same Person.IdInfo object reference. As a result, modifying the clone's Person.IdInfo property changes the original object's Person.IdInfo property. On the other hand, when a deep copy operation is performed, the cloned Person object, including its Person.IdInfo property, can be modified without affecting the original object.

Version Information
Universal Windows Platform
Available since 4.5
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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