Object.MemberwiseClone Method

Updated: September 2010

Creates a shallow copy of the current Object.

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

'Declaration
Protected Function MemberwiseClone As Object
'Usage
Dim returnValue As Object 

returnValue = Me.MemberwiseClone()

Return Value

Type: System.Object
A shallow copy of the current Object.

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.

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.

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(Me.IdInfo.IdNumber)
       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 = DirectCast(p1.ShallowCopy(), Person)

        ' 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.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

September 2010

Revised the Remarks section and replaced the example.

Customer feedback.

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