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Object.MemberwiseClone Method

Updated: October 2010

Creates a shallow copy of the current Object.

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

'Declaration
<SecuritySafeCriticalAttribute> _
Protected Function MemberwiseClone As Object

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 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(ByVal 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 Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      ' 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
      outputBlock.Text &= "Original values of p1 and p2:" & vbCrLf
      outputBlock.Text &= "   p1 instance values: " & vbCrLf
      DisplayValues(outputBlock, p1)
      outputBlock.Text &= "   p2 instance values:" & vbCrLf
      DisplayValues(outputBlock, p2)
      outputBlock.Text &= vbCrLf

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

      ' 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
      outputBlock.Text &= "Values of p1 and p3 after changes to p1:" & vbCrLf
      outputBlock.Text &= "   p1 instance values: " & vbCrLf
      DisplayValues(outputBlock, p1)
      outputBlock.Text &= "   p3 instance values:" & vbCrLf
      DisplayValues(outputBlock, p3)
   End Sub

   Public Sub DisplayValues(ByVal outputBlock As System.Windows.Controls.TextBlock, ByVal p As Person)
      outputBlock.Text += String.Format("      Name: {0:s}, Age: {1:d}", p.Name, p.Age) & vbCrLf
      outputBlock.Text += String.Format("      Value: {0:d}", p.IdInfo.IdNumber) & vbCrLf
   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.

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Date

History

Reason

October 2010

Revised the Remarks section and replaced the example.

Customer feedback.

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