Properties of Object Passed ByVal Can Be Modified

ID: Q138517


The information in this article applies to:
  • Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 16-bit and 32-bit, for Windows, version 4.0


SUMMARY

When you pass an object by value to a procedure, you can modify its properties in the procedure. Using ByVal with an object parameter affects how the object can be redefined in the procedure. If an object variable is passed to a procedure by using the ByVal keyword and the object parameter is set to a different object, the object variable still references the original object. Conversely, if an object variable is passed to a procedure by reference and the object parameter is set to a different object, the object variable references this different object. This article provides examples that highlight the functionality of the ByVal keyword.


MORE INFORMATION

Consider the following sample code where Class1 has one public text property called Description:


   Private Sub PassByVal(ByVal C As Class1)
      C.Description = "Modified Value"
   End Sub

   Private Sub Form_Load()
      Dim MyClass As Class1
      Set MyClass = New Class1
      MyClass.Description = "Original Value"
      PassByVal MyClass
      MsgBox MyClass.Description
   End Sub 
In this case, the message box displays "Modified Value" even though the object is passed by value. You might expect that the ByVal would prevent the procedure from modifying the property. However, when used with objects, ByVal affects the procedure's ability to redefine the object.

If an object variable is passed to a procedure by using the ByVal keyword and the object parameter is set to a different object, the object variable still references the original object. Consider the following example code with the same definition of Class1:

   Private Sub PassByValSet(ByVal C As Class1)
      Dim A As Class1
      Set A = New Class1
      A.Description = "New Value"
      Set C = A
   End Sub

   Private Sub Form_Load()
      Dim MyClass As Class1
      Set MyClass = New Class1
      MyClass.Description = "Original Value"
      PassByValSet MyClass
      MsgBox MyClass.Description
   End Sub 
In this case, the message box displays "Original Value" because the ByVal prevents the procedure from redefining the object with "Set C = A." This is how ByVal works with objects.

Conversely, if an object variable is passed to a procedure by reference and the object parameter is set to a different object, the object variable references this different object. The following example, using the same Class1, illustrates this:

   Private Sub PassByRefSet(C As Class1)
      Dim A As Class1
      Set A = New Class1
      A.Description = "New Value"
      Set C = A
   End Sub

   Private Sub Form_Load()
      Dim MyClass As Class1
      Set MyClass = New Class1
      MyClass.Description = "Original Value"
      PassByRefSet MyClass
      MsgBox MyClass.Description
   End Sub 
In this example, the message box displays "New Value" because the object is passed by reference and the procedure is allowed to redefine it with "Set C = A."

If you need to modify an object parameter's properties within a procedure without modifying the object passed, you need to create a copy of the object in the procedure. The following example of this method includes all the points previously discussed.

Step-by-Step Example

  1. Start a new project. Form1 is created by default.


  2. Add a command button to Form1.


  3. Add the following code to the General declarations section of Form1:
    
       ' This procedure modifies the description of the object
       ' passed in as expected
       Private Sub PassByRef(C As Class1)
          C.Description = "Modified Value"
       End Sub
    
       ' You might expect that this procedure would not modify the
       ' Description property of the object that was passed in.
       ' However, it does modify it.
       Private Sub PassByVal(ByVal C As Class1)
          C.Description = "Modified Value"
       End Sub
    
       ' This procedure redefines the original object to the new
       ' object with a new value for the description field.
       Private Sub PassByRefAndSet(C As Class1)
          Dim A As Class1
          Set A = New Class1
          A.Description = "New Value"
          Set C = A
       End Sub
    
       ' This procedure does not redefine the original object.
       ' However, within the scope of the procedure it is redefined.
       Private Sub PassByValAndSet(ByVal C As Class1)
          Dim A As Class1
          Set A = New Class1
          A.Description = "New Value"
          Set C = A
       End Sub
    
       ' This procedure shows how to redefine the object within the
       ' procedure to a copy of itself. Changes made in the locally
       ' redefined parameter will not be reflected back to the
       ' original object.
       Private Sub PassByValAndCopy(ByVal C As Class1)
          Set C = C.Copy
          C.Description = "Modified Value"
       End Sub
    
       Private Sub Command1_Click()
          Me.AutoRedraw = True
    
          Print "Passing object by reference to modify property ";
          Dim MyClass1 As Class1
          Set MyClass1 = New Class1
          MyClass1.Description = "Original Value"
          PassByRef MyClass1
          Print "results in: "; MyClass1.Description
          Set MyClass1 = Nothing
    
          Print "Passing object by value to modify property ";
          Dim MyClass2 As Class1
          Set MyClass2 = New Class1
          MyClass2.Description = "Original Value"
          PassByVal MyClass2
          Print "results in: "; MyClass2.Description
          Set MyClass2 = Nothing
    
          Print "Passing object by reference to set object ";
          Dim MyClass3 As Class1
          Set MyClass3 = New Class1
          MyClass3.Description = "Original Value"
          PassByRefAndSet MyClass3
          Print "results in: "; MyClass3.Description
          Set MyClass3 = Nothing
    
          Print "Passing object by value to set object ";
          Dim MyClass4 As Class1
          Set MyClass4 = New Class1
          MyClass4.Description = "Original Value"
          PassByValAndSet MyClass4
          Print "results in: "; MyClass4.Description
          Set MyClass4 = Nothing
    
          Print "Passing object by value to set copy of object ";
          Dim MyClass5 As Class1
          Set MyClass5 = New Class1
          MyClass5.Description = "Original Value"
          PassByValAndCopy MyClass5
          Print "results in: "; MyClass5.Description
          Set MyClass5 = Nothing
       End Sub 


  4. Add a Class Module (Class1) to the project.


  5. Put the following code in the Class1 module:
    
       Public Description As String
    
       ' Create a member by member copy of Class1
       Public Function Copy() As Class1
          Dim Ret As Class1
          Set Ret = New Class1
          Ret.Description = Me.Description
          Set Copy = Ret
       End Function 


  6. Save the project.


  7. Run the project, and click the button. The results of the five tests should be printed on the form.


Additional query words: 4.00 vb4win vb4all

Keywords : kbcode PrgOther
Version : 4.00
Platform : WINDOWS
Issue type :


Last Reviewed: September 17, 1999
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