Microsoft KB Archive/138517

= Properties of Object Passed ByVal Can Be Modified =

Article ID: 138517

Article Last Modified on 12/9/2003

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APPLIES TO


 * Microsoft Visual Basic 4.0 Standard Edition
 * Microsoft Visual Basic 4.0 Professional Edition
 * Microsoft Visual Basic 4.0 Professional Edition
 * Microsoft Visual Basic 4.0 16-bit Enterprise Edition
 * Microsoft Visual Basic 4.0 32-Bit Enterprise Edition

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This article was previously published under Q138517



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
 Start a new project. Form1 is created by default. Add a command button to Form1.  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  Add a Class Module (Class1) to the project.  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  Save the project. Run the project, and click the button. The results of the five tests should be printed on the form.</li></ol>

Additional query words: 4.00 vb4win vb4all

Keywords: kbcode KB138517

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