Q161308: HOWTO: Modify Properties of an Object Passed ByVal
Article: Q161308
Product(s): Microsoft Visual Basic for Windows
Version(s):
Operating System(s):
Keyword(s): kbusage kbVBp500 kbVBp600 kbGrpDSVB
Last Modified: 11-JAN-2001
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The information in this article applies to:
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0
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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 example of this method below includes all the points previously
discussed.
Step-by-Step Example
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1. Start a new Standard EXE project. Form1 is added by default.
2. Add a CommandButton 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.
RESULT: The five tests are printed on the form as follows:
- Passing object by reference to modify property results in: Modified Value
- Passing object by value to modify property results in: Modified Value
- Passing object by reference to set object results in: New Value
- Passing object by value to set object results in: Original Value
- Passing object by value to set copy of object results in: Original Value
Additional query words:
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Keywords : kbusage kbVBp500 kbVBp600 kbGrpDSVB
Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Issue type : kbhowto
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