How To Modify Properties of an Object Passed ByVal

• Microsoft Visual Basic Professional and Enterprise Editions for Windows, version 5.0

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

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

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

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

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. The results of the five tests should be printed on the form. They should look like this: