Properties of Object Passed ByVal Can Be Modified

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic, 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

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 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.