Q79597: PRB:Parameter Mismatch Error When Pass Properties by Reference

Article: Q79597
Product(s): Microsoft Visual Basic for Windows
Version(s): 1.0,2.0,3.0
Operating System(s): 
Keyword(s): 
Last Modified: 02-NOV-2001

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The information in this article applies to:

- Microsoft Visual Basic Professional Edition for Windows, versions 2.0, 3.0 
- Microsoft Visual Basic Standard Edition for Windows, versions 1.0, 2.0, 3.0 
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SYMPTOMS
========

When trying to pass a control property value by reference to a Sub or Function
procedure, you receive a "Parameter Mismatch" error.

CAUSE
=====

Control property values in Visual Basic are stored in a formatted form whose
location is periodically changed as part of Windows memory management. The
values are accessed by handles, not addresses. Although the values behave as
their prescribed types when used directly, they cannot be passed by reference to
a Sub or Function procedure. Any attempt to do so will generate a "Parameter
Mismatch" error.

Passing by reference, the default parameter passing method in Visual Basic,
places the address of the variable on the stack. The Sub or Function procedure
then accesses the address on the stack and uses it to refer to that variable.

Sending a control property as a parameter to a Sub or Function procedure will
place its handle on the stack instead of an address. Because the handle uses a
different form from an address, the Sub or Function procedure finds a value that
it is not expecting, and will generate a "Parameter Mismatch" error.

RESOLUTION
==========

Pass the property by value instead of by reference. To pass by value, place a
set of parentheses around the property variable in the Sub or Function call.
This syntax will place the actual value of the property on the stack and tell
the Sub or Function procedure to treat it as such. Because an actual memory
location is not transferred to the Sub or Function procedure, any changes to the
value of the property are localized to that Sub or Function procedure.

As an alternative resolution, assign the property value to a temporary variable.
The temporary variable has an actual address and can be passed to a Sub or
Function procedure in the usual manner. Because an actual address is sent, any
change to the temporary variable will be permanent. In order for the actual
property variable to reflect this change, you must assign the value of the
temporary variable to the property variable upon return from the Sub or Function
procedure.

Step-by-Step Example
--------------------

1. Start a new project in Visual Basic. Form1 is created by default.

2. Place two command buttons (Command1 and Command2), and one text box (Text1)
  on Form1.

3. Add the two command Click events as follows:

     Sub Command1_Click ()
        Text1.text = "passed by value"
        CALL Mysub ((Text1.text))
        ' Notice Text1.text did not change.
     End Sub

     Sub Command2_Click()
        Text1.text = "passed temporary variable"
        temp$ = Text1.text
        CALL Mysub (temp$)
        Text1.text = temp$
        ' Notice Text1.text did change when assigned to temp$.
     End Sub

4. In the General section of Form1, add the following:

     Sub Mysub(A$)
        A$ = "Changed"
     End Sub

5. Run the program by pressing the F5 key. Click the command buttons to observe
  the behavior.

Additional query words: 2.00 3.00

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Keywords          :  
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB300Search kbVB300 kbVB200
Version           : :1.0,2.0,3.0
Issue type        : kbprb

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