How to Avoid the "ByRef Argument Type Mismatch" Error

• Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 16-bit and 32-bit, for Windows, version 4.0
• Microsoft Visual Basic Standard and Professional Editions for Windows, version 3.0

SUMMARY

If you pass a parameter to a function or sub procedure by reference, the type of the actual parameter passed and the corresponding function argument must match. Otherwise, you will get a "ByRef argument type mismatch" error.

MORE INFORMATION

The reason types have to match with ByRef parameters is that the called procedure is working on the original outside variable through a reference pointer. In the following step-by-step example, if the procedure thinks it is modifying a variant but the outside variable is really a control, the data will probably be ruined.

If something is passed by value, Visual Basic can do automatic type conversion. When you pass by value, the inside procedure is working on a copy and can therefore modify it in any way, such as converting the passed object to a temporary Variant and working on that.

This applies to simple built-in types as well as objects. Problems like this are easier to understand and debug if you set Option Explicit and declare every variable type explicitly.

Step-by-Step Example

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



2. Add the following code to the General Declarations section of Form1:

   
      Sub test(x As Control)
          x.Text = "hello"
      End Sub 




3. Add the following code to the Form_Click event of Form1:

   
      Private Sub Form_Click()
          For Each object In Controls
              test object
          Next
      End Sub 




4. Press F5 to run the program. Click Form1. The "ByRef argument type mismatch" error appears because the type of object passed is a Variant by default and the argument to test() is of type Control.



5. To avoid this error, you can do one of the following:
   
   
   • Pass the control ByVal:

     
           Sub test(ByVal x As Control)
               x.Text = "hello"
           End Sub 

   
   
   -or-
   
   
   • Dimension the object as a Control variable so that the types match:

     
           Private Sub Form_Click()
               Dim object As Control
               For Each object In Controls
                  test object
               Next
           End Sub 

   
   
   

Additional query words: 3.00 4.00 vb4win vb4all

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