Microsoft KB Archive/138535

= How to Avoid the "ByRef Argument Type Mismatch" Error =

Article ID: 138535

Article Last Modified on 1/8/2003

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APPLIES TO


 * Microsoft Visual Basic 4.0 Standard Edition
 * Microsoft Visual Basic 4.0 Professional Edition
 * Microsoft Visual Basic 4.0 Professional Edition
 * Microsoft Visual Basic 4.0 16-bit Enterprise Edition
 * Microsoft Visual Basic 4.0 32-Bit Enterprise Edition
 * Microsoft Visual Basic 3.0 Professional Edition
 * Microsoft Visual Basic 3.0 Professional Edition

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This article was previously published under Q138535



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
 Start a new project in Visual Basic. Form1 is created by default.  Add the following code to the General Declarations section of Form1: Sub test(x As Control) x.Text = "hello" End Sub   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  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. 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 </li></ul> </li></ol>

Additional query words: 3.00 4.00 vb4win vb4all

Keywords: KB138535

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