PRB: "Hash Marks" on ActiveX Controls After Run-Time Error

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

SYMPTOMS

After receiving an error message, all instances of the ActiveX control are disabled and drawn with "hash marks."

CAUSE

This behavior occurs after an unhandled error in event code.

RESOLUTION

The conditions listed below must be met to avoid this problem:

1. Ensure that the event code never generates a run-time error or that it handles all errors with an error handler.

2. Never use Err.Raise within a UserControl.

STATUS

This behavior is by design.

MORE INFORMATION

Unhandled errors in event procedures are fatal to your control component's operation and the application using it because there is no procedure on the call stack to handle the error.

It is safe to raise errors in property procedures and methods, however, because properties and methods are always invoked by other procedures and errors you raise can be handled by the user in those procedures.

Steps to Reproduce Behavior

1. Create an ActiveX control called "UserCtrl" with no constituent controls and the following code:

         Option Explicit
   

         Private Procedure UserControl_Click()
           Err.Raise 999, "UserControl", "Don't Click on Me"
         End Sub
   
         Public Sub BackColor(ByVal NewVal As Long)
           UserCtrl.BackColor = NewVal
           Err.Raise 555, "UserControl", "Don't invoke this method"
         End Sub
   
   

2. Make an OCX.

3. Create a new project and add the OCX via the Project|Components menu.

4. Add your control and a CommandButton to the form.

5. Add the following code to the CommandButton's Click event:

         Sub Command1_Click()
           On Error Resume Next
           UserCtrl1.BackColor 0
         End Sub
   
   

6. Run the form and click the button. Then click your control.

   When you click the CommandButton, nothing happens; the error is trapped and dealt with (ignored in this case).

   When you click your control, you see the error message and, when dismissed, the control is disabled and becomes hashed out.

REFERENCES

Microsoft Visual Basic User's Guide, Chapter 9, Building ActiveX Controls, Creating Robust Controls.

(c) Microsoft Corporation 1996, All Rights Reserved. Contributions by Malcolm Stewart, Microsoft Corporation