KB149032 - PRB: VB 3.0 OLE Clients Do not Repaint Before OLE Call Returns

Article ID: 149032

Article Last Modified on 7/15/2004

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

This article was previously published under Q149032

SUMMARY

When calling a method or accessing a property of an OLE server from Visual Basic 3.0, no painting takes place on any of the Visual Basic 3.0 forms that are showing. This behavior remains until the call to the OLE server method or property returns. This behavior occurs with OLE servers built in any environment, including, but not limited to, Visual Basic 4.0. OLE automation clients built with Visual Basic 4.0 do not show this characteristic.

CAUSE

To receive Windows paint messages while in a synchronous OLE call, an OLE automation client must implement the IMessageFilter interface. Visual Basic 3.0 does not implement this interface, and so Visual Basic 3.0 windows cannot receive any paint messages until a synchronous call returns. Because Visual Basic 4.0 does implement this interface, painting can continue without problem before a call has returned.

STATUS

This behavior is by design.

MORE INFORMATION

Steps To Reproduce Problem

Following the instructions in 129801, "How to Create and Use a Minimal OLE Automation Server," create a simple OLE server with Visual Basic 4.0. To obtain this article, query on the following words here in the Microsoft Knowledge Base:
vbwin and chocolates and step
Add a single form to this OLE server project by clickng Form on the Insert menu.

Add this procedure to the class module in the OLE server project:

      Public Sub ShowForm()
         Form1.Show vbModal
      End Sub

Compile the OLE server project and exit Visual Basic 4.0.
Start Visual Basic 3.0. Form1 is created by default.

Add this code to the Form_Click event in the Visual Basic 3.0 project:

      Sub Form_Click()
         Dim o As Object

         ' If the ProgID has been changed then modify it below.
         Set o = CreateObject("Project1.Class1")
         o.ShowForm
         MsgBox "Method returned"
      End Sub

Press F5 or click Start on the Run menu to run the application. Click once on the form and notice how no repainting occurs while the modal form displays. Unloading the modal form causes the message box to display and also resumes all Windows messages to the Visual Basic 3.0 project.

Keywords: kbprb KB149032