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PRB: VB5.0 OLE Automation Error --2147418107 (80010005)

Article ID: 176399 - View products that this article applies to.

This article was previously published under Q176399

SYMPTOMS

When a compiled Visual Basic application makes calls to an ActiveX server via a timer event, the following error may occur:

Run-time error '-2147418107 (80010005)': Automation error

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CAUSE

After the first WM_TIMER event is fired, subsequent WM_TIMER events cannot be processed until the first event is completed. The error message equates to:

It is illegal to call out while inside message filter.

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STATUS

This behavior is by design.

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Timers in Microsoft Windows are fired by Windows placing WM_TIMER messages in a windows message queue. For example, an application receives the first timer's WM_TIMER message in its window proc. This causes Visual Basic to fire the first timer's Timer event, which calls the ActiveX server through the Remote Procedure Call engine (RPC) because ActiveX EXE servers run in a separate process.

While the Visual Basic application is waiting for the RPC call to complete, the RPC engine allows the Visual Basic application to sit and poll its message queue. This is allowed primarily for screen redraw (if WM_PAINT messages come in because the user was switching around windows), so the application can update its windows while it is waiting for the RPC call to complete.

If, while Visual Basic waits for the first RPC call to complete, it processes a second WM_TIMER message coming in through the RPC message filter, this causes Visual Basic to fire the second timer's Timer event, which attempts to call the ActiveX server again through the RPC engine. However, the RPC engine explicitly does not allow an application to make an RPC call when it is in the RPC message filter, so it returns an error that Visual Basic returns to the user.

At the point at which Visual Basic gets the WM_TIMER message, there is no way for Visual Basic to know whether firing the Timer event will cause an RPC call to be made. In fact, because ActiveX servers can be transparently remote, there is no way for Visual Basic to know, when it is calling an object method, whether that method will be routed through the RPC engine. Thus, there is no way for Visual Basic to know ahead of time that the RPC engine is going to throw an error.

Steps to Reproduce Behavior

NOTE: The error will not reproduce unless you have compiled the projects.

Server

Start a new ActiveX EXE project.

Change the following properties:

      Project Name:TestDelayServer
      Class Name: cTestClass

Place the following code into cTestClass:

      Option Explicit

      Public Function Test(Optional delay As Long = 1000) as String
         Dim index As Long
         Dim Y As String
         Dim X As String
         Y = "a;lskdjf;laksdjf;lkasjd;flkajsmd;lfkjma;sldkfj;laskdj"
         For index = 1 To delay
             X = Right(Y, CInt(Len(Y) / 2))
             X = ""
         Next
         Test = "I'm here"
      End Function

Compile the EXE.
Run the EXE to register it.

Client

Start a new Standard EXE.
Select TestDelayServer from the Project - References menu.
Insert a Command Button, Timer Control and TextBox Control.

Place the following code into Form1's code window:

      Option Explicit

      Dim t As New TestDelayServer.cTestClass
      Dim f As Form
      Public RetStr As String

      Private Sub Command1_Click()
          Set f = New Form1
          f.Show
      End Sub

      Private Sub Form_Load()
          Timer1.Interval = 1000
          Timer1.Enabled = True
      End Sub

      Private Sub Timer1_Timer()
         RetStr =  t.Test(100000)
         Text1.Text = RetStr
      End Sub