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

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

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.

Resolution

Three workarounds have been implemented with success:

Workaround 1

Set a boolean flag in the timer event (see code listed in the Steps to Reproduce Behavior section below).

  1. Select Project .Add Module and add a standard module
  2. Type the following code into the module:
          Public flag As Boolean

  3. Change the Timer1_Timer event code to read as follows:
          Private Sub Timer1_Timer()
    If Not flag Then
    flag = True
    RetStr = t.Test(100000)
    flag = False
    End If
    Text1.Text = RetStr
    End Sub

By doing this, the timer can't call the server until the previous call is completed.

Workaround 2

Create the server as a DLL, not an EXE. ActiveX DLLs are in-process servers. An in-process server would not use Remote Procedure Call engine (RPC). (See the MORE INFORMATION below.)

Workaround 3

Place error handling in effect to ignore the error:

   Private Sub Timer1_Timer()
On Error Resume Next
RetStr = t.Test(100000)
End Sub

Status

This behavior is by design.

More Information

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

  1. Start a new ActiveX EXE project.
  2. Change the following properties:

    Project Name:TestDelayServer
    Class Name: cTestClass

  3. 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

  4. Compile the EXE.
  5. Run the EXE to register it.

Client

  1. Start a new Standard EXE.
  2. Select TestDelayServer from the Project - References menu.
  3. Insert a Command Button, Timer Control and TextBox Control.
  4. 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
  5. Save the project.
  6. Compile the EXE.
  7. Run the compiled EXE. Note that the Automation error occurs.
Properties

Article ID: 176399 - Last Review: May 12, 2003 - Revision: 1

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