After the first WM_TIMER event is fired, subsequent WM_TIMER events cannotbe processed until the first event is completed. The error message equatesto:
It is illegal to call out while inside message filter.
Three workarounds have been implemented with success:
Set a boolean flag in the timer event (see code listed in the Steps toReproduce Behavior section below).
Select Project .Add Module and add a standard module
Type the following code into the module:
Public flag As Boolean
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 iscompleted.
Create the server as a DLL, not an EXE. ActiveX DLLs are in-processservers. An in-process server would not use Remote Procedure Call engine(RPC). (See the MORE INFORMATION below.)
Place error handling in effect to ignore the error:
Private Sub Timer1_Timer() On Error Resume Next RetStr = t.Test(100000) End Sub
This behavior is by design.
Timers in Microsoft Windows are fired by Windows placing WM_TIMER messagesin a windows message queue. For example, an application receives the firsttimer's WM_TIMER message in its window proc. This causes Visual Basic tofire the first timer's Timer event, which calls the ActiveX server throughthe Remote Procedure Call engine (RPC) because ActiveX EXE servers run in aseparate 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 itsmessage queue. This is allowed primarily for screen redraw (if WM_PAINTmessages come in because the user was switching around windows), so theapplication can update its windows while it is waiting for the RPC call tocomplete.
If, while Visual Basic waits for the first RPC call to complete, itprocesses a second WM_TIMER message coming in through the RPC messagefilter, 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 anRPC call when it is in the RPC message filter, so it returns an error thatVisual Basic returns to the user.
At the point at which Visual Basic gets the WM_TIMER message, there is noway for Visual Basic to know whether firing the Timer event will cause anRPC call to be made. In fact, because ActiveX servers can be transparentlyremote, there is no way for Visual Basic to know, when it is calling anobject 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 RPCengine is going to throw an error.
Steps to Reproduce Behavior
NOTE: The error will not reproduce unless you have compiled the projects.
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.
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
Save the project.
Compile the EXE.
Run the compiled EXE. Note that the Automation error occurs.