BUG: RPC_E_CANTCALLOUT_ININPUTSYNCCALL Error When System Menu Is Shown in Taskbar

Article ID: 198996 - View products that this article applies to.
This article was previously published under Q198996
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SYMPTOMS

When an application tries to call a Component Object Model (COM) method while the user has displayed the system menu from the application's taskbar button, the COM call fails with RPC_E_CANTCALLOUT_ININPUTSYNCCALL.

The following list outlines in detail what occurs:
  1. A thread creates a window that has a corresponding taskbar button.
  2. The thread makes outgoing COM calls in response to posted window messages, such as WM_TIMER messages.
  3. The user right-clicks the taskbar button or sets the keyboard focus on the button. Then the user presses the SHIFT+F10 key combination or the Application key.
  4. The shell sends a message to the thread using the SendNotifyMessage function. The default message handler displays the system menu and runs a message loop to service the menu.
  5. The thread receives and processes a posted message that causes it to attempt an outgoing COM call.
  6. The COM call fails with RPC_E_CANTCALLOUT_ININPUTSYNCCALL.
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CAUSE

COM does not allow normal outgoing COM method calls from a thread that is currently servicing a SendMessage request. For additional information, click the article number below to view the article in the Microsoft Knowledge Base:
131056
(http://support.microsoft.com/kb/131056/EN-US/ )
PRB: Synch OLE Call Fails in Inter-Process/Thread SendMessage
When the system menu is displayed in the taskbar, the thread that owns the window is servicing a SendNotifyMessage request. On Windows 95, Windows 98, and Windows NT 4.0, COM does not distinguish between SendMessage and SendNotifyMessage.
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RESOLUTION

This bug was corrected in Microsoft Windows 2000. In Windows 2000, COM uses the InSendMessageEx function to detect a true SendMessage request.

To work around this problem if the application is running Windows 95, Windows 98, or Windows NT 4.0, use one of the following methods:
  • Allow outgoing COM calls to fail with RPC_E_CANTCALLOUT_ININPUTSYNCCALL.
  • Call InSendMessage before you make any outbound COM calls in response to posted window messages. If InSendMessage returns TRUE, do not attempt an outgoing COM call.
  • Make outgoing COM calls on another thread.
  • Prevent the taskbar from creating a button for the window. To do this, create the window with the WS_EX_TOOLWINDOW extended style, or create a hidden window, and make the hidden window the owner of the visible window.
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STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.

This bug was corrected in Windows 2000.
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Properties

Article ID: 198996 - Last Review: December 1, 2003 - Revision: 3.2
APPLIES TO
  • Microsoft COM+ 2.0 Standard Edition, when used with:
    • Microsoft Windows NT 4.0
    • Microsoft Windows 98 Standard Edition
    • Microsoft Windows 95
Keywords: 
kbbug kbfix KB198996
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