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shutdownblockreasoncreate (user32)
Indicates that the system cannot be shut down and sets a reason string to be displayed to the user if system shutdown is initiated.

C# Signature:

[DllImport("user32.dll", SetLastError=true)]
static extern bool ShutdownBlockReasonCreate(IntPtr hWnd, [MarshalAs(UnmanagedType.LPWStr)] string reason);

VB Signature:

Declare Function ShutdownBlockReasonCreate Lib "user32.dll" (ByVal hWnd As IntPtr, <Runtime.InteropServices.MarshalAs(Runtime.InteropServices.UnmanagedType.LPWStr)> ByVal reason As String) As Boolean

User-Defined Types:


Alternative Managed API:

Do you know one? Please contribute it!


hWnd - A handle to the main window of the application.

reason - The reason the application must block system shutdown.

Tips & Tricks:

  • Invisible windows won't show up in Vista's "Running Applications" dialog on shutdown/logoff and therefore the reason set with this method will not be visible to the end-user
  • Make sure you also block/delay the WM_QUERYENDSESSION message.

Sample Code:

IntPtr handleForShutdownBlocker = shutdownBlockingForm.Handle;
if (!ShutdownBlockReasonCreate(handleForShutdownBlocker, UIMessages.GetString("ShutdownBlockReason"))) {
    // failed, log error code and handle used.
    int error = Marshal.GetLastWin32Error();
    log.Error(string.Format("Failed calling ShutdownBlockReasonCreate. Error code: {0}, blocking form handle: {1}", error, handleForShutdownBlocker));


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