Why does a .NET application prevent Windows from shutting down?

Question

One of my applications prevents windows from shutting down if it is running.

The only spot where I suspect the cause may be is FormClosing event handler, which is however quite standard:

EDIT: Removing this handler does not change the situation at all so the cause is somewhere else.

private void MainForm_FormClosing(object sender, FormClosingEventArgs e)
{
    if (e.CloseReason != CloseReason.UserClosing)
    {
        StopAllThreads(); 
        //let close
        return;
    }
    //ask user whether he wants to save his work
}

I have not been able to reproduce this with the simplest possible application containing only this FormClosing handler - the simple application is closed correctly when windows starts shutting down.

What else can prevent windows from shutting down? Where should I look in the code to debug this issue?

I have no custom WndProc implementation in my main form. This is a .NET 2.0 application.

When I run the app in debugger and initiate shutdown, I get only "This program is preventing Windows from shutting down" (Windows 7) for a short while. Windows then shuts down Visual Studio which shuts down the debugger which shuts down the application being debugged.

---

EDIT: StopAllThreads method

public static void StopAllThreads()
{
    lock (syncLock)
    {
        foreach (IStop stoppable in stoppables)
        {
            try
            {
                stoppable.Stop(); //stops a running thread by setting a volatile boolean flag
            }
            catch (Exception ex)
            {
                Debug.Fail("Error stopping a stoppable instance: " + ex.ToString());
            }
        }
        stoppables.Clear();
        disposed = true;
    }
}

Please note: The application can be closed normally when the user closes it manually.

Answer 1

Problem solved: One of the child forms was overriding OnClosing, with the following code:

    protected override void OnClosing(CancelEventArgs e)
    {
        e.Cancel = true;
        this.Hide();
    }