cross-user C# mutex

Question

My app is forced to use a 3rd party module which will blue-screen Windows if two instances are started at the same time on the same machine. To work around the issue, my C# app has a mutex:

static Mutex mutex = new Mutex(true, "{MyApp_b9d19f99-b83e-4755-9b11-d204dbd6d096}");

And I check if it's present - and if so I show an error message and close the app:

bool IsAnotherInstanceRunning()
{
    if (mutex.WaitOne(TimeSpan.Zero, true))
        return (true);
    else
        return (false);
}

The problem is if two users can log in and open the application at the same time, and IsAnotherInstanceRunning() will return false.

How do I get around this?

How does the 3rd party module do that? Have you contacted their support? — SLaks, May 13 '10 at 21:46
Unless it is a Kernel-level driver, it is surprisingly difficult to make Windows blue-screen these days - what in the world is that module doing? — BlueRaja - Danny Pflughoeft, May 13 '10 at 21:52
@BlueRaja: It's quite easy to blue-screen Windows from user mode. Did you ever try to kill csrss.exe? — Dirk Vollmar, May 13 '10 at 21:57
The 3rd party module is a hardware device I shall not name :) — Warpin, May 13 '10 at 22:35

score 43 · Accepted Answer · edited Jun 04 '20 at 10:55

43

Prefix the name of the mutex with "Global\".

static Mutex mutex = new Mutex(true, "Global\MyApp_b9d19f99-b83e-4755-9b11-d204dbd6d096");

From http://msdn.microsoft.com/en-us/library/system.threading.mutex.aspx:

If its name begins with the prefix "Global\", the mutex is visible in all terminal server sessions. If its name begins with the prefix "Local\", the mutex is visible only in the terminal server session where it was created. In that case, a separate mutex with the same name can exist in each of the other terminal server sessions on the server. If you do not specify a prefix when you create a named mutex, it takes the prefix "Local\".

edited Jun 04 '20 at 10:55

Dan Atkinson

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answered May 13 '10 at 21:50

Paul Kearney - pk

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1

I tried this method, but encounter an unhandled exception when trying to debug,I run VS as administrator, ----------- An unhandled exception of type 'System.UnauthorizedAccessException' occurred in mscorlib.dll Additional information: Access to the path 'Global\MDProduction' is denied. – Mac Lee Apr 27 '20 at 01:29

score 15 · Answer 2 · answered May 13 '10 at 21:47

Change the mutex name to begin with Global\.

Source

On a server that is running Terminal Services, a named system mutex can have two levels of visibility. If its name begins with the prefix "Global\", the mutex is visible in all terminal server sessions. If its name begins with the prefix "Local\", the mutex is visible only in the terminal server session where it was created. In that case, a separate mutex with the same name can exist in each of the other terminal server sessions on the server. If you do not specify a prefix when you create a named mutex, it takes the prefix "Local\". Within a terminal server session, two mutexes whose names differ only by their prefixes are separate mutexes, and both are visible to all processes in the terminal server session. That is, the prefix names "Global\" and "Local\" describe the scope of the mutex name relative to terminal server sessions, not relative to processes.

cross-user C# mutex

2 Answers2

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