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    <p>Edit: By popular demand, I'll point out that this answer is in response to the questionioner's mentioning of mutex, so I'm assuming he/she wants to limit the number of users per machine. If he/she instead wants to limit total usage in the network, then this is not the way to do it.</p> <p>Here's what I'm currently using:</p> <pre><code> // Mutex object used to determine if there are multiple instances of this program running. // Note that this is a reference to a .Net Mutex object, not the Windows mutex itself. private static Mutex _onlyOneInstanceMutex; /// &lt;summary&gt; /// Method to test that there is not another instance of the program already running on this /// machine, or at least in this Terminal Services session or Windows Vista / Windows 7 /// concurrent sessions session. If there is, a message box-style localized error message is /// displayed and the value false is returned. This implies that this method should only be /// used in WinForms programs. /// /// This implementation uses a .Net Mutex object in public storage to prevent it from being /// garbage-collected. The name of the associated Windows mutex is simply the program name as /// provided by the caller. Neither the .Net Mutex object nor the Windows mutex are ever /// explicitly released; they remain in existence, perhaps in an "abandoned" state, until the /// process that created them terminates. /// &lt;/summary&gt; /// &lt;returns&gt;false if another instance running, otherwise true&lt;/returns&gt; [SuppressMessage("Microsoft.Reliability", "CA2004:RemoveCallsToGCKeepAlive", Justification = "Not sure if this is correct or not.")] public static bool TestOnlyOneInstance(string programName) { // Funny construct to prevent the Mutex from being garbage collected GC.KeepAlive(_onlyOneInstanceMutex); // Test if we are the first instance, and if so create the Windows mutex, making it // impossible for subsequent instances to successfully create their mutex bool firstInstance; _onlyOneInstanceMutex = new Mutex(false, programName, out firstInstance); if (firstInstance) return true; // Display a (possibly localized) error message, then return string errorMessage = MLocalizer.GetString("Error1", "Another instance of this program is already running on this machine.") + "\n" + MLocalizer.GetString("Error2", "You cannot run two instances at the same time.") + "\n" + MLocalizer.GetString("Error3", "Please use the other instance."); MessageBox.Show(errorMessage, programName, MessageBoxButtons.OK, MessageBoxIcon.Error); return false; } </code></pre>
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    1. POprevent multiple instance of an application
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    1. COPrevents multiple copies on one computer, not multiple copies on multiple computers.
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    2. CO@Corey: Yes, I know. But the question is ambiguous, and the questioner even mentioned mutex.
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    3. CO@RenniePet - if you know that there are many ways to read the question please consider adding "this answer for XXXXXX interpretation of the question. Does[not] cover YYYYY and ZZZZ interpretations".
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