How Can I Prevent Multiple Instances of My App and Notify the User of an Existing Instance?

Single-Instance Application Launch with User Notification using Mutex

This article demonstrates how to use a mutex to prevent multiple instances of an application from running simultaneously, and importantly, how to notify the user if an instance is already running.

The Mutex Approach

The core of the solution involves using a mutex (mutual exclusion) object. A mutex acts as a lock; only one process can acquire ownership of it at a time. Attempting to create a mutex that already exists will fail, allowing us to detect if another instance is running.

Code Enhancement for User Notification

The provided code snippet uses Mutex.OpenExisting to check for a running instance. To improve this, we add user notification:

static void Main(string[] args)
{
    bool createdNew;
    Mutex m = new Mutex(true, AppDomain.CurrentDomain.FriendlyName, out createdNew);

    if (!createdNew)
    {
        // Notify the user that an instance is already running.
        MessageBox.Show("An instance of this application is already running.", "Application Already Running", MessageBoxButtons.OK, MessageBoxIcon.Information);
        return; // Exit the new instance.
    }
    else
    {
        // Continue application execution.
    }

    // ... rest of your application code ...

    m.Dispose(); // Release the mutex when the application closes.
}

This enhanced code utilizes the out createdNew parameter of the Mutex constructor. If createdNew is false, it indicates that the mutex already exists, meaning another instance is running. A MessageBox informs the user, and the new instance gracefully exits using return. Finally, the m.Dispose() call ensures proper resource cleanup. This method provides a clear and user-friendly way to handle multiple application launches.

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