How to Ensure Only One Instance of a WPF Application Runs at a Time?

Implementing Single-Instance Behavior in WPF Applications

This guide explains how to build a WPF application that allows only one instance to run concurrently. This is achieved using mutexes.

Understanding Mutexes

A mutex (mutual exclusion) is a synchronization primitive. It ensures that only a single process can access a shared resource at any given moment. In WPF, we leverage mutexes to prevent multiple application instances from running simultaneously.

Implementation Details

Utilizing Named Mutexes:

The preferred method involves creating a named mutex within your application's main entry point:

static class Program
{
    static Mutex mutex = new Mutex(true, "{8F6F0AC4-B9A1-45fd-A8CF-72F04E6BDE8F}");
    // ...
}

Named mutexes enable synchronization across multiple processes and threads.

Acquiring the Mutex:

To determine if another instance is already running, we use the WaitOne method:

if (mutex.WaitOne(TimeSpan.Zero, true))
{
    // Mutex acquired; application can start
    Application.Run(new MainWindow());
    mutex.ReleaseMutex();
}
else
{
    // Mutex unavailable; another instance is active
    MessageBox.Show("An instance is already running.");
}

Optional: Notifying the Existing Instance:

For enhanced user experience, you can notify the running instance when a new launch is attempted. This is done using PostMessage:

if (!mutex.WaitOne(TimeSpan.Zero, true))
{
    NativeMethods.PostMessage(
        (IntPtr)NativeMethods.HWND_BROADCAST,
        NativeMethods.WM_SHOWME,
        IntPtr.Zero,
        IntPtr.Zero);
}

Handling the Custom Win32 Message:

In your main window, override WndProc to listen for the custom message and bring the existing window to the foreground:

protected override void WndProc(ref Message m)
{
    if (m.Msg == NativeMethods.WM_SHOWME)
    {
        Activate(); // Bring to front
    }
    base.WndProc(ref m);
}

This comprehensive approach ensures a robust single-instance WPF application, preventing multiple instances from running concurrently. Remember to include necessary NativeMethods definitions (not shown here for brevity).

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