Prevent the app to run twice, instead show the first window

Question

I have a wpf app and I have to make the app to be run only once. There shouldnt be more than one instance on one machine.

Here's my code in App.xaml.cs, in the OnStartup() method:

var runningProcess = GetAnotherAppInstanceIfExists(); if (runningProcess != null) { HandleMultipleInstances(runningProcess); Environment.Exit(0); } public Process GetAnotherAppInstanceIfExists() { var currentProcess = Process.GetCurrentProcess(); var appProcesses = new List<string> { "myApp", "myApp.vshost" }; var allProcesses = Process.GetProcesses(); var myAppProcess = (from process in allProcesses where process.Id != currentProcess.Id && appProcesses.Contains(process.ProcessName) select process).FirstOrDefault(); if (myAppProcess != null) { return currentProcess; } return myAppProcess; } public void HandleMultipleInstances(Process runningProcess) { SetForegroundWindow(runningProcess.MainWindowHandle); ShowWindow(runningProcess.MainWindowHandle, SW_RESTORE); }

So the app, when run for the second time, doesnt open a new instance, which is great. However, I need to find the first instance and show the window again, if it is minimized. This line is for that:

ShowWindow(runningProcess.MainWindowHandle, SW_RESTORE);

and it doesn't work. What am I doing wrong? I looked at a lot of examples online and my code is the same as them.

Abdurrahman Alp Köken · Accepted Answer · 2014-12-25 14:22:40Z

First change your App.xaml file's build action to Page. Then you can use this code snippet in App.xaml.cs :

private static readonly Mutex Mutex = new Mutex(true, "put your unique value here or GUID"); private static MainWindow _mainWindow; [STAThread] static void Main() { if (Mutex.WaitOne(TimeSpan.Zero, true)) { var app = new App(); _mainWindow = new MainWindow(); app.Run(_mainWindow); Mutex.ReleaseMutex(); } else { //MessageBox.Show("You can only run one instance!"); _mainWindow.WindowState = WindowState.Maximized; } }

I edited the code snippet. You can set WindowState to Maximized to achieve this.

A. Morel · Accepted Answer · 2020-05-28 19:23:31Z

For me the solution was to do this:

change your App.xaml file's build action to Page. (right click on the file then properties => Build Action => dropdown to page)

in App.xaml.cs:

 private static readonly Mutex Mutex = new Mutex(true, "42ae83c2-03a0-472e-a2ea-41d69524a85b"); // GUI can be what you want ! private static App _app; [STAThread] static void Main() { if (Mutex.WaitOne(TimeSpan.Zero, true)) { _app = new App(); _app.InitializeComponent(); _app.Run(); Mutex.ReleaseMutex(); } else { //MessageBox.Show("You can only run one instance!"); _app.MainWindow.WindowState = WindowState.Maximized; } }

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