Implement a single-instance restriction to gracefully inform the user that the application is already running

Documentation/Settings.md

@@ -10,6 +10,7 @@ Back to [Home](https://github.com/dankrusi/WindowsVirtualDesktopHelper)
 
 |Config|Default|Description|
 | --- | --- | --- |
+| debug.singleInstance | ``true`` | If true, the app will prevent multiple instances of the app from starting.  Most users won't need to change this option. |
 | general.startupWithWindows | ``false`` | If true, the app will register itself with Windows to startup when Windows starts (via the registry). |
 | general.theme | ``"auto"`` | Can be either auto, dark or light. If set to auto, the theme is derived from the current windows theme (dark or light). |
 | theme.icons.disabledOpacity | ``"0.5"`` | Defines the opacity to use for icons which are disabled. |

Source/App/Settings.cs

@@ -24,10 +24,13 @@ class Settings {
 		public static void LoadDefaults() {
 			// Register any known default settings here
 
+			// Debug
+			RegisterDefault("debug.singleInstance", true, "true - only allow a single instance of the application to run (default); false - allow multiple instances (expect errors registering hotkeys)");
+
 			// General
 			RegisterDefault("general.startupWithWindows", false, "If true, the app will register itself with Windows to startup when Windows starts (via the registry).");
 			RegisterDefault("general.theme", "auto", "Can be either auto, dark or light. If set to auto, the theme is derived from the current windows theme (dark or light).");
-			
+
 			// Theme
 			RegisterDefault("theme.icons.disabledOpacity", 0.5, "Defines the opacity to use for icons which are disabled.");
 			RegisterDefault("theme.icons.font", "Segoe UI", "Defines the font name to use for the icons (for regular numbers, characters).");

Source/Main.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Threading;
 using System.Windows.Forms;
 
 namespace WindowsVirtualDesktopHelper {
@@ -14,10 +15,17 @@ public static void Main(string[] args) {
 				// Load config
 				Settings.LoadConfig();
 				// Start app
-				Application.EnableVisualStyles();
-				Application.SetCompatibleTextRenderingDefault(false);
-				var app = new App();
-				Application.Run(app.AppForm);
+				using (Mutex mutex = new Mutex(false, "Mutex/WindowsVirtualDesktopHelper")) {
+					if (Settings.GetBool("debug.singleInstance") && !mutex.WaitOne(0)) {
+						MessageBox.Show("WindowsVirtualDesktopHelper is already running.", "Error", MessageBoxButtons.OK);
+						return;
+					}
+
+					Application.EnableVisualStyles();
+					Application.SetCompatibleTextRenderingDefault(false);
+					var app = new App();
+					Application.Run(app.AppForm);
+				}
 			} catch (Exception e) {
 				// Global error handler
 				Console.Error.WriteLine(e);