Add check for ensuring there's only single instance to avoid endless (feeling) locked screensaver.

Screensaver.cs

@@ -23,6 +23,8 @@ using System.Runtime.InteropServices;
 using System.Threading;
 using System.Diagnostics;
 using System.Collections.Generic;
+using System.Security.AccessControl;
+using System.Security.Principal;
 
 namespace Screensavers
 {
@@ -291,7 +293,7 @@ namespace Screensavers
 				switch (screensaverMode)
 				{
 					case ScreensaverMode.Windowed:
-						RunWindowed();
+						RunWithSingleInstanceMutex(screensaverMode);
 						break;
 					case ScreensaverMode.Settings:
 						ShowSettingsDialog();
@@ -304,7 +306,7 @@ namespace Screensavers
 						if (!closeOnKeyboardInputOverride)
 							closeOnKeyboardInput = true;
 
-						RunNormal();
+						RunWithSingleInstanceMutex(screensaverMode);
 						break;
 					case ScreensaverMode.Preview:
 						RunPreview();
@@ -504,6 +506,81 @@ namespace Screensavers
 			set { windowTitle = value; }
 		}
 
+		private void RunWithSingleInstanceMutex(ScreensaverMode screensaverMode)
+        {
+			// From: https://stackoverflow.com/questions/229565/what-is-a-good-pattern-for-using-a-global-mutex-in-c/229567
+			// get application GUID as defined in AssemblyInfo.cs
+			string appGuid =
+				((GuidAttribute)System.Reflection.Assembly.GetExecutingAssembly().
+					GetCustomAttributes(typeof(GuidAttribute), false).
+						GetValue(0)).Value.ToString();
+
+			// unique id for global mutex - Global prefix means it is global to the machine
+			string mutexId = string.Format("Global\\{{{0}}}", appGuid);
+
+			// Need a place to store a return value in Mutex() constructor call
+			bool createdNew;
+
+			// edited by Jeremy Wiebe to add example of setting up security for multi-user usage
+			// edited by 'Marc' to work also on localized systems (don't use just "Everyone") 
+			var allowEveryoneRule =
+				new MutexAccessRule(new SecurityIdentifier(WellKnownSidType.WorldSid
+														   , null)
+								   , MutexRights.FullControl
+								   , AccessControlType.Allow
+								   );
+			var securitySettings = new MutexSecurity();
+			securitySettings.AddAccessRule(allowEveryoneRule);
+
+			// edited by MasonGZhwiti to prevent race condition on security settings via VanNguyen
+			using (var mutex = new Mutex(false, mutexId, out createdNew, securitySettings))
+			{
+				// edited by acidzombie24
+				var hasHandle = false;
+				try
+				{
+					try
+					{
+						// note, you may want to time out here instead of waiting forever
+						// edited by acidzombie24
+						// mutex.WaitOne(Timeout.Infinite, false);
+						hasHandle = mutex.WaitOne(5000, false);
+						if (hasHandle == false)
+							throw new TimeoutException("Timeout waiting for exclusive access");
+					}
+					catch (AbandonedMutexException)
+					{
+						// Log the fact that the mutex was abandoned in another process,
+						// it will still get acquired
+						hasHandle = true;
+					}
+
+					// Actuall Run.
+					switch (screensaverMode)
+					{
+						case ScreensaverMode.Windowed:
+							RunWindowed();
+							break;
+						case ScreensaverMode.Settings:
+							ShowSettingsDialog();
+							break;
+						case ScreensaverMode.Normal:
+							RunNormal();
+							break;
+						case ScreensaverMode.Preview:
+							RunPreview();
+							break;
+					}
+				}
+				finally
+				{
+					// edited by acidzombie24, added if statement
+					if (hasHandle)
+						mutex.ReleaseMutex();
+				}
+			}
+        }
+
 		private void RunNormal()
 		{
 			Cursor.Hide();