What is the correct way to create a single-instance WPF application

Question

Using C  and WPF under  NET  rather than Windows Forms or console   what is the correct way to create an application that can only be run as a single instance   I know it has something to do with some mythical thing called a mutex  rarely can I find someone that bothers to stop and explain what one of these are   The code needs to also inform the already-running instance that the user tried to start a second one  and maybe also pass any command-line arguments if any existed

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Update 2017-01-25  After trying few things  I decided to go with VisualBasic dll it is easier and works better  at least for me   I let my previous answer just as reference     Just as reference  this is how I did without passing arguments  which I can t find any reason to do so    I mean a single app with arguments that as to be passed out from one instance to another one   If file association is required  then an app should  per users standard expectation  be instanciated for each doc  If you have to pass args to existing app  I  think I would used vb dll   Not passing args  just single instance app   I prefer not registering a new Window message and not override the message loop as defined in Matt Davis Solution  Although it s not a big deal to add a VisualBasic dll  but I prefer not add a new reference just to do single instance app  Also  I do prefer instanciate a new class with Main instead of calling Shutdown from App Startup override to ensure to exit as soon as possible   In hope that anybody will like it    or will inspire a little bit  -    Project startup class should be set as  SingleInstanceApp    public class SingleInstanceApp        STAThread      public static void Main string   args                Mutex  mutexSingleInstance   new Mutex true   MonitorMeSingleInstance             if   mutexSingleInstance WaitOne TimeSpan Zero  true                         try                               var app   new App                    app InitializeComponent                    app Run                               finally                                mutexSingleInstance ReleaseMutex                     mutexSingleInstance Close                                    else                       MessageBox Show  One instance is already running                  var processes   Process GetProcessesByName Assembly GetEntryAssembly   GetName   Name                                 if  processes Length  gt  1                                        foreach  var process in processes                                                if  process Id    Process GetCurrentProcess   Id                                                        WindowHelper SetForegroundWindow process MainWindowHandle                                                                                                       WindowHelper   using System  using System Runtime InteropServices  using System Windows  using System Windows Interop  using System Windows Threading   namespace HQ Util Unmanaged       public class WindowHelper                DllImport  user32 dll             return  MarshalAs UnmanagedType Bool           public static extern bool SetForegroundWindow IntPtr hWnd

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Here is a very good article regarding the Mutex solution   The approach described by the article is advantageous for two reasons   First  it does not require a dependency on the Microsoft VisualBasic assembly   If my project already had a dependency on that assembly  I would probably advocate using the approach shown in another answer   But as it is  I do not use the Microsoft VisualBasic assembly  and I d rather not add an unnecessary dependency to my project   Second  the article shows how to bring the existing instance of the application to the foreground when the user tries to start another instance   That s a very nice touch that the other Mutex solutions described here do not address     UPDATE  As of 8 1 2014  the article I linked to above is still active  but the blog hasn t been updated in a while   That makes me worry that eventually it might disappear  and with it  the advocated solution   I m reproducing the content of the article here for posterity   The words belong solely to the blog owner at Sanity Free Coding      Today I wanted to refactor some code that prohibited my application   from running multiple instances of itself       Previously I had use System Diagnostics Process to search for an   instance of my myapp exe in the process list  While this works  it   brings on a lot of overhead  and I wanted something cleaner       Knowing that I could use a mutex for this  but never having done it   before  I set out to cut down my code and simplify my life       In the class of my application main I created a static named Mutex    static class Program       static Mutex mutex   new Mutex true    8F6F0AC4-B9A1-45fd-A8CF-72F04E6BDE8F          STAThread                 Having a named mutex allows us to stack synchronization across   multiple threads and processes which is just the magic I m looking   for       Mutex WaitOne has an overload that specifies an amount of time for us   to wait  Since we re not actually wanting to synchronizing our code    more just check if it is currently in use  we use the overload with   two parameters  Mutex WaitOne Timespan timeout  bool exitContext     Wait one returns true if it is able to enter  and false if it wasn t    In this case  we don t want to wait at all  If our mutex is being   used  skip it  and move on  so we pass in TimeSpan Zero  wait 0   milliseconds   and set the exitContext to true so we can exit the   synchronization context before we try to aquire a lock on it  Using   this  we wrap our Application Run code inside something like this    static class Program       static Mutex mutex   new Mutex true    8F6F0AC4-B9A1-45fd-A8CF-72F04E6BDE8F          STAThread      static void Main             if mutex WaitOne TimeSpan Zero  true                 Application EnableVisualStyles                Application SetCompatibleTextRenderingDefault false               Application Run new Form1                 mutex ReleaseMutex              else               MessageBox Show  only one instance at a time                           So  if our app is running  WaitOne will return false  and we ll get a   message box       Instead of showing a message box  I opted to utilize a little Win32 to   notify my running instance that someone forgot that it was already   running  by bringing itself to the top of all the other windows   To   achieve this I used PostMessage to broadcast a custom message to every   window  the custom message was registered with RegisterWindowMessage   by my running application  which means only my application knows what   it is  then my second instance exits  The running application instance   would receive that notification and process it  In order to do that  I   overrode WndProc in my main form and listened for my custom   notification  When I received that notification I set the form s   TopMost property to true to bring it up on top       Here is what I ended up with          Program cs      static class Program       static Mutex mutex   new Mutex true    8F6F0AC4-B9A1-45fd-A8CF-72F04E6BDE8F          STAThread      static void Main             if mutex WaitOne TimeSpan Zero  true                 Application EnableVisualStyles                Application SetCompatibleTextRenderingDefault false               Application Run new Form1                 mutex ReleaseMutex              else                  send our Win32 message to make the currently running instance                jump on top of all the other windows             NativeMethods PostMessage                   IntPtr NativeMethods HWND BROADCAST                  NativeMethods WM SHOWME                  IntPtr Zero                  IntPtr Zero                             NativeMethods cs         this class just wraps some Win32 stuff that we re going to use internal class NativeMethods       public const int HWND BROADCAST   0xffff      public static readonly int WM SHOWME   RegisterWindowMessage  WM SHOWME         DllImport  user32        public static extern bool PostMessage IntPtr hwnd  int msg  IntPtr wparam  IntPtr lparam        DllImport  user32        public static extern int RegisterWindowMessage string message             Form1 cs  front side partial       public partial class Form1   Form       public Form1                 InitializeComponent              protected override void WndProc ref Message m                if m Msg    NativeMethods WM SHOWME                ShowMe                      base WndProc ref m             private void ShowMe                 if WindowState    FormWindowState Minimized                WindowState   FormWindowState Normal                       get our current  TopMost  value  ours will always be false though          bool top   TopMost             make our form jump to the top of everything         TopMost   true             set it back to whatever it was         TopMost   top

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Not using Mutex though  simple answer   System Diagnostics          string thisprocessname   Process GetCurrentProcess   ProcessName   if  Process GetProcesses   Count p   gt  p ProcessName    thisprocessname   gt  1                  return    Put it inside the Program Main    Example   using System  using System Collections Generic  using System Linq  using System Threading Tasks  using System Windows Forms  using System Diagnostics   namespace Sample       static class Program                    lt summary gt              The main entry point for the application               lt  summary gt           STAThread          static void Main                           simple add Diagnostics namespace  and these 3 lines below              string thisprocessname   Process GetCurrentProcess   ProcessName              if  Process GetProcesses   Count p   gt  p ProcessName    thisprocessname   gt  1                  return               Application EnableVisualStyles                Application SetCompatibleTextRenderingDefault false               Application Run new Sample                         You can add MessageBox Show to the if-statement and put  Application already running   This might be helpful to someone

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Well  I have a disposable Class for this that works easily for most use cases   Use it like this   static void Main         using  SingleInstanceMutex sim   new SingleInstanceMutex                  if  sim IsOtherInstanceRunning                        Application Exit                          Initialize program here            Here it is        lt summary gt      Represents a  lt see cref  SingleInstanceMutex   gt  class       lt  summary gt  public partial class SingleInstanceMutex   IDisposable        region Fields           lt summary gt          Indicator whether another instance of this application is running or not           lt  summary gt      private bool isNoOtherInstanceRunning            lt summary gt          The  lt see cref  Mutex   gt  used to ask for other instances of this application           lt  summary gt      private Mutex singleInstanceMutex   null            lt summary gt          An indicator whether this object is beeing actively disposed or not           lt  summary gt      private bool disposed        endregion       region Constructor           lt summary gt          Initializes a new instance of the  lt see cref  SingleInstanceMutex   gt  class           lt  summary gt      public SingleInstanceMutex                 this singleInstanceMutex   new Mutex true  Assembly GetCallingAssembly   FullName  out this isNoOtherInstanceRunning               endregion       region Properties           lt summary gt          Gets an indicator whether another instance of the application is running or not           lt  summary gt      public bool IsOtherInstanceRunning               get                       return  this isNoOtherInstanceRunning                        endregion       region Methods           lt summary gt          Closes the  lt see cref  SingleInstanceMutex   gt            lt  summary gt      public void Close                 this ThrowIfDisposed            this singleInstanceMutex Close               public void Dispose                 this Dispose true           GC SuppressFinalize this              private void Dispose bool disposing                if   this disposed                           Release unmanaged ressources                 if  disposing                                   Release managed ressources                    this Close                               this disposed   true                            lt summary gt          Throws an exception if something is tried to be done with an already disposed object           lt  summary gt           lt remarks gt          All public methods of the class must first call this           lt  remarks gt      public void ThrowIfDisposed                 if  this disposed                        throw new ObjectDisposedException this GetType   Name                         endregion

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Please check the proposed solution from here that uses a semaphore to determine if an existing instance is already running  works for a WPF application and can pass arguments from second instance to the first already running instance by using a TcpListener and a TcpClient   It works also for  NET Core  not only for  NET Framework

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Here is my 2 cents   static class Program                STAThread          static void Main                         bool createdNew              using  new Mutex true   MyApp   out createdNew                                 if  createdNew                        Application EnableVisualStyles                        Application SetCompatibleTextRenderingDefault false                       var mainClass   new SynGesturesLogic                        Application ApplicationExit    mainClass tray exit                      Application Run                                      else                                       var current   Process GetCurrentProcess                        foreach  var process in Process GetProcessesByName current ProcessName  Where process   gt  process Id    current Id                                                 NativeMethods SetForegroundWindow process MainWindowHandle                           break

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I found the simpler solution  similar to Dale Ragan s  but slightly modified  It does practically everything you need and based on the standard Microsoft WindowsFormsApplicationBase class   Firstly  you create SingleInstanceController class  which you can use in all other single-instance applications  which use Windows Forms   using System  using System Collections Generic  using System Linq  using System Text  using System Windows Forms  using Microsoft VisualBasic ApplicationServices    namespace SingleInstanceController NET       public class SingleInstanceController       WindowsFormsApplicationBase               public delegate Form CreateMainForm            public delegate void StartNextInstanceDelegate Form mainWindow           CreateMainForm formCreation          StartNextInstanceDelegate onStartNextInstance          public SingleInstanceController CreateMainForm formCreation  StartNextInstanceDelegate onStartNextInstance                           Set whether the application is single instance             this formCreation   formCreation              this onStartNextInstance   onStartNextInstance              this IsSingleInstance   true               this StartupNextInstance    new StartupNextInstanceEventHandler this StartupNextInstance                                            void this StartupNextInstance object sender  StartupNextInstanceEventArgs e                        if  onStartNextInstance    null                                onStartNextInstance this MainForm      This code will be executed when the user tries to start the running program again                                                         for example  by clicking on the exe file                                                         This code can determine how to re-activate the existing main window of the running application                     protected override void OnCreateMainForm                            Instantiate your main application form             this MainForm   formCreation                       public void Run                         string   commandLine   new string 0               base Run commandLine                       Then you can use it in your program as follows   using System  using System Collections Generic  using System Linq  using System Windows Forms  using SingleInstanceController NET   namespace SingleInstance       static class Program                    lt summary gt              The main entry point for the application               lt  summary gt          static Form CreateForm                         return new Form1       Form1 is used for the main window                     static void OnStartNextInstance Form mainWindow     When the user tries to restart the application again                                                              the main window is activated again                        mainWindow WindowState   FormWindowState Maximized                     STAThread          static void Main                         Application EnableVisualStyles                Application SetCompatibleTextRenderingDefault false                           SingleInstanceController controller   new SingleInstanceController CreateForm  OnStartNextInstance               controller Run                                 Both the program and the SingleInstanceController NET solution should reference Microsoft VisualBasic   If you just want to reactivate the running application as a normal window when the user tries to restart the running program  the second parameter in the SingleInstanceController can be null  In the given example  the window is maximized

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MSDN actually has a sample application for both C  and VB to do exactly this  http   msdn microsoft com en-us library ms771662 v VS 90  aspx     The most common and reliable technique   for developing single-instance   detection is to use the Microsoft  NET   Framework remoting infrastructure    System Remoting   The Microsoft  NET   Framework  version 2 0  includes a   type  WindowsFormsApplicationBase    which encapsulates the required   remoting functionality  To incorporate   this type into a WPF application  a   type needs to derive from it  and be   used as a shim between the application   static entry point method  Main  and   the WPF application s Application   type  The shim detects when an   application is first launched  and   when subsequent launches are   attempted  and yields control the WPF   Application type to determine how to   process the launches     For C  people just take a deep breath and forget about  the whole  I don t wanna include VisualBasic DLL   Because of this and what Scott Hanselman says and the fact that this pretty much is the cleanest solution to the problem and is designed by people who know a lot more about the framework than you do  From a usability standpoint the fact is if your user is loading an application and it is already open and you re giving them an error message like  Another instance of the app is running  Bye  then they re not gonna be a very happy user  You simply MUST  in a GUI application  switch to that application and pass in the arguments provided - or if command line parameters have no meaning then you must pop up the application which may have been minimized    The framework already has support for this - its just that some idiot named the DLL Microsoft VisualBasic and it didn t get put into Microsoft ApplicationUtils or something like that  Get over it - or open up Reflector   Tip  If you use this approach exactly as is  and you already have an App xaml with resources etc  you ll want to take a look at this too

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Named-mutex-based approaches are not cross-platform because named mutexes are not global in Mono  Process-enumeration-based approaches don t have any synchronization and may result in incorrect behavior  e g  multiple processes started at the same time may all self-terminate depending on timing   Windowing-system-based approaches are not desirable in a console application  This solution  built on top of Divin s answer  addresses all these issues   using System  using System IO   namespace TestCs       public class Program                  The app id must be unique  Generate a new guid for your application           public static string AppId    01234567-89ab-cdef-0123-456789abcdef               The stream is stored globally to ensure that it won t be disposed before the application terminates          public static FileStream UniqueInstanceStream           public static int Main string   args                        EnsureUniqueInstance                    Your code here               return 0                     private static void EnsureUniqueInstance                            Note  If you want the check to be per-user  use Environment SpecialFolder ApplicationData instead              string lockDir   Path Combine                  Environment GetFolderPath Environment SpecialFolder CommonApplicationData                    UniqueInstanceApps                string lockPath   Path Combine lockDir     AppId  unique                 Directory CreateDirectory lockDir                try                                  Create the file with exclusive write access  If this fails  then another process is executing                  UniqueInstanceStream   File Open lockPath  FileMode Create  FileAccess Write  FileShare None                       Although only the line above should be sufficient  when debugging with a vshost on Visual Studio                     that acts as a proxy   the IO exception isn t passed to the application before a Write is executed                  UniqueInstanceStream Write new byte     0    0  1                   UniqueInstanceStream Flush                              catch                               throw new Exception  Another instance of the application is already running

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Here s a lightweight solution I use which allows the application to bring an already existing window to the foreground without resorting to custom windows messages or blindly searching process names    DllImport  user32 dll    static extern bool SetForegroundWindow IntPtr hWnd    static readonly string guid     lt Application Guid gt     static void Main         Mutex mutex   null      if   CreateMutex out mutex           return          Application startup code       Environment SetEnvironmentVariable guid  null  EnvironmentVariableTarget User      static bool CreateMutex out Mutex mutex        bool createdNew   false      mutex   new Mutex false  guid  out createdNew        if  createdNew                Process process   Process GetCurrentProcess            string value   process Id ToString             Environment SetEnvironmentVariable guid  value  EnvironmentVariableTarget User             else               string value   Environment GetEnvironmentVariable guid  EnvironmentVariableTarget User           Process process   null          int processId   -1           if  int TryParse value  out processId               process   Process GetProcessById processId            if  process    null     SetForegroundWindow process MainWindowHandle               MessageBox Show  Unable to start application  An instance of this application is already running                return createdNew      Edit  You can also store and initialize mutex and createdNew statically  but you ll need to explicitly dispose release the mutex once you re done with it  Personally  I prefer keeping the mutex local as it will be automatically disposed of even if the application closes without ever reaching the end of Main

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I have provided sample code for console and wpf applications below    You only have to check the value of the createdNew variable  example below    after you create the named Mutex instance   The boolean createdNew will return false      if the Mutex instance named  YourApplicationNameHere  was already   created on the system somewhere    The boolean createdNew will return true      if this is the first Mutex named  YourApplicationNameHere  on the   system     Console application - Example   static Mutex m   null   static void Main string   args        const string mutexName    YourApplicationNameHere       bool createdNew   false       try                  Initializes a new instance of the Mutex class with a Boolean value that indicates             whether the calling thread should have initial ownership of the mutex  a string that is the name of the mutex              and a Boolean value that  when the method returns  indicates whether the calling thread was granted initial ownership of the mutex           using  m   new Mutex true  mutexName  out createdNew                         if   createdNew                                Console WriteLine  instance is alreday running    shutting down                        Console Read                    return     Exit the application                               Run your windows forms app here             Console WriteLine  Single instance app is running                 Console ReadLine                          catch  Exception ex                 Console WriteLine ex Message           Console ReadLine              WPF-Example   public partial class App   Application   static Mutex m   null   protected override void OnStartup StartupEventArgs e         const string mutexName    YourApplicationNameHere       bool createdNew   false       try                  Initializes a new instance of the Mutex class with a Boolean value that indicates             whether the calling thread should have initial ownership of the mutex  a string that is the name of the mutex              and a Boolean value that  when the method returns  indicates whether the calling thread was granted initial ownership of the mutex           m   new Mutex true  mutexName  out createdNew            if   createdNew                        Current Shutdown       Exit the application                      catch  Exception                throw             base OnStartup e       protected override void OnExit ExitEventArgs e        if  m    null                m Dispose              base OnExit e

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Here is what I use  It combined process enumeration to perform switching and mutex to safeguard from  active clickers    public partial class App        DllImport  user32        private static extern int OpenIcon IntPtr hWnd         DllImport  user32 dll        private static extern bool SetForegroundWindow IntPtr hWnd        protected override void OnStartup StartupEventArgs e                base OnStartup e           var p   Process             GetProcessesByName Process GetCurrentProcess   ProcessName               foreach  var t in p Where t   gt  t MainWindowHandle    IntPtr Zero                                 OpenIcon t MainWindowHandle                   SetForegroundWindow t MainWindowHandle                   Current Shutdown                    return                                there is a chance the user tries to click on the icon repeatedly                and the process cannot be discovered yet             bool createdNew              var mutex   new Mutex true   MyAwesomeApp                   out createdNew       must be a variable  though it is unused -                 we just need a bit of time until the process shows up             if   createdNew                                Current Shutdown                    return                             new Bootstrapper   Run

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Look at the folllowing code  It is a great and simple solution to prevent multiple instances of a WPF application   private void Application Startup object sender  StartupEventArgs e        Process thisProc   Process GetCurrentProcess        if  Process GetProcessesByName thisProc ProcessName  Length  gt  1                MessageBox Show  Application running            Application Current Shutdown            return             var wLogin   new LoginWindow         if  wLogin ShowDialog      true                var wMain   new Main            wMain WindowState   WindowState Maximized          wMain Show              else               Application Current Shutdown

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A time saving solution for C  Winforms     Program cs   using System  using System Windows Forms     needs reference to Microsoft VisualBasic using Microsoft VisualBasic ApplicationServices     namespace YourNamespace       public class SingleInstanceController   WindowsFormsApplicationBase               public SingleInstanceController                         this IsSingleInstance   true                     protected override void OnStartupNextInstance StartupNextInstanceEventArgs e                        e BringToForeground   true              base OnStartupNextInstance e                      protected override void OnCreateMainForm                         this MainForm   new Form1                         static class Program                STAThread          static void Main                         Application EnableVisualStyles                Application SetCompatibleTextRenderingDefault false               string   args   Environment GetCommandLineArgs                SingleInstanceController controller   new SingleInstanceController                controller Run args

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Here s the same thing implemented via Event   public enum ApplicationSingleInstanceMode       CurrentUserSession      AllSessionsOfCurrentUser      Pc    public class ApplicationSingleInstancePerUser  IDisposable       private readonly EventWaitHandle  event            lt summary gt          Shows if the current instance of ghost is the first          lt  summary gt      public bool FirstInstance   get  private set              lt summary gt          Initializes           lt  summary gt           lt param name  applicationName  gt The application name lt  param gt           lt param name  mode  gt The single mode lt  param gt      public ApplicationSingleInstancePerUser string applicationName  ApplicationSingleInstanceMode mode   ApplicationSingleInstanceMode CurrentUserSession                string name          if  mode    ApplicationSingleInstanceMode CurrentUserSession              name     Local   applicationName            else if  mode    ApplicationSingleInstanceMode AllSessionsOfCurrentUser              name     Global   applicationName  Environment UserDomainName            else             name     Global   applicationName             try                       bool created               event   new EventWaitHandle false  EventResetMode ManualReset  name  out created               FirstInstance   created                    catch                                public void Dispose                  event Dispose

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This is how I ended up taking care of this issue   Note that debug code is still in there for testing   This code is within the OnStartup in the App xaml cs file    WPF                Process already running            if  Process GetProcessesByName Process GetCurrentProcess   ProcessName  Length  gt  1                            Show your error message             MessageBox Show  xxx is already running    r n r nIf the original process is hung up you may need to restart your computer  or kill the current xxx process using the task manager     xxx is already running    MessageBoxButton OK  MessageBoxImage Exclamation                   This process              Process currentProcess   Process GetCurrentProcess                    Get all processes running on the local computer              Process   localAll   Process GetProcessesByName Process GetCurrentProcess   ProcessName                   ID of this process                 int temp   currentProcess Id              MessageBox Show  This Process ID       temp ToString                  for  int i   0  i  lt  localAll Length  i                                     Find the other process                  if  localAll i  Id    currentProcess Id                                        MessageBox Show  Original Process ID  Switching to        localAll i  Id ToString                             Switch to it                         SetForegroundWindow localAll i  MainWindowHandle                                                 Application Current Shutdown                 This may have issues that I have not caught yet   If I run into any I ll update my answer

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Here is a solution   Protected Overrides Sub OnStartup e As StartupEventArgs      Const appName As String    TestApp      Dim createdNew As Boolean      mutex   New Mutex True  appName  createdNew      If Not createdNew Then          app is already running  Exiting the application         MessageBox Show  Application is already running            Application Current Shutdown       End If     MyBase OnStartup e  End Sub

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I use Mutex in my solution for preventing multiple instances   static Mutex mutex   null    A string that is the name of the mutex string mutexName     Global test     Prevent Multiple Instances of Application bool onlyInstance   false  mutex   new Mutex true  mutexName  out onlyInstance    if   onlyInstance      MessageBox Show  You are already running this application in your system     Already Running     MessageBoxButton OK     Application Current Shutdown

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You could use the Mutex class  but you will soon find out that you will need to implement the code to pass the arguments and such yourself   Well  I learned a trick when programming in WinForms when I read Chris Sell s book   This trick uses logic that is already available to us in the framework   I don t know about you  but when I learn about stuff I can reuse in the framework  that is usually the route I take instead of reinventing the wheel   Unless of course it doesn t do everything I want   When I got into WPF  I came up with a way to use that same code  but in a WPF application   This solution should meet your needs based off your question   First  we need to create our application class   In this class we are going override the OnStartup event and create a method called Activate  which will be used later   public class SingleInstanceApplication   System Windows Application       protected override void OnStartup System Windows StartupEventArgs e                   Call the OnStartup event on our base class         base OnStartup e               Create our MainWindow and show it         MainWindow window   new MainWindow            window Show               public void Activate                    Reactivate the main window         MainWindow Activate              Second  we will need to create a class that can manage our instances   Before we go through that  we are actually going to reuse some code that is in the Microsoft VisualBasic assembly   Since  I am using C  in this example  I had to make a reference to the assembly   If you are using VB NET  you don t have to do anything   The class we are going to use is WindowsFormsApplicationBase and inherit our instance manager off of it and then leverage properties and events to handle the single instancing   public class SingleInstanceManager   Microsoft VisualBasic ApplicationServices WindowsFormsApplicationBase       private SingleInstanceApplication  application      private System Collections ObjectModel ReadOnlyCollection lt string gt   commandLine       public SingleInstanceManager                 IsSingleInstance   true             protected override bool OnStartup Microsoft VisualBasic ApplicationServices StartupEventArgs eventArgs                   First time  application is launched          commandLine   eventArgs CommandLine           application   new SingleInstanceApplication             application Run            return false             protected override void OnStartupNextInstance StartupNextInstanceEventArgs eventArgs                   Subsequent launches         base OnStartupNextInstance eventArgs            commandLine   eventArgs CommandLine           application Activate              Basically  we are using the VB bits to detect single instance s and process accordingly   OnStartup will be fired when the first instance loads   OnStartupNextInstance is fired when the application is re-run again   As you can see  I can get to what was passed on the command line through the event arguments   I set the value to an instance field   You could parse the command line here  or you could pass it to your application through the constructor and the call to the Activate method   Third  it s time to create our EntryPoint   Instead of newing up the application like you would normally do  we are going to take advantage of our SingleInstanceManager   public class EntryPoint        STAThread      public static void Main string   args                SingleInstanceManager manager   new SingleInstanceManager            manager Run args             Well  I hope you are able to follow everything and be able use this implementation and make it your own

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I added a sendMessage Method to the NativeMethods Class   Apparently the postmessage method dosent work  if the application is not show in the taskbar  however using the sendmessage method solves this   class NativeMethods       public const int HWND BROADCAST   0xffff      public static readonly int WM SHOWME   RegisterWindowMessage  WM SHOWME         DllImport  user32        public static extern bool PostMessage IntPtr hwnd  int msg  IntPtr wparam  IntPtr lparam        DllImport  user32 dll   CharSet   CharSet Auto       public static extern IntPtr SendMessage IntPtr hWnd  int Msg  IntPtr wParam  IntPtr lParam        DllImport  user32        public static extern int RegisterWindowMessage string message

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My favourite solution is from MVP Daniel Vaughan  Enforcing Single Instance Wpf Applications It use MemoryMappedFile to send command line arguments to the first instance       lt summary gt      This class allows restricting the number of executables in execution  to one       lt  summary gt  public sealed class SingletonApplicationEnforcer       readonly Action lt IEnumerable lt string gt  gt  processArgsFunc      readonly string applicationId      Thread thread      string argDelimiter    quot      quot             lt summary gt          Gets or sets the string that is used to join          the string array of arguments in memory           lt  summary gt           lt value gt The arg delimeter  lt  value gt      public string ArgDelimeter               get                       return argDelimiter                    set                       argDelimiter   value                            lt summary gt          Initializes a new instance of the  lt see cref  quot SingletonApplicationEnforcer quot   gt  class           lt  summary gt           lt param name  quot processArgsFunc quot  gt A handler for processing command line args          when they are received from another application instance  lt  param gt           lt param name  quot applicationId quot  gt The application id used          for naming the  lt seealso cref  quot EventWaitHandle quot   gt   lt  param gt      public SingletonApplicationEnforcer Action lt IEnumerable lt string gt  gt  processArgsFunc           string applicationId    quot DisciplesRock quot                 if  processArgsFunc    null                        throw new ArgumentNullException  quot processArgsFunc quot                      this processArgsFunc   processArgsFunc          this applicationId   applicationId                  lt summary gt          Determines if this application instance is not the singleton instance          If this application is not the singleton  then it should exit           lt  summary gt           lt returns gt  lt c gt true lt  c gt  if the application should shutdown           otherwise  lt c gt false lt  c gt   lt  returns gt      public bool ShouldApplicationExit                 bool createdNew          string argsWaitHandleName    quot ArgsWaitHandle  quot    applicationId          string memoryFileName    quot ArgFile  quot    applicationId           EventWaitHandle argsWaitHandle   new EventWaitHandle              false  EventResetMode AutoReset  argsWaitHandleName  out createdNew            GC KeepAlive argsWaitHandle            if  createdNew                           This is the main  or singleton application                     A thread is created to service the MemoryMappedFile                     We repeatedly examine this file each time the argsWaitHandle                    is Set by a non-singleton application instance                 thread   new Thread      gt                                        try                                               using  MemoryMappedFile file   MemoryMappedFile CreateOrOpen memoryFileName  10000                                                         while  true                                                                argsWaitHandle WaitOne                                    using  MemoryMappedViewStream stream   file CreateViewStream                                                                          var reader   new BinaryReader stream                                       string args                                      try                                                                               args   reader ReadString                                                                              catch  Exception ex                                                                                Debug WriteLine  quot Unable to retrieve string   quot    ex                                           continue                                                                            string   argsSplit   args Split new string     argDelimiter                                                                         StringSplitOptions RemoveEmptyEntries                                       processArgsFunc argsSplit                                                                                                                                        catch  Exception ex                                                Debug WriteLine  quot Unable to monitor memory file   quot    ex                                                          thread IsBackground   true              thread Start                      else                          Non singleton application instance                     Should exit  after passing command line args to singleton process                     via the MemoryMappedFile                 using  MemoryMappedFile mmf   MemoryMappedFile OpenExisting memoryFileName                                 using  MemoryMappedViewStream stream   mmf CreateViewStream                                          var writer   new BinaryWriter stream                       string   args   Environment GetCommandLineArgs                        string joined   string Join argDelimiter  args                       writer Write joined                                               argsWaitHandle Set                       return  createdNew

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A new one that uses Mutex and IPC stuff  and also passes any command line arguments to the running instance  is WPF Single Instance Application

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This code should go to the main method  Look at here for more information about the main method in WPF    DllImport  user32 dll    private static extern Boolean ShowWindow IntPtr hWnd  Int32 nCmdShow    private const int SW SHOWMAXIMIZED   3   static void Main          Process currentProcess   Process GetCurrentProcess        var runningProcess    from process in Process GetProcesses                             where                             process Id    currentProcess Id  amp  amp                              process ProcessName Equals                                currentProcess ProcessName                                StringComparison Ordinal                            select process  FirstOrDefault        if  runningProcess    null                ShowWindow runningProcess MainWindowHandle  SW SHOWMAXIMIZED          return             Method 2  static void Main         string procName   Process GetCurrentProcess   ProcessName         get the list of all processes by that name      Process   processes Process GetProcessesByName procName        if  processes Length  gt  1                MessageBox Show procName     already running              return             else                  Application Run                 Note   Above methods assumes your process application has a unique name  Because it uses process name to find if any existing processors  So  if your application has a very common name  ie  Notepad   above approach won t work

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The following code is my WCF named pipes solution to register a single-instance application  It s nice because it also raises an event when another instance attempts to start  and receives the command line of the other instance   It s geared toward WPF because it uses the System Windows StartupEventHandler class  but this could be easily modified   This code requires a reference to PresentationFramework  and System ServiceModel   Usage   class Program       static void Main                 var applicationId   new Guid  b54f7b0d-87f9-4df9-9686-4d8fd76066dc             if  SingleInstanceManager VerifySingleInstance applicationId                         SingleInstanceManager OtherInstanceStarted    OnOtherInstanceStarted                  Start the application                      static void OnOtherInstanceStarted object sender  StartupEventArgs e                   Do something in response to another instance starting up            Source Code        lt summary gt      A class to use for single-instance applications       lt  summary gt  public static class SingleInstanceManager          lt summary gt        Raised when another instance attempts to start up         lt  summary gt    public static event StartupEventHandler OtherInstanceStarted          lt summary gt        Checks to see if this instance is the first instance running on this machine   If it is not  this method will       send the main instance this instance s startup information         lt  summary gt         lt param name  guid  gt The application s unique identifier  lt  param gt         lt returns gt True if this instance is the main instance  lt  returns gt    public static bool VerifySingleInstace Guid guid          if   AttemptPublishService guid               NotifyMainInstance guid          return false             return true              lt summary gt        Attempts to publish the service         lt  summary gt         lt param name  guid  gt The application s unique identifier  lt  param gt         lt returns gt True if the service was published successfully  lt  returns gt    private static bool AttemptPublishService Guid guid          try             ServiceHost serviceHost   new ServiceHost typeof SingleInstance          NetNamedPipeBinding binding   new NetNamedPipeBinding NetNamedPipeSecurityMode None         serviceHost AddServiceEndpoint typeof ISingleInstance   binding  CreateAddress guid          serviceHost Open           return true            catch             return false                    lt summary gt        Notifies the main instance that this instance is attempting to start up         lt  summary gt         lt param name  guid  gt The application s unique identifier  lt  param gt    private static void NotifyMainInstance Guid guid          NetNamedPipeBinding binding   new NetNamedPipeBinding NetNamedPipeSecurityMode None       EndpointAddress remoteAddress   new EndpointAddress CreateAddress guid        using  ChannelFactory lt ISingleInstance gt  factory   new ChannelFactory lt ISingleInstance gt  binding  remoteAddress               ISingleInstance singleInstance   factory CreateChannel          singleInstance NotifyMainInstance Environment GetCommandLineArgs                       lt summary gt        Creates an address to publish contact the service at based on a globally unique identifier         lt  summary gt         lt param name  guid  gt The identifier for the application  lt  param gt         lt returns gt The address to publish contact the service  lt  returns gt    private static string CreateAddress Guid guid          return string Format CultureInfo CurrentCulture   net pipe   localhost  0    guid               lt summary gt        The interface that describes the single instance service         lt  summary gt     ServiceContract    private interface ISingleInstance              lt summary gt          Notifies the main instance that another instance of the application attempted to start           lt  summary gt           lt param name  args  gt The other instance s command-line arguments  lt  param gt       OperationContract      void NotifyMainInstance string   args               lt summary gt        The implementation of the single instance service interface         lt  summary gt    private class SingleInstance   ISingleInstance              lt summary gt          Notifies the main instance that another instance of the application attempted to start           lt  summary gt           lt param name  args  gt The other instance s command-line arguments  lt  param gt      public void NotifyMainInstance string   args              if  OtherInstanceStarted    null                  Type type   typeof StartupEventArgs           ConstructorInfo constructor   type GetConstructor BindingFlags Instance   BindingFlags NonPublic  null  Type EmptyTypes  null           StartupEventArgs e    StartupEventArgs constructor Invoke null           FieldInfo argsField   type GetField   args   BindingFlags Instance   BindingFlags NonPublic           Debug Assert argsField    null           argsField SetValue e  args            OtherInstanceStarted null  e

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You can also use the CodeFluent Runtime which is free set of tools  It provides a SingleInstance class to implement a single instance application

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I like a solution to allow multiple Instances  if the exe is called from an other path  I modified CharithJ solution Method 1      static class Program        DllImport  user32 dll        private static extern bool ShowWindow IntPtr hWnd  Int32 nCmdShow        DllImport  User32 dll        public static extern Int32 SetForegroundWindow IntPtr hWnd        STAThread      static void Main             Process currentProcess   Process GetCurrentProcess            foreach  var process in Process GetProcesses                  try                   if   process Id    currentProcess Id   amp  amp                        process ProcessName    currentProcess ProcessName   amp  amp                       process MainModule FileName    currentProcess MainModule FileName                         ShowWindow process MainWindowHandle  5      const int SW SHOW   5    Activates the window and displays it in its current size and position                       SetForegroundWindow process MainWindowHandle                       return                                  catch  Exception ex                      ignore Exception  Access denied                                    Application EnableVisualStyles            Application SetCompatibleTextRenderingDefault false           Application Run new Form1

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It looks like there is a really good way to handle this   WPF Single Instance Application  This provides a class you can add that manages all the mutex and messaging cruff to simplify the your implementation to the point where it s simply trivial

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Simply using a StreamWriter  how about this   System IO File StreamWriter OpenFlag   null      globally   and  try       OpenFlag   new StreamWriter Path GetTempPath      OpenedIfRunning      catch  System IO IOException    file in use       Environment Exit 0

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So many answers to such a seemingly simple question   Just to shake things up a little bit here is my solution to this problem   Creating a Mutex can be troublesome because the JIT-er only sees you using it for a small portion of your code and wants to mark it as ready for garbage collection   It pretty much wants to out-smart you thinking you are not going to be using that Mutex for that long   In reality you want to hang onto this Mutex for as long as your application is running   The best way to tell the garbage collector to leave you Mutex alone is to tell it to keep it alive though out the different generations of garage collection   Example   var m   new Mutex           GC KeepAlive m     I lifted the idea from this page  http   www ai uga edu  mc SingleInstance html

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Just some thoughts  There are cases when requiring that only one instance of an application is not  lame  as some would have you believe  Database apps  etc  are an order of magnitude more difficult if one allows multiple instances of the app for a single user to access a database  you know  all that updating all the records that are open in multiple instances of the app on the users machine  etc     First  for the  name collision thing  don t use a human readable name - use a GUID instead or  even better a GUID   the human readable name  Chances of name collision just dropped off the radar and the Mutex doesn t care  As someone pointed out  a DOS attack would suck  but if the malicious person has gone to the trouble of getting the mutex name and incorporating it into their app  you are pretty much a target anyway and will have to do MUCH more to protect yourself than just fiddle a mutex name  Also  if one uses the variant of  new Mutex true   some GUID plus Name   out AIsFirstInstance   you already have your indicator as to whether or not the Mutex is the first instance

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Here is an example that allows you to have a single instance of an application  When any new instances load  they pass their arguments to the main instance that is running   public partial class App   Application       private static Mutex SingleMutex      public static uint MessageId       private void Application Startup object sender  StartupEventArgs e                IntPtr Result          IntPtr SendOk          Win32 COPYDATASTRUCT CopyData          string   Args          IntPtr CopyDataMem          bool AllowMultipleInstances   false           Args   Environment GetCommandLineArgs                TODO  Replace  00000000-0000-0000-0000-000000000000  with your application s GUID         MessageId     Win32 RegisterWindowMessage   00000000-0000-0000-0000-000000000000             SingleMutex   new Mutex false   AppName             if   AllowMultipleInstances       AllowMultipleInstances  amp  amp  SingleMutex WaitOne 1  true                          new Main                      else if  Args Length  gt  1                        foreach  Process Proc in Process GetProcesses                                  SendOk   Win32 SendMessageTimeout Proc MainWindowHandle  MessageId  IntPtr Zero  IntPtr Zero                      Win32 SendMessageTimeoutFlags SMTO BLOCK   Win32 SendMessageTimeoutFlags SMTO ABORTIFHUNG                      2000  out Result                    if  SendOk    IntPtr Zero                      continue                  if   uint Result    MessageId                      continue                   CopyDataMem   Marshal AllocHGlobal Marshal SizeOf typeof Win32 COPYDATASTRUCT                      CopyData dwData   IntPtr Zero                  CopyData cbData   Args 1  Length 2                  CopyData lpData   Marshal StringToHGlobalUni Args 1                     Marshal StructureToPtr CopyData  CopyDataMem  false                    Win32 SendMessageTimeout Proc MainWindowHandle  Win32 WM COPYDATA  IntPtr Zero  CopyDataMem                      Win32 SendMessageTimeoutFlags SMTO BLOCK   Win32 SendMessageTimeoutFlags SMTO ABORTIFHUNG                      5000  out Result                    Marshal FreeHGlobal CopyData lpData                   Marshal FreeHGlobal CopyDataMem                              Shutdown 0                      public partial class Main   Window       private void Window Loaded object sender  RoutedEventArgs e                HwndSource Source           Source   HwndSource FromHwnd new WindowInteropHelper this  Handle           Source AddHook new HwndSourceHook Window Proc               private IntPtr Window Proc IntPtr hWnd  int Msg  IntPtr wParam  IntPtr lParam  ref bool Handled                Win32 COPYDATASTRUCT CopyData          string Path           if  Msg    Win32 WM COPYDATA                        CopyData    Win32 COPYDATASTRUCT Marshal PtrToStructure lParam  typeof Win32 COPYDATASTRUCT                Path   Marshal PtrToStringUni CopyData lpData  CopyData cbData   2                if  WindowState    WindowState Minimized                                   Restore window from tray                               Do whatever we want with information              Activate                Focus                       if  Msg    App MessageId                        Handled   true              return new IntPtr App MessageId                      return IntPtr Zero           public class Win32       public const uint WM COPYDATA   0x004A       public struct COPYDATASTRUCT               public IntPtr dwData          public int    cbData          public IntPtr lpData              Flags      public enum SendMessageTimeoutFlags   uint               SMTO NORMAL               0x0000          SMTO BLOCK                0x0001          SMTO ABORTIFHUNG          0x0002          SMTO NOTIMEOUTIFNOTHUNG   0x0008             DllImport  user32 dll   SetLastError true  CharSet CharSet Auto       public static extern uint RegisterWindowMessage string lpString        DllImport  user32 dll        public static extern IntPtr SendMessageTimeout          IntPtr hWnd  uint Msg  IntPtr wParam  IntPtr lParam          SendMessageTimeoutFlags fuFlags  uint uTimeout  out IntPtr lpdwResult

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From here   A common use for a cross-process Mutex is to ensure that only instance of a program can run at a time  Here s how it s done   class OneAtATimePlease         Use a name unique to the application  eg include your company URL    static Mutex mutex   new Mutex  false   oreilly com OneAtATimeDemo       static void Main              Wait 5 seconds if contended     in case another instance        of the program is in the process of shutting down      if   mutex WaitOne TimeSpan FromSeconds  5   false                 Console WriteLine  Another instance of the app is running  Bye             return             try                   Console WriteLine  Running - press Enter to exit            Console ReadLine              finally               mutex ReleaseMutex                          A good feature of Mutex is that if the application terminates without ReleaseMutex first being called  the CLR will release the Mutex automatically

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You should never use a named mutex to implement a single-instance application  or at least not for production code   Malicious code can easily DoS  Denial of Service  your ass

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Use mutex solution   using System  using System Windows Forms  using System Threading   namespace OneAndOnlyOne   static class Program       static String  mutexID        generate guid           lt summary gt          The main entry point for the application           lt  summary gt       STAThread      static void Main                 Application EnableVisualStyles            Application SetCompatibleTextRenderingDefault false            Boolean  isNotRunning          using  Mutex  mutex   new Mutex true   mutexID  out  isNotRunning                         if   isNotRunning                                Application Run new Form1                               else                               MessageBox Show  An instance is already running                     return

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The code C   NET Single Instance Application that is the reference for the marked answer is a great start   However  I found it doesn t handle very well the cases when the instance that already exist has a modal dialog open  whether that dialog is a managed one  like another Form such as an about box   or an unmanaged one  like the OpenFileDialog even when using the standard  NET class   With the original code  the main form is activated  but the modal one stays unactive  which looks strange  plus the user must click on it to keep using the app   So  I have create a SingleInstance utility class to handle all this quite automatically for Winforms and WPF applications   Winforms   1  modify the Program class like this   static class Program       public static readonly SingleInstance Singleton   new SingleInstance typeof Program  FullName         STAThread      static void Main string   args                   NOTE  if this always return false  close  amp  restart Visual Studio            this is probably due to the vshost exe thing         Singleton RunFirstInstance      gt                        SingleInstanceMain args                          public static void SingleInstanceMain string   args                   standard code that was in Main now goes here         Application EnableVisualStyles            Application SetCompatibleTextRenderingDefault false           Application Run new Form1               2  modify the main window class like this   public partial class Form1   Form       public Form1                 InitializeComponent               protected override void WndProc ref Message m                   if needed  the singleton will restore this window         Program Singleton OnWndProc this  m  true               TODO  handle specific messages here if needed         base WndProc ref m             WPF   1  modify the App page like this  and make sure you set its build action to page to be able to redefine the Main method    public partial class App   Application       public static readonly SingleInstance Singleton   new SingleInstance typeof App  FullName         STAThread      public static void Main string   args                   NOTE  if this always return false  close  amp  restart Visual Studio            this is probably due to the vshost exe thing         Singleton RunFirstInstance      gt                        SingleInstanceMain args                          public static void SingleInstanceMain string   args                   standard code that was in Main now goes here         App app   new App            app InitializeComponent            app Run              2  modify the main window class like this   public partial class MainWindow   Window       private HwndSource  source       public MainWindow                 InitializeComponent               protected override void OnSourceInitialized EventArgs e                base OnSourceInitialized e            source    HwndSource PresentationSource FromVisual this            source AddHook HwndSourceHook              protected virtual IntPtr HwndSourceHook IntPtr hwnd  int msg  IntPtr wParam  IntPtr lParam  ref bool handled                   if needed  the singleton will restore this window         App Singleton OnWndProc hwnd  msg  wParam  lParam  true  true               TODO  handle other specific message         return IntPtr Zero          And here is the utility class   using System  using System ComponentModel  using System Runtime InteropServices  using System Threading   namespace SingleInstanceUtilities       public sealed class SingleInstance               private const int HWND BROADCAST   0xFFFF            DllImport  user32 dll            private static extern bool PostMessage IntPtr hwnd  int msg  IntPtr wparam  IntPtr lparam             DllImport  user32 dll   CharSet   CharSet Unicode           private static extern int RegisterWindowMessage string message             DllImport  user32 dll            private static extern bool SetForegroundWindow IntPtr hWnd            public SingleInstance string uniqueName                        if  uniqueName    null                  throw new ArgumentNullException  uniqueName                 Mutex   new Mutex true  uniqueName               Message   RegisterWindowMessage  WM     uniqueName                      public Mutex Mutex   get  private set            public int Message   get  private set             public void RunFirstInstance Action action                        RunFirstInstance action  IntPtr Zero  IntPtr Zero                         NOTE  if this always return false  close  amp  restart Visual Studio            this is probably due to the vshost exe thing         public void RunFirstInstance Action action  IntPtr wParam  IntPtr lParam                        if  action    null                  throw new ArgumentNullException  action                 if  WaitForMutext wParam  lParam                                 try                                       action                                      finally                                       ReleaseMutex                                                       public static void ActivateWindow IntPtr hwnd                        if  hwnd    IntPtr Zero                  return               FormUtilities ActivateWindow FormUtilities GetModalWindow hwnd                       public void OnWndProc IntPtr hwnd  int m  IntPtr wParam  IntPtr lParam  bool restorePlacement  bool activate                        if  m    Message                                if  restorePlacement                                        WindowPlacement placement   WindowPlacement GetPlacement hwnd  false                       if  placement IsValid  amp  amp  placement IsMinimized                                                const int SW SHOWNORMAL   1                          placement ShowCmd   SW SHOWNORMAL                          placement SetPlacement hwnd                                                            if  activate                                        SetForegroundWindow hwnd                       FormUtilities ActivateWindow FormUtilities GetModalWindow hwnd                                                if WINFORMS    define this for Winforms apps         public void OnWndProc System Windows Forms Form form  int m  IntPtr wParam  IntPtr lParam  bool activate                        if  form    null                  throw new ArgumentNullException  form                 if  m    Message                                if  activate                                        if  form WindowState    System Windows Forms FormWindowState Minimized                                                form WindowState   System Windows Forms FormWindowState Normal                                             form Activate                        FormUtilities ActivateWindow FormUtilities GetModalWindow form Handle                                                       public void OnWndProc System Windows Forms Form form  System Windows Forms Message m  bool activate                        if  form    null                  throw new ArgumentNullException  form                 OnWndProc form  m Msg  m WParam  m LParam  activate              endif          public void ReleaseMutex                         Mutex ReleaseMutex                       public bool WaitForMutext bool force  IntPtr wParam  IntPtr lParam                        bool b   PrivateWaitForMutext force               if   b                                PostMessage  IntPtr HWND BROADCAST  Message  wParam  lParam                             return b                     public bool WaitForMutext IntPtr wParam  IntPtr lParam                        return WaitForMutext false  wParam  lParam                      private bool PrivateWaitForMutext bool force                        if  force                  return true               try                               return Mutex WaitOne TimeSpan Zero  true                             catch  AbandonedMutexException                                return true                                        NOTE  don t add any field or public get set property  as this must exactly map to Windows  WINDOWPLACEMENT structure      StructLayout LayoutKind Sequential       public struct WindowPlacement               public int Length   get  set            public int Flags   get  set            public int ShowCmd   get  set            public int MinPositionX   get  set            public int MinPositionY   get  set            public int MaxPositionX   get  set            public int MaxPositionY   get  set            public int NormalPositionLeft   get  set            public int NormalPositionTop   get  set            public int NormalPositionRight   get  set            public int NormalPositionBottom   get  set              DllImport  user32 dll   SetLastError   true           private static extern bool SetWindowPlacement IntPtr hWnd  ref WindowPlacement lpwndpl             DllImport  user32 dll   SetLastError   true           private static extern bool GetWindowPlacement IntPtr hWnd  ref WindowPlacement lpwndpl            private const int SW SHOWMINIMIZED   2           public bool IsMinimized                       get                               return ShowCmd    SW SHOWMINIMIZED                                   public bool IsValid                       get                               return Length    Marshal SizeOf typeof WindowPlacement                                     public void SetPlacement IntPtr windowHandle                        SetWindowPlacement windowHandle  ref this                      public static WindowPlacement GetPlacement IntPtr windowHandle  bool throwOnError                        WindowPlacement placement   new WindowPlacement                if  windowHandle    IntPtr Zero                  return placement               placement Length   Marshal SizeOf typeof WindowPlacement                if   GetWindowPlacement windowHandle  ref placement                                 if  throwOnError                      throw new Win32Exception Marshal GetLastWin32Error                      return new WindowPlacement                              return placement                       public static class FormUtilities                DllImport  user32 dll            private static extern IntPtr GetWindow IntPtr hWnd  int uCmd             DllImport  user32 dll   SetLastError   true           private static extern IntPtr SetActiveWindow IntPtr hWnd             DllImport  user32 dll            private static extern bool IsWindowVisible IntPtr hWnd             DllImport  kernel32 dll            public static extern int GetCurrentThreadId             private delegate bool EnumChildrenCallback IntPtr hwnd  IntPtr lParam             DllImport  user32 dll            private static extern bool EnumThreadWindows int dwThreadId  EnumChildrenCallback lpEnumFunc  IntPtr lParam            private class ModalWindowUtil                       private const int GW OWNER   4              private int  maxOwnershipLevel              private IntPtr  maxOwnershipHandle               private bool EnumChildren IntPtr hwnd  IntPtr lParam                                int level   1                  if  IsWindowVisible hwnd   amp  amp  IsOwned lParam  hwnd  ref level                                         if  level  gt   maxOwnershipLevel                                                 maxOwnershipHandle   hwnd                           maxOwnershipLevel   level                                                          return true                             private static bool IsOwned IntPtr owner  IntPtr hwnd  ref int level                                IntPtr o   GetWindow hwnd  GW OWNER                   if  o    IntPtr Zero                      return false                   if  o    owner                      return true                   level                    return IsOwned owner  o  ref level                              public static void ActivateWindow IntPtr hwnd                                if  hwnd    IntPtr Zero                                        SetActiveWindow hwnd                                                public static IntPtr GetModalWindow IntPtr owner                                ModalWindowUtil util   new ModalWindowUtil                    EnumThreadWindows GetCurrentThreadId    util EnumChildren  owner                   return util  maxOwnershipHandle     may be IntPtr Zero                                  public static void ActivateWindow IntPtr hwnd                        ModalWindowUtil ActivateWindow hwnd                      public static IntPtr GetModalWindow IntPtr owner                        return ModalWindowUtil GetModalWindow owner

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Normally  this is the code I use for single-instance Windows Forms applications    STAThread  public static void Main         String assemblyName   Assembly GetExecutingAssembly   GetName   Name       using  Mutex mutex   new Mutex false  assemblyName                 if   mutex WaitOne 0  false                         Boolean shownProcess   false              Process currentProcess   Process GetCurrentProcess                 foreach  Process process in Process GetProcessesByName currentProcess ProcessName                                 if   process Id Equals currentProcess Id   amp  amp  process MainModule FileName Equals currentProcess MainModule FileName   amp  amp   process MainWindowHandle Equals IntPtr Zero                                         IntPtr windowHandle   process MainWindowHandle                       if  NativeMethods IsIconic windowHandle                           NativeMethods ShowWindow windowHandle  ShowWindowCommand Restore                        NativeMethods SetForegroundWindow windowHandle                        shownProcess   true                                               if   shownProcess                  MessageBox Show String Format CultureInfo CurrentCulture   An instance of  0  is already running    assemblyName   assemblyName  MessageBoxButtons OK  MessageBoxIcon Asterisk  MessageBoxDefaultButton Button1   MessageBoxOptions 0                     else                       Application EnableVisualStyles                Application SetCompatibleTextRenderingDefault false               Application Run new Form                         Where native components are    DllImport  User32 dll   CharSet   CharSet Unicode  ExactSpelling   true  SetLastError   true    return  MarshalAs UnmanagedType Bool   internal static extern Boolean IsIconic  In  IntPtr windowHandle     DllImport  User32 dll   CharSet   CharSet Unicode  ExactSpelling   true  SetLastError   true    return  MarshalAs UnmanagedType Bool   internal static extern Boolean SetForegroundWindow  In  IntPtr windowHandle     DllImport  User32 dll   CharSet   CharSet Unicode  ExactSpelling   true  SetLastError   true    return  MarshalAs UnmanagedType Bool   internal static extern Boolean ShowWindow  In  IntPtr windowHandle   In  ShowWindowCommand command    public enum ShowWindowCommand   int       Hide                     0x0      ShowNormal               0x1      ShowMinimized            0x2      ShowMaximized            0x3      ShowNormalNotActive      0x4      Minimize                 0x6      ShowMinimizedNotActive   0x7      ShowCurrentNotActive     0x8      Restore                  0x9      ShowDefault              0xA      ForceMinimize            0xB

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I can t find a short solution here so I hope someone will like this   UPDATED 2018-09-20  Put this code in your Program cs   using System Diagnostics   static void Main         Process thisProcess   Process GetCurrentProcess        Process   allProcesses   Process GetProcessesByName thisProcess ProcessName       if  allProcesses Length  gt  1                   Don t put a MessageBox in here because the user could spam this MessageBox          return                Optional code  If you don t want that someone runs your   exe  with a different name       string exeName   AppDomain CurrentDomain FriendlyName         in debug mode  don t forget that you don t use your normal  exe name         Debug uses the  vshost exe      if  exeName     the name of your executable exe                     You can add a MessageBox here if you want             To point out to users that the name got changed and maybe what the name should be or something like that            MessageBox Show  The executable name should be   the name of your executable exe                   Wrong executable name   MessageBoxButtons OK  MessageBoxIcon Error           return                Following code is default code      Application EnableVisualStyles        Application SetCompatibleTextRenderingDefault false       Application Run new MainForm

[c#] What is the correct way to create a single-instance WPF application?

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