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.
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.
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;
}
}
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<string> _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.
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.
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:
/// <summary>
/// Represents a <see cref="SingleInstanceMutex"/> class.
/// </summary>
public partial class SingleInstanceMutex : IDisposable
{
#region Fields
/// <summary>
/// Indicator whether another instance of this application is running or not.
/// </summary>
private bool isNoOtherInstanceRunning;
/// <summary>
/// The <see cref="Mutex"/> used to ask for other instances of this application.
/// </summary>
private Mutex singleInstanceMutex = null;
/// <summary>
/// An indicator whether this object is beeing actively disposed or not.
/// </summary>
private bool disposed;
#endregion
#region Constructor
/// <summary>
/// Initializes a new instance of the <see cref="SingleInstanceMutex"/> class.
/// </summary>
public SingleInstanceMutex()
{
this.singleInstanceMutex = new Mutex(true, Assembly.GetCallingAssembly().FullName, out this.isNoOtherInstanceRunning);
}
#endregion
#region Properties
/// <summary>
/// Gets an indicator whether another instance of the application is running or not.
/// </summary>
public bool IsOtherInstanceRunning
{
get
{
return !this.isNoOtherInstanceRunning;
}
}
#endregion
#region Methods
/// <summary>
/// Closes the <see cref="SingleInstanceMutex"/>.
/// </summary>
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;
}
}
/// <summary>
/// Throws an exception if something is tried to be done with an already disposed object.
/// </summary>
/// <remarks>
/// All public methods of the class must first call this.
/// </remarks>
public void ThrowIfDisposed()
{
if (this.disposed)
{
throw new ObjectDisposedException(this.GetType().Name);
}
}
#endregion
}
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.
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...
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 > 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);
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();
}
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 = "<Application Guid>";
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.
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);
}
}
}
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.
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) &&
(process.ProcessName == currentProcess.ProcessName) &&
(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());
}
}
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:
/// <summary>
/// This class allows restricting the number of executables in execution, to one.
/// </summary>
public sealed class SingletonApplicationEnforcer
{
readonly Action<IEnumerable<string>> processArgsFunc;
readonly string applicationId;
Thread thread;
string argDelimiter = "_;;_";
/// <summary>
/// Gets or sets the string that is used to join
/// the string array of arguments in memory.
/// </summary>
/// <value>The arg delimeter.</value>
public string ArgDelimeter
{
get
{
return argDelimiter;
}
set
{
argDelimiter = value;
}
}
/// <summary>
/// Initializes a new instance of the <see cref="SingletonApplicationEnforcer"/> class.
/// </summary>
/// <param name="processArgsFunc">A handler for processing command line args
/// when they are received from another application instance.</param>
/// <param name="applicationId">The application id used
/// for naming the <seealso cref="EventWaitHandle"/>.</param>
public SingletonApplicationEnforcer(Action<IEnumerable<string>> processArgsFunc,
string applicationId = "DisciplesRock")
{
if (processArgsFunc == null)
{
throw new ArgumentNullException("processArgsFunc");
}
this.processArgsFunc = processArgsFunc;
this.applicationId = applicationId;
}
/// <summary>
/// Determines if this application instance is not the singleton instance.
/// If this application is not the singleton, then it should exit.
/// </summary>
/// <returns><c>true</c> if the application should shutdown,
/// otherwise <c>false</c>.</returns>
public bool ShouldApplicationExit()
{
bool createdNew;
string argsWaitHandleName = "ArgsWaitHandle_" + applicationId;
string memoryFileName = "ArgFile_" + 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(() =>
{
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("Unable to retrieve string. " + ex);
continue;
}
string[] argsSplit = args.Split(new string[] { argDelimiter },
StringSplitOptions.RemoveEmptyEntries);
processArgsFunc(argsSplit);
}
}
}
}
catch (Exception ex)
{
Debug.WriteLine("Unable to monitor memory file. " + 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;
}
}
Here is my entire App.xaml.cs, this code also brings the launched program instance to the foreground:
public partial class App : Application
{
private static Mutex _mutex = null;
[DllImport("user32.dll")]
static extern bool SetForegroundWindow(IntPtr hWnd);
[DllImport("user32.dll")]
static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);
protected override void OnStartup(StartupEventArgs e)
{
_mutex = new Mutex(true, "YourMutexName", out var createdNew);
if (!createdNew)
{
using (var currentProcess = Process.GetCurrentProcess())
{
foreach (var process in Process.GetProcessesByName(currentProcess.ProcessName))
{
if (process.Id != currentProcess.Id)
{
ShowWindow(process.MainWindowHandle, 9);
SetForegroundWindow(process.MainWindowHandle);
}
process.Dispose();
}
}
// app is already running! Exiting the application
Shutdown();
}
base.OnStartup(e);
}
protected override void OnExit(ExitEventArgs e)
{
_mutex.Dispose();
base.OnExit(e);
}
}
Based Matt Davis' answer, wrapped into a class for convenience.
public static class SingleAppInstanceChecker
{
/// <summary>
/// Arbitrary unique string
/// </summary>
private static Mutex _mutex = new Mutex(true, "0d12ad74-026f-40c3-bdae-e178ddee8602");
public static bool IsNotRunning()
{
return _mutex.WaitOne(TimeSpan.Zero, true);
}
}
Example usage:
private void Application_Startup(object sender, StartupEventArgs e)
{
if (!SingleAppInstanceChecker.IsNotRunning())
{
MessageBox.Show("Application is already running.");
// Exit application using:
// Environment.Exit(1);
// Application.Current.Shutdown();
// Etc...
return;
}
// Allow startup and continue with normal processing
// ...
}