C# Mutex
last modified July 5, 2023
In this article we show how to use the Mutex primitive to ensure thread safety in C#.
A thread is a unique execution path of a program. The
Thread class represents a thread in C#. It creates and controls a
thread, sets its priority, and gets its status.
A deadlock occurs when a set of threads share the same resource and are blocked by each other by attempting to access a resource already locked by another thread.
Mutex
In order to ensure thread safety, we need a synchronization mechanizm to grant access to a shared resource. A Mutex is a synchronization primitive that grants access to a shared resource to only one thread. Other threads wanting to access the resource are blocked until the one holding the mutex releases it.
A mutex is similar to a lock, but it can work across multiple
processes; that is, it can be computer-wide as well as application-wide. The
Mutex class is located in the System.Threading
namespace.
The section of code that accesses the shared resource is called a critical or
protected section. The resource can be a data structure or a device. A thread
acquires a mutex with WaitOne and releases it with
ReleaseMutex.
C# Mutex simple example
The following is a simple example that uses Mutex.
class Program
{
private static Mutex mutex = new Mutex();
private const int nThreads = 4;
private static void Worker()
{
UseResource();
}
private static void UseResource()
{
mutex.WaitOne();
string? name = Thread.CurrentThread.Name;
Console.WriteLine($"{name} has entered protected section");
Thread.Sleep(800);
Console.WriteLine($"{name} has left protected section");
mutex.ReleaseMutex();
}
static void Main(string[] args)
{
for (int i = 0; i < nThreads; i++)
{
var t = new Thread(new ThreadStart(Worker));
t.Name = $"Thread {i + 1}";
t.Start();
}
Console.Read();
}
}
In the program, we create four threads that access a shared resource. Each of the threads has exclusive access to the resource.
private static Mutex mutex = new Mutex();
The Mutex is a static field.
private static void Worker()
{
UseResource();
}
Each of the threads calls this worker.
private static void UseResource()
{
mutex.WaitOne();
string? name = Thread.CurrentThread.Name;
Console.WriteLine($"{name} has entered protected section");
Thread.Sleep(800);
Console.WriteLine($"{name} has left protected section");
mutex.ReleaseMutex();
}
This is the critical section of the code that is protected by mutex using
WaitOne and ReleaseMutex methods.
for (int i = 0; i < nThreads; i++)
{
var t = new Thread(new ThreadStart(Worker));
t.Name = $"Thread {i + 1}";
t.Start();
}
In a for loop, we generate four threads.
Console.Read();
To ensure that all threads have time to finish, we call the
Console.Read method. Just press enter after the threads finish
running.
$ dotnet run Thread 1 has entered protected section Thread 1 has left protected section Thread 2 has entered protected section Thread 2 has left protected section Thread 3 has entered protected section Thread 3 has left protected section Thread 4 has entered protected section Thread 4 has left protected section
C# Mutex single instance
In the following example, we create a single instance application. Only one application is allowed to run on computer.
To create a single-instance application, we use a named mutext. In order for the
mutex to apply to all terminal sessions, the name of the mutex is prefixed with
Global\.
public class Program
{
public static void Main()
{
using var mutex = new Mutex(false, @"Global\MySingletonApp");
bool IsRunning = !mutex.WaitOne(TimeSpan.Zero);
if (IsRunning)
{
Console.WriteLine("application is already running");
return;
}
Console.WriteLine("application started");
Console.ReadKey();
mutex.ReleaseMutex();
}
}
In the program, we create a global named mutex. This prevents us to create more than one instances of the application on a computer. Another instance of the program can be run only after the running application finishes.
Try to run the application in two different terminals.
Source
Mutex class - language reference
In this article we have used Mutex primitive to synchronize access to a protected resource in C#.
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