What is a Mutex?

The mutex (In fact, the term mutex is short for mutual exclusion) also known as spinlock is the simplest synchronization tool that is used to protect critical regions and thus prevent race conditions. That is a thread must acquire a lock before entering into a critical section (In critical section multi threads share a common variable, updating a table, writing a file and so on), it releases the lock when it leaves critical section.

What is a Race Condition?

A race condition occurs when two or more threads can access shared data and they try to change it at the same time. Because the thread scheduling algorithm can swap between threads at any time, you don't know the order in which the threads will attempt to access the shared data. Therefore, the result of the change in data is dependent on the thread scheduling algorithm, i.e. both threads are "racing" to access/change the data.

Real life example:

When I am having a big heated discussion at work, I use a rubber chicken which I keep in my desk for just such occasions. The person holding the chicken is the only person who is allowed to talk. If you don't hold the chicken you cannot speak. You can only indicate that you want the chicken and wait until you get it before you speak. Once you have finished speaking, you can hand the chicken back to the moderator who will hand it to the next person to speak. This ensures that people do not speak over each other, and also have their own space to talk. Replace Chicken with Mutex and person with thread and you basically have the concept of a mutex. @Xetius

Usage in C#:

This example shows how a local Mutex object is used to synchronize access to a protected resource. Because each calling thread is blocked until it acquires ownership of the mutex, it must call the ReleaseMutex method to release ownership of the thread.

using System; using System.Threading; class Example { // Create a new Mutex. The creating thread does not own the mutex. private static Mutex mut = new Mutex(); private const int numIterations = 1; private const int numThreads = 3; static void Main() { // Create the threads that will use the protected resource. for(int i = 0; i < numThreads; i++) { Thread newThread = new Thread(new ThreadStart(ThreadProc)); newThread.Name = String.Format("Thread{0}", i + 1); newThread.Start(); } // The main thread exits, but the application continues to // run until all foreground threads have exited. } private static void ThreadProc() { for(int i = 0; i < numIterations; i++) { UseResource(); } } // This method represents a resource that must be synchronized // so that only one thread at a time can enter. private static void UseResource() { // Wait until it is safe to enter. Console.WriteLine("{0} is requesting the mutex", Thread.CurrentThread.Name); mut.WaitOne(); Console.WriteLine("{0} has entered the protected area", Thread.CurrentThread.Name); // Place code to access non-reentrant resources here. // Simulate some work. Thread.Sleep(500); Console.WriteLine("{0} is leaving the protected area", Thread.CurrentThread.Name); // Release the Mutex. mut.ReleaseMutex(); Console.WriteLine("{0} has released the mutex", Thread.CurrentThread.Name); } } // The example displays output like the following: // Thread1 is requesting the mutex // Thread2 is requesting the mutex // Thread1 has entered the protected area // Thread3 is requesting the mutex // Thread1 is leaving the protected area // Thread1 has released the mutex // Thread3 has entered the protected area // Thread3 is leaving the protected area // Thread3 has released the mutex // Thread2 has entered the protected area // Thread2 is leaving the protected area // Thread2 has released the mutex

MSDN Reference Mutex