C# 4 and .NET 4

synchronization ❘ 523
The classes Mutex, EventWaitHandle, and Semaphore are derived from the base class WaitHandle, so you
can use all of these with waits.
mutex
Mutex (mutual exclusion) is one of the classes of the .NET Framework that offers synchronization across
multiple processes. It is very similar to the Monitor class in that there is just one owner. Just one thread can
get a lock on the mutex and access the synchronized code regions that are secured by the mutex.
With the constructor of the Mutex class, you can define if the mutex should initially be owned by the calling
thread, define a name for the mutex, and get the information if the mutex already exists. In the sample
code, the third parameter is defined as an out parameter to receive a Boolean value if the mutex was newly
created. If the value returned is false, the mutex was already defined. The mutex might be defined in
a different process, because a mutex with a name is known to the operating system and is shared among
different processes. If there is no name assigned to the mutex, the mutex is unnamed and not shared
among different processes.
bool createdNew;
Mutex mutex = new Mutex(false, "ProCSharpMutex", out createdNew);
To open an existing mutex, you can also use the method Mutex.OpenExisting(), which doesn’t require the
same .NET privileges as creating the mutex with the constructor.
Because the Mutex class derives from the base class WaitHandle, you can do a WaitOne() to acquire
the mutex lock and be the owner of the mutex during that time. The mutex is released by invoking the
ReleaseMutex() method.
if (mutex.WaitOne())
{
try
{
// synchronized region
}
finally
{
mutex.ReleaseMutex();
}
}
else
{
// some problem happened while waiting
}
Because a named mutex is known system-wide, you can use it to keep an application from being started
twice. In the following Windows Forms application, the constructor of the Mutex object is invoked.
Then, it is verified if the mutex with the name SingletonWinAppMutex exists already. If it does, the
application exits.
static class Program
{
[STAThread]
static void Main()
{
bool createdNew;
Mutex mutex = new Mutex(false, "SingletonWinAppMutex",
out createdNew);
if (!createdNew)
{
MessageBox.Show("You can only start one instance " +
"of the application");
Application.Exit();
return;
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