C# 4 and .NET 4

524 ❘ ChaPTer 20 threAds, tAsks, And synchrOnizAtiOn
}
}
}
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
Application.Run(new Form1());
semaphore
A semaphore is very similar to a mutex, but, in contrast, the semaphore can be used by multiple threads
at once. A semaphore is a counting mutex, meaning that with a semaphore you can define the number of
threads that are allowed to access the resource guarded by the semaphore simultaneously. This can be used
if you have several of the resources available and can allow only a specific number of threads access to the
resource. For example, say that you want to access physical I/O ports on the system and there are three
ports available. So, three threads can access the I/O ports simultaneously, but the fourth thread needs to
wait until the resource is released by one of the other threads.
.NET 4 gives you two classes with semaphore functionality: Semaphore and SemaphoreSlim. Semaphore
can be named, use system-wide resources, and allow synchronization between different processes.
SemaphoreSlim is lightweight version that is optimized for shorter wait times.
In the sample application, in the Main() method six threads are created and one semaphore with a count
of 4. In the constructor of the Semaphore class, you can define the count for the number of locks that can be
acquired with the semaphore (the second parameter) and the number of locks that are free initially (the first
parameter). If the first parameter has a lower value than the second parameter, the difference between the
values defines the already allocated semaphore count. As with the mutex, you can also assign a name to
the semaphore to share it among different processes. Here, no name is defined with the semaphore, so it is
used only within this process. After the SemaphoreSlim object is created, six threads are started, and they
all get the same semaphore.
using System;
using System.Threading;
using System.Diagnostics;
namespace Wrox.ProCSharp.Threading
{
class Program
{
static void Main()
{
int threadCount = 6;
int semaphoreCount = 4;
var semaphore = new SemaphoreSlim(semaphoreCount, semaphoreCount);
var threads = new Thread[threadCount];
for (int i = 0; i < threadCount; i++)
{
threads[i] = new Thread(ThreadMain);
threads[i].Start(semaphore);
}
}
for (int i = 0; i < threadCount; i++)
{
threads[i].Join();
}
Console.WriteLine("All threads finished");
code snippet Semaphore/Program.cs
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