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Semaphore Class

July 28, 2014

Limits the number of threads that can access a resource or pool of resources concurrently.

Namespace:  System.Threading
Assembly:  System (in System.dll)

'Declaration
Public NotInheritable Class Semaphore _
	Inherits WaitHandle

The Semaphore type exposes the following members.

  NameDescription
Public methodSemaphore(Int32, Int32)Initializes a new instance of the Semaphore class, specifying the maximum number of concurrent entries and optionally reserving some entries.
Public methodSemaphore(Int32, Int32, String)Security Critical. Initializes a new instance of the Semaphore class, specifying the maximum number of concurrent entries, optionally reserving some entries for the calling thread, and optionally specifying the name of a system semaphore object.
Public methodSemaphore(Int32, Int32, String, Boolean)Security Critical. Initializes a new instance of the Semaphore class, specifying the maximum number of concurrent entries, optionally reserving some entries for the calling thread, optionally specifying the name of a system semaphore object, and specifying a variable that receives a value indicating whether a new system semaphore was created.
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  NameDescription
Public propertySafeWaitHandleGets or sets the native operating-system handle. (Inherited from WaitHandle.)
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  NameDescription
Public methodCloseWhen overridden in a derived class, releases all resources held by the current WaitHandle. (Inherited from WaitHandle.)
Public methodDisposeReleases all resources used by the current instance of the WaitHandle class. (Inherited from WaitHandle.)
Protected methodDispose(Boolean)When overridden in a derived class, releases the unmanaged resources used by the WaitHandle, and optionally releases the managed resources. (Inherited from WaitHandle.)
Public methodEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodStatic memberOpenExistingSecurity Critical. Opens the specified named semaphore, if it already exists.
Public methodReleaseExits the semaphore and returns the previous count.
Public methodRelease(Int32)Exits the semaphore a specified number of times and returns the previous count.
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodStatic memberTryOpenExistingSecurity Critical. Opens the specified named semaphore, if it already exists, and returns a value that indicates whether the operation succeeded.
Public methodWaitOneBlocks the current thread until the current WaitHandle receives a signal. (Inherited from WaitHandle.)
Public methodWaitOne(Int32)Blocks the current thread until the current WaitHandle receives a signal, using 32-bit signed integer to specify the time interval. (Inherited from WaitHandle.)
Public methodWaitOne(TimeSpan)Blocks the current thread until the current instance receives a signal, using a TimeSpan to specify the time interval. (Inherited from WaitHandle.)
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Use the Semaphore class to control access to a pool of resources. Threads enter the semaphore by calling the WaitOne method, which is inherited from the WaitHandle class, and release the semaphore by calling the Release method.

The count on a semaphore is decremented each time a thread enters the semaphore, and incremented when a thread releases the semaphore. When the count is zero, subsequent requests block until other threads release the semaphore. When all threads have released the semaphore, the count is at the maximum value specified when the semaphore was created.

There is no guaranteed order, such as FIFO or LIFO, in which blocked threads enter the semaphore.

A thread can enter the semaphore multiple times, by calling the WaitOne method repeatedly. To release some or all of these entries, the thread can call the parameterless Release method overload multiple times, or it can call the Release(Int32) method overload that specifies the number of entries to be released.

The Semaphore class does not enforce thread identity on calls to WaitOne or Release. It is the programmer's responsibility to ensure that threads do not release the semaphore too many times. For example, suppose a semaphore has a maximum count of two, and that thread A and thread B both enter the semaphore. If a programming error in thread B causes it to call Release twice, both calls succeed. The count on the semaphore is full, and when thread A eventually calls Release, a SemaphoreFullException is thrown.

Semaphores are of two types: local semaphores and named system semaphores. If you create a Semaphore object using a constructor that accepts a name, it is associated with an operating-system semaphore of that name. Named system semaphores are visible throughout the operating system, and can be used to synchronize the activities of processes. You can create multiple Semaphore objects that represent the same named system semaphore, and you can use the OpenExisting method to open an existing named system semaphore.

A local semaphore exists only within your process. It can be used by any thread in your process that has a reference to the local Semaphore object. Each Semaphore object is a separate local semaphore.

The following code example creates a semaphore with a maximum count of three and an initial count of zero. The example starts five threads, which block waiting for the semaphore. The main thread uses the Release(Int32) method overload to increase the semaphore count to its maximum, allowing three threads to enter the semaphore. Each thread uses the Thread.Sleep method to wait for one second, to simulate work, and then calls the Release method overload to release the semaphore. Each time the semaphore is released, the previous semaphore count is displayed. Console messages track semaphore use. The simulated work interval is increased slightly for each thread, to make the output easier to read.

Windows Phone OS

Supported in: 8.1, 8.0

This type is thread safe.

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