Access to this page requires authorization. You can try signing in or changing directories . Access to this page requires authorization. You can try changing directories . Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here. Limits the number of threads that can access a resource or pool of resources concurrently. 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. 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. By default, a named semaphore is not restricted to the user that created it. Other users may be able to open and use the semaphore, including interfering with the semaphore by acquiring the semaphore multiple times and not releasing it. To restrict access to specific users, you can use a constructor overload or SemaphoreAcl and pass in a SemaphoreSecurity when creating the named semaphore. Avoid using named semaphores without access restrictions on systems that might have untrusted users running code. Initializes a new instance of the Semaphore class, specifying the initial number of entries and the maximum number of concurrent entries, optionally specifying the name of a system semaphore object, specifying a variable that receives a value indicating whether a new system semaphore was created, and specifying security access control for the system semaphore. Initializes a new instance of the Semaphore class, specifying the initial number of entries and the maximum number of concurrent entries, 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. Initializes a new instance of the Semaphore class, specifying the initial number of entries and the maximum number of concurrent entries, optionally specifying the name of a system semaphore object and options to set user-scope and session-scope access, and specifying a variable that receives a value indicating whether a new system semaphore was created. Initializes a new instance of the Semaphore class, specifying the initial number of entries and the maximum number of concurrent entries, and optionally specifying the name of a system semaphore object and options to set user-scope and session-scope access. Initializes a new instance of the Semaphore class, specifying the initial number of entries and the maximum number of concurrent entries, and optionally specifying the name of a system semaphore object. Initializes a new instance of the Semaphore class, specifying the initial number of entries and the maximum number of concurrent entries. Indicates that a WaitAny(WaitHandle[], Int32, Boolean) operation timed out before any of the wait handles were signaled. This field is constant. Gets or sets the native operating system handle. Gets or sets the native operating system handle. Releases all resources held by the current WaitHandle . Creates an object that contains all the relevant information required to generate a proxy used to communicate with a remote object. Releases all resources used by the current instance of the WaitHandle class. When overridden in a derived class, releases the unmanaged resources used by the WaitHandle , and optionally releases the managed resources. Determines whether the specified object is equal to the current object. Gets the access control security for a named system semaphore. Serves as the default hash function. Retrieves the current lifetime service object that controls the lifetime policy for this instance. Gets the Type of the current instance.

Obtains a lifetime service object to control the lifetime policy for this instance. Creates a shallow copy of the current Object . Creates a shallow copy of the current MarshalByRefObject object. Opens the specified named semaphore, if it already exists. If the options are set to the current user only, the object's access controls are verified for the calling user. Opens the specified named semaphore, if it already exists, with the desired security access. Opens the specified named semaphore, if it already exists. Exits the semaphore and returns the previous count. Exits the semaphore a specified number of times and returns the previous count. Sets the access control security for a named system semaphore. Returns a string that represents the current object. Opens the specified named semaphore, if it already exists, and returns a value that indicates whether the operation succeeded. If the options are set to the current user only, the object's access controls are verified for the calling user. Opens the specified named semaphore, if it already exists, and returns a value that indicates whether the operation succeeded. Opens the specified named semaphore, if it already exists, with the desired security access, and returns a value that indicates whether the operation succeeded. Blocks the current thread until the current WaitHandle receives a signal. Blocks the current thread until the current WaitHandle receives a signal, using a 32-bit signed integer to specify the time interval and specifying whether to exit the synchronization domain before the wait. Blocks the current thread until the current WaitHandle receives a signal, using a 32-bit signed integer to specify the time interval in milliseconds. Blocks the current thread until the current instance receives a signal, using a TimeSpan to specify the time interval and specifying whether to exit the synchronization domain before the wait. Blocks the current thread until the current instance receives a signal, using a TimeSpan to specify the time interval. Explicit Interface Implementations This API supports the product infrastructure and is not intended to be used directly from your code. Releases all resources used by the WaitHandle . Extension Methods Returns the security descriptors for the specified semaphore . Gets the safe handle for a native operating system wait handle. Sets the security descriptors for the specified semaphore. Sets a safe handle for a native operating system wait handle. This type is thread safe. Managed Threading Was this page helpful? Need help with this topic? Want to try using Ask Learn to clarify or guide you through this topic?