Export (0) Print
Expand All

Semaphore.Release Method

Exits the semaphore and returns the previous count.

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

'Declaration
Public Function Release As Integer

Return Value

Type: System.Int32
The count on the semaphore before the Release method was called.

ExceptionCondition
SemaphoreFullException

The semaphore count is already at the maximum value.

IOException

A Win32 error occurred with a named semaphore.

UnauthorizedAccessException

The current semaphore represents a named system semaphore, but the user does not have SemaphoreRights.Modify.

-or-

The current semaphore represents a named system semaphore, but it was not opened with SemaphoreRights.Modify.

Threads typically use the WaitOne method to enter the semaphore, and they typically use this method overload to exit.

If a SemaphoreFullException is thrown by the Release method, it does not necessarily indicate a problem with the calling thread. A programming error in another thread might have caused that thread to exit the semaphore more times than it entered.

If the current Semaphore object represents a named system semaphore, the user must have SemaphoreRights.Modify rights and the semaphore must have been opened with SemaphoreRights.Modify rights.

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.

Imports System
Imports System.Threading

Public Class Example

    ' A semaphore that simulates a limited resource pool. 
    
    Private Shared _pool As Semaphore

    ' A padding interval to make the output more orderly. 
    Private Shared _padding As Integer

    <MTAThread> _
    Public Shared Sub Main()
        ' Create a semaphore that can satisfy up to three 
        ' concurrent requests. Use an initial count of zero, 
        ' so that the entire semaphore count is initially 
        ' owned by the main program thread. 
        '
        _pool = New Semaphore(0, 3)

        ' Create and start five numbered threads.  
        
        For i As Integer = 1 To 5
            Dim t As New Thread(New ParameterizedThreadStart(AddressOf Worker))
            'Dim t As New Thread(AddressOf Worker) 

            ' Start the thread, passing the number. 
            '
            t.Start(i)
        Next i

        ' Wait for half a second, to allow all the 
        ' threads to start and to block on the semaphore. 
        '
        Thread.Sleep(500)

        ' The main thread starts out holding the entire 
        ' semaphore count. Calling Release(3) brings the  
        ' semaphore count back to its maximum value, and 
        ' allows the waiting threads to enter the semaphore, 
        ' up to three at a time. 
        '
        Console.WriteLine("Main thread calls Release(3).")
        _pool.Release(3)

        Console.WriteLine("Main thread exits.")
    End Sub 

    Private Shared Sub Worker(ByVal num As Object)
        ' Each worker thread begins by requesting the 
        ' semaphore.
        Console.WriteLine("Thread {0} begins " _
            & "and waits for the semaphore.", num)
        _pool.WaitOne()

        ' A padding interval to make the output more orderly. 
        Dim padding As Integer = Interlocked.Add(_padding, 100)

        Console.WriteLine("Thread {0} enters the semaphore.", num)

        ' The thread's "work" consists of sleeping for  
        ' about a second. Each thread "works" a little  
        ' longer, just to make the output more orderly. 
        '
        Thread.Sleep(1000 + padding)

        Console.WriteLine("Thread {0} releases the semaphore.", num)
        Console.WriteLine("Thread {0} previous semaphore count: {1}", _
            num, _
            _pool.Release())
    End Sub 
End Class

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
Show:
© 2015 Microsoft