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Semaphore Constructor (Int32, Int32, String, Boolean)

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.

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

'Declaration
Public Sub New ( _
	initialCount As Integer, _
	maximumCount As Integer, _
	name As String, _
	<OutAttribute> ByRef createdNew As Boolean _
)

Parameters

initialCount
Type: System.Int32

The initial number of requests for the semaphore that can be satisfied concurrently.

maximumCount
Type: System.Int32

The maximum number of requests for the semaphore that can be satisfied concurrently.

name
Type: System.String

The name of a named system semaphore object.

createdNew
Type: System.Boolean

When this method returns, contains true if a local semaphore was created (that is, if name is Nothing or an empty string) or if the specified named system semaphore was created; false if the specified named system semaphore already existed. This parameter is passed uninitialized.

ExceptionCondition
ArgumentException

initialCount is greater than maximumCount.

-or-

name is longer than 260 characters.

ArgumentOutOfRangeException

maximumCount is less than 1.

-or-

initialCount is less than 0.

IOException

A Win32 error occurred.

UnauthorizedAccessException

The named semaphore exists and has access control security, and the user does not have SemaphoreRights.FullControl.

WaitHandleCannotBeOpenedException

The named semaphore cannot be created, perhaps because a wait handle of a different type has the same name.

This constructor initializes a Semaphore object that represents a named system semaphore. You can create multiple Semaphore objects that represent the same named system semaphore.

If the named system semaphore does not exist, it is created with the initial count and maximum count specified by initialCount and maximumCount. If the named system semaphore already exists, initialCount and maximumCount are not used, although invalid values still cause exceptions. Use createdNew to determine whether the system semaphore was created.

If initialCount is less than maximumCount, and createdNew is true, the effect is the same as if the current thread had called WaitOne (maximumCount minus initialCount) times.

If you specify Nothing or an empty string for name, a local semaphore is created, as if you had called the Semaphore(Int32, Int32) constructor overload. In this case, createdNew is always true.

Because named semaphores are visible throughout the operating system, they can be used to coordinate resource use across process boundaries.

The following code example demonstrates the cross-process behavior of a named semaphore. The example creates a named semaphore with a maximum count of five and an initial count of two. That is, it reserves three entries for the thread that calls the constructor. If createNew is false, the program makes three calls to the WaitOne method. Thus, if you run the compiled example from two command windows, the second copy will block on the third call to WaitOne. Release one or more entries in the first copy of the program to unblock the second.

Imports System
Imports System.Threading

Public Class Example

    <MTAThread> _
    Public Shared Sub Main()
        ' The value of this variable is set by the semaphore 
        ' constructor. It is True if the named system semaphore was 
        ' created, and False if the named semaphore already existed. 
        
        Dim semaphoreWasCreated As Boolean 

        ' Create a Semaphore object that represents the named  
        ' system semaphore "SemaphoreExample". The semaphore has a 
        ' maximum count of five, and an initial count of two. The 
        ' Boolean value that indicates creation of the underlying  
        ' system object is placed in semaphoreWasCreated. 
        
        Dim sem As New Semaphore(2, 5, "SemaphoreExample", _
            semaphoreWasCreated)

        If semaphoreWasCreated Then 
            ' If the named system semaphore was created, its count is 
            ' set to the initial count requested in the constructor. 
            ' In effect, the current thread has entered the semaphore 
            ' three times. 
            ' 
            Console.WriteLine("Entered the semaphore three times.")
        Else 
            ' If the named system semaphore was not created,   
            ' attempt to enter it three times. If another copy of 
            ' this program is already running, only the first two 
            ' requests can be satisfied. The third blocks. 
            '
            sem.WaitOne()
            Console.WriteLine("Entered the semaphore once.")
            sem.WaitOne()
            Console.WriteLine("Entered the semaphore twice.")
            sem.WaitOne()
            Console.WriteLine("Entered the semaphore three times.")
        End If 

        ' The thread executing this program has entered the  
        ' semaphore three times. If a second copy of the program 
        ' is run, it will block until this program releases the  
        ' semaphore at least once. 
        '
        Console.WriteLine("Enter the number of times to call Release.")
        Dim n As Integer 
        If Integer.TryParse(Console.ReadLine(), n) Then
            sem.Release(n)
        End If 

        Dim remaining As Integer = 3 - n
        If (remaining) > 0 Then
            Console.WriteLine("Press Enter to release the remaining " _
                & "count ({0}) and exit the program.", remaining)
            Console.ReadLine()
            sem.Release(remaining)
        End If 

    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

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