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Barrier (.NET Framework)

A barrier is a user-defined synchronization primitive that enables multiple threads (known as participants) to work concurrently on an algorithm in phases. Each participant executes until it reaches the barrier point in the code. The barrier represents the end of one phase of work. When a participant reaches the barrier, it blocks until all participants have reached the same barrier. After all participants have reached the barrier, you can optionally invoke a post-phase action. This post-phase action can be used to perform actions by a single thread while all other threads are still blocked. After the action has been executed, the participants are all unblocked.

The following code snippet shows a basic barrier pattern.



' Create the Barrier object, and supply a post-phase delegate 
' to be invoked at the end of each phase.
Dim barrier = New Barrier(2, Sub(bar)
                                 ' Examine results from all threads, determine 
                                 ' whether to continue, create inputs for next phase, etc. 
                                 If (someCondition) Then
                                     success = True
                                 End If
                             End Sub)



' Define the work that each thread will perform. (Threads do not
' have to all execute the same method.)
Sub CrunchNumbers(ByVal partitionNum As Integer)

    ' Up to System.Int64.MaxValue phases are supported. We assume
    ' in this code that the problem will be solved before that.
    While (success = False)

        ' Begin phase:
        ' Process data here on each thread, and optionally
        ' store results, for example:
        results(partitionNum) = ProcessData(myData(partitionNum))

        ' End phase:
        ' After all threads arrive,post-phase delegate
        ' is invoked, then threads are unblocked. Overloads
        ' accept a timeout value and/or CancellationToken.
        barrier.SignalAndWait()
    End While
End Sub

' Perform n tasks to run in in parallel. For simplicity
' all threads execute the same method in this example.
Shared Sub Main()

    Dim app = New BarrierDemo()
    Dim t1 = New Thread(Sub() app.CrunchNumbers(0))
    Dim t2 = New Thread(Sub() app.CrunchNumbers(1))
    t1.Start()
    t2.Start()
End Sub


For a complete example, see How to: Synchronize Concurrent Operations with a Barrier.

When you create a Barrier, specify the number of participants. You can also add or remove participants dynamically at any time. For example, if one participant solves its part of the problem, you can store the result, stop execution on that thread, and call RemoveParticipant to decrement the number of participants in the barrier. When you add a participant by calling AddParticipant, the return value specifies the current phase number, which may be useful in order to initialize the work of the new participant.

Deadlocks can occur if one participant fails to reach the barrier. To avoid these deadlocks, use the overloads of the SignalAndWait method to specify a time-out period and a cancellation token. These overloads return a Boolean value that every participant can check before it continues to the next phase.

If the post-phase delegate throws an exception, it is wrapped in a BarrierPostPhaseException object which is then propagated to all participants.

Barriers are especially useful when the threads are performing multiple phases in loops. If your code requires only one or two phases of work, consider whether to use System.Threading.Tasks.Task objects with any kind of implicit join, including:

For more information, see How to: Chain Multiple Tasks with Continuations.

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