Using SyncLock to Synchronize Threads

To synchronize threads, you can use the SyncLock statement. This statement lets you block access to shared resources by other threads. Here's how you use this statement:

SyncLock Expression
    [sensitive code]
End SyncLock

To use that statement, you pass it an expression to use to lock access, such as an object. (The type of this expression must be a reference type, such as a class, a module, an array, or an interface.) For example, if you lock an object, no other thread can access the object until it's unlocked.

For example, we saw how to use SyncLock in the SynchronizeThreads example in the In Depth section of this chapter. In that example, we used two threads to increment a value. The two threads paused for a millisecond each before updating the incremented value, which meant that the threads interfered with each other, as discussed in the In Depth section of this chapter. By using SyncLock, we were able to lock the object being updated, which stopped the threads from interfering with each other:

    Private Sub Button1_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button1.Click
        Thread1.Start()
        Thread2.Start()
        ⋮
    End Sub

    Private Sub Counter1()
        Dim LoopIndex As Integer

        For LoopIndex = 1 To 100
            SyncLock c
                Dim temp = c.Total
                Thread1.Sleep(1)
                c.Total = temp + 1
            End SyncLock
        Next LoopIndex
    End Sub

    Private Sub Counter2()
        Dim LoopIndex As Integer

        For LoopIndex = 1 To 100
            SyncLock c
                Dim temp = c.Total
                Thread2.Sleep(1)
                c.Total = temp + 1
            End SyncLock
        Next LoopIndex
    End Sub

For more on this example, see the In Depth section of this chapter.