`SyncLock`

Threads in VB.NET programs run all at the same time. This causes problems when they access sensitive parts of code—it creates conflicts.

With SyncLock, we restrict all except one thread from running, based on a lock object. This keyword is the same as the "lock" keyword in the C# language.

In Main, we iterate through ten numbers and create a new Thread each time. When each thread starts, control flow moves to the Sub A. In Sub A, we encounter a SyncLock.

Info The SyncLock statement uses an argument. Each call to Sub A will SyncLock on the same Object reference.

Result The statements within the SyncLock will always be sequentially run. This can help with correct threading.

Imports System.Threading

Module Module1
    Dim field As Object = New Object

    Sub A()
        ' Lock on the field.
        SyncLock field
            Thread.Sleep(100)
            Console.WriteLine(Environment.TickCount)
        End SyncLock
    End Sub

    Sub Main()
        ' Create ten new threads.
        ' ... They finish one after the other.
        For i As Integer = 1 To 10
            ' Call A() in new thread.
            Dim ts As ThreadStart = New ThreadStart(AddressOf A)
            Dim t As Thread = New Thread(ts)
            t.Start()
        Next
    End Sub
End Module1320870012
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Output notes

Environment.TickCount evaluates to a number about 100 milliseconds higher each time. This means that the statements within the SyncLock are not simultaneously run.

Detail The statements are sequentially run. The Thread.Sleep() calls evaluate one after another.

Discussion

It is important to use SyncLock when accessing fields from multiple threads. If you do not, it is possible for fields to end up in an invalid state.

And Imagine if many threads read a field and then change it based on some condition.

So One thread could read the field and then, before it changes it, another thread could read it. Then the first thread changes the field.

Thus By using SyncLock, the loads and stores to the field would be kept always in a valid state.

SyncLock allows one thread to execute the code at once. The runtime uses the identity of the object argument of SyncLock. That identity is used to restrict access.