ArrayList.Synchronized Method (ArrayList)


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Returns an ArrayList wrapper that is synchronized (thread safe).

Namespace:   System.Collections
Assembly:  mscorlib (in mscorlib.dll)

<HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization := True)>
Public Shared Function Synchronized (
	list As ArrayList
) As ArrayList


Type: System.Collections.ArrayList

The ArrayList to synchronize.

Return Value

Type: System.Collections.ArrayList

An ArrayList wrapper that is synchronized (thread safe).

Exception Condition

list is null.

To guarantee the thread safety of the ArrayList, all operations must be done through this wrapper.

Enumerating through a collection is intrinsically not a thread-safe procedure. Even when a collection is synchronized, other threads can still modify the collection, which causes the enumerator to throw an exception. To guarantee thread safety during enumeration, you can either lock the collection during the entire enumeration or catch the exceptions resulting from changes made by other threads.

The following code example shows how to lock the collection using the SyncRoot during the entire enumeration.

Dim myCollection As New ArrayList()

SyncLock myCollection.SyncRoot
    For Each item As Object In myCollection
        ' Insert your code here.
    Next item
End SyncLock

This method is an O(1) operation.

The following code example shows how to synchronize an ArrayList, determine if an ArrayList is synchronized and use a synchronized ArrayList.

Imports System
Imports System.Collections

Public Class SamplesArrayList

    Public Shared Sub Main()

        ' Creates and initializes a new ArrayList.
        Dim myAL As New ArrayList()

        ' Creates a synchronized wrapper around the ArrayList.
        Dim mySyncdAL As ArrayList = ArrayList.Synchronized(myAL)

        ' Displays the sychronization status of both ArrayLists.
        Dim str As String
        If myAL.IsSynchronized Then
            str = "synchronized"
            str = "not synchronized"
        End If
        Console.WriteLine("myAL is {0}.", str)
        If mySyncdAL.IsSynchronized Then
            str = "synchronized"
            str = "not synchronized"
        End If
        Console.WriteLine("mySyncdAL is {0}.", str)
    End Sub
End Class

' This code produces the following output.
' myAL is not synchronized.
' mySyncdAL is synchronized. 

Universal Windows Platform
Available since 10
.NET Framework
Available since 1.1
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