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ArrayList.Synchronized Method (ArrayList)

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).


list is Nothing.

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 HostProtectionAttribute attribute applied to this type or member has the following Resources property value: Synchronization. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

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. 

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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