Synchronized Method

Hashtable.Synchronized Method

Returns a synchronized (thread-safe) wrapper for the Hashtable.

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

<HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization := True)> _
Public Shared Function Synchronized ( _
	table As Hashtable _
) As Hashtable


Type: System.Collections.Hashtable
The Hashtable to synchronize.

Return Value

Type: System.Collections.Hashtable
A synchronized (thread-safe) wrapper for the Hashtable.


table is Nothing.

The Synchronized method is thread safe for multiple readers and writers. Furthermore, the synchronized wrapper ensures that there is only one writer writing at a time.

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 Hashtable()
SyncLock myCollection.SyncRoot
    For Each item In myCollection
        ' Insert your code here.
End SyncLock

This method is an O(1) operation.


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 example shows how to synchronize a Hashtable, determine if a Hashtable is synchronized, and use a synchronized Hashtable.

Imports System
Imports System.Collections

Public Class SamplesHashtable    

    Public Shared Sub Main()

        ' Creates and initializes a new Hashtable.
        Dim myHT As New Hashtable()
        myHT.Add(0, "zero")
        myHT.Add(1, "one")
        myHT.Add(2, "two")
        myHT.Add(3, "three")
        myHT.Add(4, "four")

        ' Creates a synchronized wrapper around the Hashtable.
        Dim mySyncdHT As Hashtable = Hashtable.Synchronized(myHT)

        ' Displays the sychronization status of both Hashtables.
        Dim msg As String
        If myHT.IsSynchronized Then
            msg = "synchronized"
            msg = "not synchronized"
        End If
        Console.WriteLine("myHT is {0}.", msg)
        If mySyncdHT.IsSynchronized Then
            msg = "synchronized"
            msg = "not synchronized"
        End If        
        Console.WriteLine("mySyncdHT is {0}.", msg)
    End Sub
End Class

' This code produces the following output.
' myHT is not synchronized.
' mySyncdHT is synchronized. 

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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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