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AsyncCompletedEventArgs.UserState Property

Gets the unique identifier for the asynchronous task.

Namespace:  System.ComponentModel
Assembly:  System (in System.dll)

'Declaration
Public ReadOnly Property UserState As Object

Property Value

Type: System.Object
An object reference that uniquely identifies the asynchronous task; otherwise, Nothing if no value has been set.

If a class supports multiple asynchronous methods, or multiple invocations of a single method, you can determine which task raised the MethodNameCompleted event by checking the value of the UserState property. Your code will have to track these tokens, known as task IDs, as their corresponding asynchronous tasks start and complete.

The value of this property is set during the original call to the asynchronous method that started the task.

The following code example demonstrates how to use UserState to track the lifetime of asynchronous operations. The local variable used to store the values tracked by UserState is named taskID in this example. This code example is part of a larger example provided for the System.ComponentModel.AsyncOperationManager class.


    ' Start GetPersons as an asynchronous process. 
    Public Sub GetPersonsAsync(ByVal itemsCount As Integer, _
                               ByVal name As String, _
                               ByVal taskId As Object)

        Dim asyncOp As AsyncOperation

        ' First make sure this is not an attempt to start the 
        ' same thread twice. Multiple threads will access the 
        ' taskID Dictionary, so it must be locked to serialize 
        ' access. 
        SyncLock Me
            If taskIDs.ContainsKey(taskId) Then
                Throw New ArgumentException _
                    ("Task ID already exists", "taskID")
            End If

            ' Create a new AsyncOperation object 
            ' and put it in the taskID Dictionary. 
            asyncOp = AsyncOperationManager.CreateOperation(taskId)
            taskIDs(taskId) = asyncOp
        End SyncLock

        ' Start the asynchronous process. 
        Dim myThread As New Thread(AddressOf StartThread)
        myThread.Start(New ThreadStartParms With _
             {.ItemsCount = itemsCount, .Name = name, .AsyncOp = asyncOp})

    End Sub

    Private Sub StartThread(ByVal parms As ThreadStartParms)
        GetPersons(parms.ItemsCount, parms.Name, parms.AsyncOp)
    End Sub




' The worker process for data retrieval. Simulates a 
' time-consuming operation by using Thread.Sleep. 
Public Sub GetPersons(ByVal itemsCount As Integer, _
                      ByVal Name As String, _
                      ByVal asyncOperation As AsyncOperation)

    Dim canceled As Boolean = False
    Dim exception As Exception = Nothing

    Dim personList As New List(Of Person)()

    Dim i As Integer = 1
    While i <= itemsCount OrElse itemsCount = 0
        Dim currentName As String = Name + CStr(i)
        personList.Add(New Person() With { _
                .Name = currentName, _
                .Age = i, _
                .Birthday = DateTime.Today.AddYears(-i), _
                .Available = (i Mod 2 = 0) _
            })

        ' Delay 1 second for each person. 
        Thread.Sleep(1000)

        ' Report progress by using AsyncOperation to raise 
        ' the ProgressChanged event. Pass in itemsCount of 
        ' zero to see effect of catching an exception. 
        Dim percentComplete As Integer
        Try
            percentComplete = Convert.ToInt32(i * 100 / itemsCount)
        Catch ex As Exception
            exception = ex
            Exit While
        End Try

        Dim progressChangedEventArgs As _
            New GetPersonsProgressChangedEventArgs _
                (currentName, _
                 percentComplete, _
                 asyncOperation.UserSuppliedState)

        asyncOperation.Post _
            (AsyncOpProgressReportHandler, progressChangedEventArgs)

        If GetPersonsCheckForCancellation _
            (asyncOperation.UserSuppliedState) Then
            canceled = True
            Exit While
        End If

        i += 1
    End While

    ' Report completion by raising the Completed event. 
    Dim completedEventArgs As _
        New GetPersonsCompletedEventArgs _
            (personList, _
             exception, _
             canceled, _
             asyncOperation.UserSuppliedState)

    asyncOperation.PostOperationCompleted _
        (AsyncOpCompletedHandler, completedEventArgs)
End Sub


Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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