AsyncCompletedEventArgs.UserState Property
Microsoft Silverlight will reach end of support after October 2021. Learn more.
Gets the unique identifier for the asynchronous task.
Namespace: System.ComponentModel
Assembly: System (in System.dll)
Syntax
'Declaration
Public ReadOnly Property UserState As Object
public Object UserState { get; }
Property Value
Type: System.Object
An object reference that uniquely identifies the asynchronous task; otherwise, nulla null reference (Nothing in Visual Basic) if no value has been set.
Remarks
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.
Examples
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
// Start GetPersons as an asynchronous process.
public void GetPersonsAsync
(int itemsCount, string name, object taskId)
{
AsyncOperation asyncOp;
// 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.
lock (this)
{
if (taskIDs.ContainsKey(taskId))
{
throw new ArgumentException
("Task ID already exists", "taskID");
}
// Create a new AsyncOperation object
// and put it in the taskID Dictionary.
asyncOp =
AsyncOperationManager.CreateOperation(taskId);
taskIDs[taskId] = asyncOp;
}
// Start the asynchronous process.
// The following line is the least amount of code to do
// this. It uses a lambda expression. Following it is
// commented code showing an alternative method.
new Thread(() =>
{ GetPersons(itemsCount, name, asyncOp); }).Start();
// The alternative is to include two lines here and
// define separately the method that they reference and
// a class you can use to pass parameters to Thread.Start:
// Thread myThread =
// new Thread(new ThreadStart(StartThread));
// myThread.Start(new ThreadStartParms
// {.ItemsCount=itemsCount,
// .Name=name,
// .AsyncOp=asyncOp });
// void StartThread()
// {
// GetPersons(itemsCount, name, asyncOp);
// }
// public class ThreadStartParms)
// {
// public int ItemsCount { get; set; }
// public string Name { get; set; }
// public AsyncOp AsyncOperation { get; set; }
// }
}
' 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
// The worker process for data retrieval. Simulates a
// time-consuming operation by using Thread.Sleep.
public void GetPersons
(int itemsCount, string Name, AsyncOperation asyncOperation)
{
bool canceled = false;
Exception exception = null;
List<Person> personList = new List<Person>();
for (int i = 1; i <= itemsCount || itemsCount == 0; i++)
{
string currentName = Name + i;
personList.Add(new Person()
{
Name = currentName,
Age = i,
Birthday = DateTime.Today.AddYears(-i),
Available = (i % 2 == 0)
});
// Delay 1 second for each person.
Thread.Sleep(1000);
// Report progress by using AsyncOperation to raise
// the ProgressChanged event.
int percentComplete = 0;
try
{
percentComplete =
Convert.ToInt32(i * 100 / itemsCount);
}
catch (Exception ex)
{
exception = ex;
break;
}
GetPersonsProgressChangedEventArgs progressChangedEventArgs =
new GetPersonsProgressChangedEventArgs(
currentName,
percentComplete,
asyncOperation.UserSuppliedState);
asyncOperation.Post(AsyncOpProgressReportHandler,
progressChangedEventArgs);
if (GetPersonsCheckForCancellation
(asyncOperation.UserSuppliedState))
{
canceled = true;
break;
}
}
// Report completion by raising the Completed event.
GetPersonsCompletedEventArgs completedEventArgs =
new GetPersonsCompletedEventArgs(
personList,
exception,
canceled,
asyncOperation.UserSuppliedState);
asyncOperation.PostOperationCompleted
(AsyncOpCompletedHandler, completedEventArgs);
}
Version Information
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
Platforms
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.