AsyncCompletedEventArgs Class (System.ComponentModel)

If you are using a class that implements the event-based asynchronous pattern that is described in the Event-based Asynchronous Pattern Overview topic of the .NET Framework documentation, the class will provide a MethodNameCompleted event. If you add an instance of the System.ComponentModel.AsyncCompletedEventHandler delegate to the event, you will receive information about the outcome of asynchronous operations in the AsyncCompletedEventArgs parameter of the corresponding event-handler method.

The client application's event-handler delegate can check the Cancelled property to determine if the asynchronous task was canceled.

The client application's event-handler delegate can check the Error property to determine if an exception occurred during execution of the asynchronous task.

If the class supports multiple asynchronous methods, or multiple calls to the same asynchronous 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.

You may want to communicate to clients more information about the outcome of an asynchronous operation than AsyncCompletedEventArgs supports. In this case, you can derive a custom class from the AsyncCompletedEventArgs class and provide additional private instance variables and corresponding read-only public properties. Call the RaiseExceptionIfNecessary method before you return the property value, in case the operation was canceled or an error occurred.

The following code example demonstrates how to derive a custom class from AsyncCompletedEventArgs to pass additional information back to the client from an asynchronous operation. This code example is part of a larger example provided for the System.ComponentModel.AsyncOperationManager class.

' Define a custom Completed-event arguments class allowing 
' the data gathered by GetPersons to be returned. 
Public Class GetPersonsCompletedEventArgs
    Inherits AsyncCompletedEventArgs
    Private m_dataListValue As List(Of Person)
    Public Sub New _
        (ByVal dataList As List(Of Person), _
         ByVal exception As Exception, _
         ByVal cancelled As Boolean, _
         ByVal userState As Object)

        MyBase.New(exception, cancelled, userState)
        m_dataListValue = dataList
    End Sub

    Public ReadOnly Property DataListValue() As List(Of Person)
        Get
            Me.RaiseExceptionIfNecessary()
            Return m_dataListValue
        End Get
    End Property
End Class

// Define a custom Completed-event arguments class allowing 
// the data gathered by GetPersons to be returned.
public class GetPersonsCompletedEventArgs : AsyncCompletedEventArgs
{
    private List<Person> dataListValue;
    public GetPersonsCompletedEventArgs(
        List<Person> dataList,
        Exception error,
        bool cancelled,
        object userState)
        : base(error, cancelled, userState)
    {
        dataListValue = dataList;
    }
    public List<Person> DataListValue
    {
        get
        {
            this.RaiseExceptionIfNecessary();
            return dataListValue;
        }
    }
}

Silverlight

Silverlight for Windows Phone

XNA Framework

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Reference

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