Artikel
11/18/2011

AsyncCompletedEventArgs.Error Property

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Gets a value that indicates which error occurred during an asynchronous operation.

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

Syntax

'Declaration
Public ReadOnly Property Error As Exception

public Exception Error { get; }

Property Value

Type: System.Exception
An Exception instance, if an error occurred during an asynchronous operation; otherwise nulla null reference (Nothing in Visual Basic).

Remarks

If an exception is raised during an asynchronous operation, the class will assign the exception to the Error property. The client application's event-handler delegate should check the Error property before accessing any properties in a class derived from AsyncCompletedEventArgs. Otherwise, the property will raise a TargetInvocationException with its InnerException property holding a reference to Error.

The value of the Error property is nulla null reference (Nothing in Visual Basic) if the operation was canceled.

Notes to Inheritors

If you provide read-only properties in a derived class, make sure that you call the RaiseExceptionIfNecessary method in your property implementation. This prevents clients from accessing properties that are potentially not valid because of a failure in the asynchronous operation.

Examples

The following code example demonstrates the using the Error property to inform a client that an asynchronous operation failed. This code example is part of a larger example provided for the System.ComponentModel.AsyncOperationManager class.

' 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.

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