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AsyncCompletedEventArgs.RaiseExceptionIfNecessary Method

Raises a user-supplied exception if an asynchronous operation failed.

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

'Declaration
Protected Sub RaiseExceptionIfNecessary

ExceptionCondition
InvalidOperationException

The Cancelled property is true.

TargetInvocationException

The Error property has been set by the asynchronous operation. The InnerException property holds a reference to Error.

Notes to Inheritors

If you have derived your own class from the AsyncCompletedEventArgs class, your read-only properties should call the RaiseExceptionIfNecessary method before returning the property value. If the component's asynchronous worker code assigns an exception to the Error property or sets the Cancelled property to true, the property will raise an exception if a client tries to read its value. This prevents clients from accessing properties that are potentially not valid due to a failure in the asynchronous operation.

The following code example demonstrates using RaiseExceptionIfNecessary in derived class properties.


Public Class CalculatePrimeCompletedEventArgs
    Inherits AsyncCompletedEventArgs
    Private numberToTestValue As Integer = 0
    Private firstDivisorValue As Integer = 1
    Private isPrimeValue As Boolean


    Public Sub New( _
    ByVal numberToTest As Integer, _
    ByVal firstDivisor As Integer, _
    ByVal isPrime As Boolean, _
    ByVal e As Exception, _
    ByVal canceled As Boolean, _
    ByVal state As Object)

        MyBase.New(e, canceled, state)
        Me.numberToTestValue = numberToTest
        Me.firstDivisorValue = firstDivisor
        Me.isPrimeValue = isPrime

    End Sub


    Public ReadOnly Property NumberToTest() As Integer
        Get
            ' Raise an exception if the operation failed 
            ' or was canceled.
            RaiseExceptionIfNecessary()

            ' If the operation was successful, return 
            ' the property value.
            Return numberToTestValue
        End Get
    End Property


    Public ReadOnly Property FirstDivisor() As Integer
        Get
            ' Raise an exception if the operation failed 
            ' or was canceled.
            RaiseExceptionIfNecessary()

            ' If the operation was successful, return 
            ' the property value.
            Return firstDivisorValue
        End Get
    End Property


    Public ReadOnly Property IsPrime() As Boolean
        Get
            ' Raise an exception if the operation failed 
            ' or was canceled.
            RaiseExceptionIfNecessary()

            ' If the operation was successful, return 
            ' the property value.
            Return isPrimeValue
        End Get
    End Property
End Class


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

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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