AsyncCompletedEventArgs.RaiseExceptionIfNecessary Method
Definition
Important
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Raises a user-supplied exception if an asynchronous operation failed.
protected:
void RaiseExceptionIfNecessary();protected void RaiseExceptionIfNecessary ();member this.RaiseExceptionIfNecessary : unit -> unitProtected Sub RaiseExceptionIfNecessary ()Exceptions
The Cancelled property is true.
The Error property has been set by the asynchronous operation. The InnerException property holds a reference to Error.
Examples
The following code example demonstrates using RaiseExceptionIfNecessary in derived class properties.
public class CalculatePrimeCompletedEventArgs :
AsyncCompletedEventArgs
{
private int numberToTestValue = 0;
private int firstDivisorValue = 1;
private bool isPrimeValue;
public CalculatePrimeCompletedEventArgs(
int numberToTest,
int firstDivisor,
bool isPrime,
Exception e,
bool canceled,
object state) : base(e, canceled, state)
{
this.numberToTestValue = numberToTest;
this.firstDivisorValue = firstDivisor;
this.isPrimeValue = isPrime;
}
public int NumberToTest
{
get
{
// Raise an exception if the operation failed or
// was canceled.
RaiseExceptionIfNecessary();
// If the operation was successful, return the
// property value.
return numberToTestValue;
}
}
public int FirstDivisor
{
get
{
// Raise an exception if the operation failed or
// was canceled.
RaiseExceptionIfNecessary();
// If the operation was successful, return the
// property value.
return firstDivisorValue;
}
}
public bool IsPrime
{
get
{
// Raise an exception if the operation failed or
// was canceled.
RaiseExceptionIfNecessary();
// If the operation was successful, return the
// property value.
return isPrimeValue;
}
}
}
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
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.
