AggregateException.Flatten Method ()

 

Flattens an AggregateException instances into a single, new instance.

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

Public Function Flatten As AggregateException

Return Value

Type: System.AggregateException

A new, flattened AggregateException.

If a task has an attached child task that throws an exception, that exception is wrapped in an AggregateException exception before it is propagated to the parent task, which wraps that exception in its own AggregateException exception before it propagates it back to the calling thread. In such cases, the InnerExceptions property of the AggregateException exception that is caught by the Task.Wait, Task(Of TResult).Wait, Task.WaitAny or Task.WaitAll method contains one or more AggregateException instances, not the original exceptions that caused the fault. To avoid having to iterate over nested AggregateException exceptions, you can use the Flatten method to remove all the nested AggregateException exceptions, so that the InnerExceptions property of the returned AggregateException object contains the original exceptions.

This method recursively flattens all instances of AggregateException exceptions that are inner exceptions of the current AggregateException instance . The inner exceptions returned in the new AggregateException are the union of all the inner exceptions from exception tree rooted at the current AggregateException instance.

In the following example, nested AggregateException instances are flattened and handled in just one loop.

Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim task1 = Task.Factory.StartNew(Sub()
                                           Dim child1 = Task.Factory.StartNew(Sub()
                                                                                 Dim child2 = Task.Factory.StartNew(Sub()
                                                                                                                       Throw New CustomException("Attached child2 faulted.")
                                                                                                                    End Sub,
                                                                                                                    TaskCreationOptions.AttachedToParent)
                                                                                                                    Throw New CustomException("Attached child1 faulted.")
                                                                              End Sub,
                                                                              TaskCreationOptions.AttachedToParent)
                                        End Sub)

      Try
         task1.Wait()
      Catch ae As AggregateException
         For Each ex In ae.Flatten().InnerExceptions
            If TypeOf ex Is CustomException Then
               Console.WriteLine(ex.Message)
            Else
               Throw
            End If
         Next
      End Try
   End Sub
End Module

Class CustomException : Inherits Exception
   Public Sub New(s As String)
      MyBase.New(s)
   End Sub
End Class
' The example displays the following output:
'       Attached child1 faulted.
'       Attached child2 faulted.

You can also use the AggregateException.Flatten method to rethrow the inner exceptions from multiple AggregateException instances thrown by multiple tasks in a single AggregateException instance, as the following example shows.

Imports System.Collections.Generic
Imports System.IO
Imports System.Threading.Tasks

Module Example
    Public Sub Main()
       Try
          ExecuteTasks()
       Catch ae As AggregateException
          For Each e In ae.InnerExceptions
             Console.WriteLine("{0}:{2}   {1}", e.GetType().Name, e.Message,
                               vbCrLf)
          Next
       End Try
    End Sub

    Sub ExecuteTasks()
        ' Assume this is a user-entered String.
        Dim path = "C:\"
        Dim tasks As New List(Of Task)

        tasks.Add(Task.Run(Function()
                             ' This should throw an UnauthorizedAccessEXception.
                              Return Directory.GetFiles(path, "*.txt",
                                                        SearchOption.AllDirectories)
                           End Function))

        tasks.Add(Task.Run(Function()
                              If path = "C:\" Then
                                 Throw New ArgumentException("The system root is not a valid path.")
                              End If
                              Return { ".txt", ".dll", ".exe", ".bin", ".dat" }
                           End Function))

        tasks.Add(Task.Run(Sub()
                              Throw New NotImplementedException("This operation has not been implemented.")
                           End Sub))

        Try
            Task.WaitAll(tasks.ToArray)
        Catch ae As AggregateException
            Throw ae.Flatten()
        End Try
    End Sub
End Module
' The example displays the following output:
'       UnauthorizedAccessException:
'          Access to the path 'C:\Documents and Settings' is denied.
'       ArgumentException:
'          The system root is not a valid path.
'       NotImplementedException:
'          This operation has not been implemented.

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 5.0
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
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