AggregateException.Handle(Func<Exception,Boolean>) Method
Definition
Important
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Invokes a handler on each Exception contained by this AggregateException.
public:
void Handle(Func<Exception ^, bool> ^ predicate);
public void Handle (Func<Exception,bool> predicate);
member this.Handle : Func<Exception, bool> -> unit
Public Sub Handle (predicate As Func(Of Exception, Boolean))
Parameters
The predicate to execute for each exception. The predicate accepts as an argument the Exception to be processed and returns a Boolean to indicate whether the exception was handled.
Exceptions
The predicate
argument is null.
An exception contained by this AggregateException was not handled.
Examples
Ordinarily, an exception handler that catches an AggregateException exception uses a foreach
loop (in C#) or For Each
loop (in Visual Basic) to handle each exception in its InnerExceptions collection. Instead, the following example uses the Handle method to handle each exception, and only rethrows exceptions that are not CustomException
instances.
using System;
using System.Threading.Tasks;
public class Example
{
public static void Main()
{
var task1 = Task.Run( () => { throw new CustomException("This exception is expected!"); } );
try {
task1.Wait();
}
catch (AggregateException ae)
{
// Call the Handle method to handle the custom exception,
// otherwise rethrow the exception.
ae.Handle(ex => { if (ex is CustomException)
Console.WriteLine(ex.Message);
return ex is CustomException;
});
}
}
}
public class CustomException : Exception
{
public CustomException(String message) : base(message)
{}
}
// The example displays the following output:
// This exception is expected!
open System
open System.Threading.Tasks
type CustomException(message) =
inherit Exception(message)
let task1 =
Task.Run(fun () -> raise (CustomException "This exception is expected!"))
try
task1.Wait()
with
| :? AggregateException as ae ->
// Call the Handle method to handle the custom exception,
// otherwise rethrow the exception.
ae.Handle (fun ex ->
if ex :? CustomException then
printfn $"{ex.Message}"
ex :? CustomException)
// The example displays the following output:
// This exception is expected!
Imports System.Threading.Tasks
Module Example
Public Sub Main()
Dim task1 = Task.Run(Sub() Throw New CustomException("This exception is expected!"))
Try
task1.Wait()
Catch ae As AggregateException
' Call the Handle method to handle the custom exception,
' otherwise rethrow the exception.
ae.Handle(Function(e)
If TypeOf e Is CustomException Then
Console.WriteLine(e.Message)
End If
Return TypeOf e Is CustomException
End Function)
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:
' This exception is expected!
The following is a more complete example that uses the Handle method to provide special handling for an UnauthorizedAccessException when enumerating files.
using System;
using System.IO;
using System.Threading.Tasks;
public class Example2
{
public static void Main()
{
// This should throw an UnauthorizedAccessException.
try {
var files = GetAllFiles(@"C:\");
if (files != null)
foreach (var file in files)
Console.WriteLine(file);
}
catch (AggregateException ae) {
foreach (var ex in ae.InnerExceptions)
Console.WriteLine("{0}: {1}", ex.GetType().Name, ex.Message);
}
Console.WriteLine();
// This should throw an ArgumentException.
try {
foreach (var s in GetAllFiles(""))
Console.WriteLine(s);
}
catch (AggregateException ae) {
foreach (var ex in ae.InnerExceptions)
Console.WriteLine("{0}: {1}", ex.GetType().Name, ex.Message);
}
}
static string[] GetAllFiles(string path)
{
var task1 = Task.Run( () => Directory.GetFiles(path, "*.txt",
SearchOption.AllDirectories));
try {
return task1.Result;
}
catch (AggregateException ae) {
ae.Handle( x => { // Handle an UnauthorizedAccessException
if (x is UnauthorizedAccessException) {
Console.WriteLine("You do not have permission to access all folders in this path.");
Console.WriteLine("See your network administrator or try another path.");
}
return x is UnauthorizedAccessException;
});
return Array.Empty<String>();
}
}
}
// The example displays the following output:
// You do not have permission to access all folders in this path.
// See your network administrator or try another path.
//
// ArgumentException: The path is not of a legal form.
open System
open System.IO
open System.Threading.Tasks
let getAllFiles path =
let task1 =
Task.Run(fun () -> Directory.GetFiles(path, "*.txt", SearchOption.AllDirectories))
try
task1.Result
with
| :? AggregateException as ae ->
ae.Handle (fun x ->
// Handle an UnauthorizedAccessException
if x :? UnauthorizedAccessException then
printfn "You do not have permission to access all folders in this path."
printfn "See your network administrator or try another path."
x :? UnauthorizedAccessException)
Array.empty
// This should throw an UnauthorizedAccessException.
try
let files = getAllFiles @"C:\"
if not (isNull files) then
for file in files do
printfn $"{file}"
with
| :? AggregateException as ae ->
for ex in ae.InnerExceptions do
printfn $"{ex.GetType().Name}: {ex.Message}"
printfn ""
// This should throw an ArgumentException.
try
for s in getAllFiles "" do
printfn $"{s}"
with
| :? AggregateException as ae ->
for ex in ae.InnerExceptions do
printfn $"{ex.GetType().Name}: {ex.Message}"
// The example displays the following output:
// You do not have permission to access all folders in this path.
// See your network administrator or try another path.
//
// ArgumentException: The path is empty. (Parameter 'path')
Imports System.IO
Imports System.Threading.Tasks
Module Example
Public Sub Main()
' This should throw an UnauthorizedAccessException.
Try
Dim files = GetAllFiles("C:\")
If files IsNot Nothing Then
For Each file In files
Console.WriteLine(file)
Next
End If
Catch ae As AggregateException
For Each ex In ae.InnerExceptions
Console.WriteLine("{0}: {1}", ex.GetType().Name, ex.Message)
Next
End Try
Console.WriteLine()
' This should throw an ArgumentException.
Try
For Each s In GetAllFiles("")
Console.WriteLine(s)
Next
Catch ae As AggregateException
For Each ex In ae.InnerExceptions
Console.WriteLine("{0}: {1}", ex.GetType().Name, ex.Message)
Next
End Try
Console.WriteLine()
End Sub
Function GetAllFiles(ByVal path As String) As String()
Dim task1 = Task.Run( Function()
Return Directory.GetFiles(path, "*.txt",
SearchOption.AllDirectories)
End Function)
Try
Return task1.Result
Catch ae As AggregateException
ae.Handle( Function(x)
' Handle an UnauthorizedAccessException
If TypeOf x Is UnauthorizedAccessException Then
Console.WriteLine("You do not have permission to access all folders in this path.")
Console.WriteLine("See your network administrator or try another path.")
End If
Return TypeOf x Is UnauthorizedAccessException
End Function)
End Try
Return Array.Empty(Of String)()
End Function
End Module
' The example displays the following output:
' You do not have permission to access all folders in this path.
' See your network administrator or try another path.
'
' ArgumentException: The path is not of a legal form.
Remarks
Each invocation of the predicate
returns true or false to indicate whether the Exception was handled. After all invocations, if any exceptions went unhandled, all unhandled exceptions will be put into a new AggregateException which will be thrown. Otherwise, the Handle method simply returns. If any invocations of the predicate
throws an exception, it will halt the processing of any more exceptions and immediately propagate the thrown exception as-is.