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System.AggregateException F# snippets (dotnet#7407)
* Add F# snippets for System.AggregateException * Update AggregateException.xml * Update exception1.fs
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‎samples/snippets/fsharp/VS_Snippets_CLR_System/system.aggregateexception.class/fs/exception1.fs

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// <Snippet1>
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open System
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open System.IO
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open System.Threading.Tasks
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let getAllFiles str =
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// Should throw an UnauthorizedAccessException exception.
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System.IO.Directory.GetFiles(str, "*.txt", System.IO.SearchOption.AllDirectories)
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// Get a folder path whose directories should throw an UnauthorizedAccessException.
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let path =
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let directory =
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Environment.SpecialFolder.UserProfile
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|> Environment.GetFolderPath
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|> Directory.GetParent
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directory.FullName
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// Use this line to throw an exception that is not handled.
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// let task1 = Task<string []>.Factory.StartNew(fun () -> raise (IndexOutOfRangeException()) )
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let task1 = Task.Factory.StartNew(fun () -> getAllFiles (path))
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let execute () =
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try
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task1.Wait()
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with
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| :? UnauthorizedAccessException -> printfn "Caught unauthorized access exception-await behavior"
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| :? AggregateException as ae ->
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printfn "Caught aggregate exception-Task.Wait behavior"
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ae.Handle (fun x ->
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match x with
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| :? UnauthorizedAccessException ->
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printfn "You do not have permission to access all folders in this path."
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printfn "See your network administrator or try another path."
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true
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| _ -> false)
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printfn $"""task1 Status: {if task1.IsCompleted then "Completed," else ""}{task1.Status}"""
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execute ()
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// The example displays the following output if the file access task is run:
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// You do not have permission to access all folders in this path.
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// See your network administrator or try another path.
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// task1 Status: Completed,Faulted
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// It displays the following output if the second task is run:
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// Unhandled exception. System.AggregateException: One or more errors occurred. (Index was outside the bounds of the array.) (Index was outside the bounds of the array.)
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// ---> System.IndexOutOfRangeException: Index was outside the bounds of the array.
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// at [email protected]()
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// at System.Threading.Tasks.Task`1.InnerInvoke()
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// at System.Threading.Tasks.Task.<>c.<.cctor>b__277_0(Object obj)
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// at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
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// --- End of stack trace from previous location ---
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// at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
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// at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
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// --- End of inner exception stack trace ---
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// at System.AggregateException.Handle(Func`2 predicate)
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// at <StartupCode$exception1>.$Exception1.main@()
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// </Snippet1>

‎samples/snippets/fsharp/VS_Snippets_CLR_System/system.aggregateexception.class/fs/exception1.fsproj

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<Project Sdk="Microsoft.NET.Sdk">
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<PropertyGroup>
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<OutputType>Exe</OutputType>
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<TargetFramework>net5.0</TargetFramework>
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</PropertyGroup>
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<ItemGroup>
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<Compile Include="exception1.fs" />
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</ItemGroup>
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</Project>

‎samples/snippets/fsharp/VS_Snippets_Misc/tpl_exceptions/fs/flatten2.fs

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module flatten2
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// <Snippet22>
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open System
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open System.Threading.Tasks
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type CustomException(message) =
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inherit Exception(message)
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let task1 =
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Task.Factory.StartNew (fun () ->
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let child1 =
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Task.Factory.StartNew(
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(fun () ->
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let child2 =
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Task.Factory.StartNew(
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(fun () -> raise (CustomException "Attached child2 faulted,")),
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TaskCreationOptions.AttachedToParent
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)
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raise (CustomException "Attached child1 faulted.")),
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TaskCreationOptions.AttachedToParent
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)
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()
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)
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try
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task1.Wait()
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with
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| :? AggregateException as ae ->
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for e in ae.Flatten().InnerExceptions do
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if e :? CustomException then
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printfn "%s" e.Message
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else
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reraise()
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// The example displays the following output:
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// Attached child1 faulted.
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// Attached child2 faulted.
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// </Snippet22>

‎samples/snippets/fsharp/VS_Snippets_Misc/tpl_exceptions/fs/handlemethod2.fs

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module handlemethod2
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// <Snippet16>
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open System
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open System.Threading.Tasks
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type CustomException(message) =
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inherit Exception(message)
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let task1 =
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Task.Run(fun () -> raise (CustomException "This exception is expected!"))
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try
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task1.Wait()
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with
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| :? AggregateException as ae ->
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// Call the Handle method to handle the custom exception,
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// otherwise rethrow the exception.
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ae.Handle (fun ex ->
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if ex :? CustomException then
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printfn $"{ex.Message}"
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ex :? CustomException)
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// The example displays the following output:
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// This exception is expected!
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// </Snippet16>

‎samples/snippets/fsharp/VS_Snippets_Misc/tpl_exceptions/fs/taskexceptions.fs

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module taskexceptions
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// <snippet12>
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open System
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open System.IO
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open System.Threading.Tasks
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let getAllFiles path =
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let task1 =
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Task.Run(fun () -> Directory.GetFiles(path, "*.txt", SearchOption.AllDirectories))
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try
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task1.Result
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with
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| :? AggregateException as ae ->
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ae.Handle (fun x ->
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// Handle an UnauthorizedAccessException
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if x :? UnauthorizedAccessException then
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printfn "You do not have permission to access all folders in this path."
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printfn "See your network administrator or try another path."
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x :? UnauthorizedAccessException)
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Array.empty
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// This should throw an UnauthorizedAccessException.
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try
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let files = getAllFiles @"C:\"
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if not (isNull files) then
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for file in files do
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printfn $"{file}"
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with
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| :? AggregateException as ae ->
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for ex in ae.InnerExceptions do
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printfn $"{ex.GetType().Name}: {ex.Message}"
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printfn ""
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// This should throw an ArgumentException.
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try
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for s in getAllFiles "" do
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printfn $"{s}"
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with
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| :? AggregateException as ae ->
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for ex in ae.InnerExceptions do
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printfn $"{ex.GetType().Name}: {ex.Message}"
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// The example displays the following output:
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// You do not have permission to access all folders in this path.
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// See your network administrator or try another path.
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//
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// ArgumentException: The path is empty. (Parameter 'path')
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// </snippet12>

‎samples/snippets/fsharp/VS_Snippets_Misc/tpl_exceptions/fs/taskexceptions2.fs

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module taskexceptions2
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// <snippet13>
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open System
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open System.IO
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open System.Threading.Tasks
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let executeTasks () =
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// Assume this is a user-entered String.
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let path = @"C:\"
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let tasks =
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[| Task.Run (fun () ->
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// This should throw an UnauthorizedAccessException.
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Directory.GetFiles(path, "*.txt", SearchOption.AllDirectories))
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:> Task
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Task.Run (fun () ->
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if path = @"C:\" then
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raise (ArgumentException "The system root is not a valid path.")
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[| ".txt"; ".dll"; ".exe"; ".bin"; ".dat" |])
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:> Task
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Task.Run(fun () -> raise (NotImplementedException "This operation has not been implemented")) |]
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try
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Task.WaitAll(tasks)
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with
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| :? AggregateException as ae -> raise (ae.Flatten())
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try
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executeTasks ()
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with
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| :? AggregateException as ae ->
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for e in ae.InnerExceptions do
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printfn $"{e.GetType().Name}:\n {e.Message}"
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// The example displays the following output:
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// UnauthorizedAccessException:
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// Access to the path 'C:\Documents and Settings' is denied.
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// ArgumentException:
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// The system root is not a valid path.
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// NotImplementedException:
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// This operation has not been implemented.
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// </Snippet13>

‎samples/snippets/fsharp/VS_Snippets_Misc/tpl_exceptions/fs/tpl_exceptions.fsproj

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<Project Sdk="Microsoft.NET.Sdk">
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<PropertyGroup>
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<OutputType>Exe</OutputType>
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<TargetFramework>net5.0</TargetFramework>
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</PropertyGroup>
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<ItemGroup>
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<Compile Include="flatten2.fs" />
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<Compile Include="taskexceptions2.fs" />
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<Compile Include="handlemethod2.fs" />
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<Compile Include="taskexceptions.fs" />
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</ItemGroup>
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</Project>

‎xml/System/AggregateException.xml

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The following example catches the <xref:System.AggregateException> exception and calls the <xref:System.AggregateException.Handle%2A> method to handle each exception it contains. Compiling and running the example with the first `task1` variable should result in an <xref:System.AggregateException> object that contains an <xref:System.UnauthorizedAccessException> exception. Commenting out that line, uncommenting the second `task1` variable, and compiling and running the example produces an <xref:System.AggregateException> object that contains an <xref:System.IndexOutOfRangeException> exception.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/system.aggregateexception.class/cs/exception1.cs" id="Snippet1":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/system.aggregateexception.class/fs/exception1.fs" id="Snippet1":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/system.aggregateexception.class/vb/exception1.vb" id="Snippet1":::
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]]></format>
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In the following example, nested <xref:System.AggregateException> instances are flattened and handled in just one loop.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_Misc/tpl_exceptions/cs/flatten2.cs" id="Snippet22":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_Misc/tpl_exceptions/fs/flatten2.fs" id="Snippet22":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_Misc/tpl_exceptions/vb/flatten2.vb" id="Snippet22":::
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You can also use the <xref:System.AggregateException.Flatten%2A?displayProperty=nameWithType> method to rethrow the inner exceptions from multiple <xref:System.AggregateException> instances thrown by multiple tasks in a single <xref:System.AggregateException> instance, as the following example shows.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_Misc/tpl_exceptions/cs/taskexceptions2.cs" id="Snippet13":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_Misc/tpl_exceptions/fs/taskexceptions2.fs" id="Snippet13":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_Misc/tpl_exceptions/vb/taskexceptions2.vb" id="Snippet13":::
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]]></format>
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Ordinarily, an exception handler that catches an <xref:System.AggregateException> exception uses a `foreach` loop (in C#) or `For Each` loop (in Visual Basic) to handle each exception in its <xref:System.AggregateException.InnerExceptions%2A> collection. Instead, the following example uses the <xref:System.AggregateException.Handle%2A> method to handle each exception, and only re-throws exceptions that are not `CustomException` instances.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_Misc/tpl_exceptions/cs/handlemethod2.cs" id="Snippet16":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_Misc/tpl_exceptions/fs/handlemethod2.fs" id="Snippet16":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_Misc/tpl_exceptions/vb/handlemethod2.vb" id="Snippet16":::
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The following is a more complete example that uses the <xref:System.AggregateException.Handle%2A> method to provide special handling for an <xref:System.UnauthorizedAccessException> when enumerating files.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_Misc/tpl_exceptions/cs/taskexceptions.cs" id="Snippet12":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_Misc/tpl_exceptions/fs/taskexceptions.fs" id="Snippet12":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_Misc/tpl_exceptions/vb/taskexceptions.vb" id="Snippet12":::
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]]></format>

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