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Async.Catch<'T> Method (F#)

Creates an asynchronous computation that executes a specified computation. If this computation completes successfully, then this method returns Choice1Of2 with the returned value. If this computation raises an exception before it completes then return Choice2Of2 with the raised exception.

Namespace/Module Path: Microsoft.FSharp.Control

Assembly: FSharp.Core (in FSharp.Core.dll)

// Signature:
static member Catch : Async<'T> -> Async<Choice<'T,exn>>

// Usage:
Async.Catch (computation)

computation

Type: Async<'T>

The input computation that returns the type 'T.

A computation that returns a Choice of type 'T or an exception.

The following code example shows how to use Async.Catch to run an asynchronous computation that might throw an exception.

open System
open System.IO

let writeToFile filename numBytes = 
    async {
        use file = File.Create(filename)
        printfn "Writing to file %s." filename
        do! file.AsyncWrite(Array.zeroCreate<byte> numBytes)
    }

let readFile filename numBytes =
    async {
        use file = File.OpenRead(filename)
        printfn "Reading from file %s." filename
        do! file.AsyncRead(numBytes) |> Async.Ignore
    }

let filename = "BigFile.dat" 
let numBytes = 100000000

let result1 = writeToFile filename numBytes
             |> Async.Catch
             |> Async.RunSynchronously
match result1 with
| Choice1Of2 _ -> printfn "Successfully wrote to file."; ()
| Choice2Of2 exn -> 
      printfn "Exception occurred writing to file %s: %s" filename exn.Message

// Start these next two operations asynchronously, forcing an exception due 
// to trying to access the file twice simultaneously.
Async.Start(readFile filename numBytes)
let result2 = writeToFile filename numBytes
             |> Async.Catch
             |> Async.RunSynchronously
match result2 with
| Choice1Of2 buffer -> printfn "Successfully read from file."
| Choice2Of2 exn ->
    printfn "Exception occurred reading from file %s: %s" filename (exn.Message)

Windows 8, Windows 7, Windows Server 2012, Windows Server 2008 R2

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

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