TaskBuilder.fs 1.2.0-rc

.NET Standard 1.6 .NET Framework 4.5
This is a prerelease version of TaskBuilder.fs.
There is a newer version of this package available.
See the version list below for details.
Install-Package TaskBuilder.fs -Version 1.2.0-rc
dotnet add package TaskBuilder.fs --version 1.2.0-rc
<PackageReference Include="TaskBuilder.fs" Version="1.2.0-rc" />
For projects that support PackageReference, copy this XML node into the project file to reference the package.
paket add TaskBuilder.fs --version 1.2.0-rc
The NuGet Team does not provide support for this client. Please contact its maintainers for support.
#r "nuget: TaskBuilder.fs, 1.2.0-rc"
#r directive can be used in F# Interactive, C# scripting and .NET Interactive. Copy this into the interactive tool or source code of the script to reference the package.
// Install TaskBuilder.fs as a Cake Addin
#addin nuget:?package=TaskBuilder.fs&version=1.2.0-rc&prerelease

// Install TaskBuilder.fs as a Cake Tool
#tool nuget:?package=TaskBuilder.fs&version=1.2.0-rc&prerelease
The NuGet Team does not provide support for this client. Please contact its maintainers for support.

Build Status NuGet


This is a single-file project that implements a computation expression for writing Tasks in F#. It is free and unencumbered software released into the public domain.

F# comes with its own Async type and functions to convert back and forth between Async and Task, but this is a bit of a hassle -- especially since now that Task has language-level support in C# and VB.NET, it's the de facto standard for asynchrony on .NET. Additionally, F#'s Async behaves a little differently from Task, which can be confusing if you're used to the latter.

The goal of this computation expression builder is to let you write asynchronous blocks that behave just like async methods in C# do.

For example, this F# method:

open System
open System.IO
open System.Linq
open FSharp.Control.Tasks

type X() =
  static member WriteFile() =
    task {
      do! Console.Out.WriteLineAsync("Enter a filename:")
      let! name = Console.In.ReadLineAsync()
      use file = File.CreateText(name)
      for i in Enumerable.Range(0, 100) do
        do! file.WriteLineAsync(String.Format("hello {0}", i))
      do! Console.Out.WriteLineAsync("Done")
      return name

Should work exactly the same as this C# method:

using System
using System.IO
using System.Linq
using System.Threading.Tasks

class X
  public static async Task<string> WriteFile()
    await Console.Out.WriteLineAsync("Enter a filename:");
    var name = await Console.In.ReadLineAsync();
    using (var file = File.CreateText(name))
      foreach (var i in Enumerable.Range(0, 100))
        await file.WriteLineAsync(String.Format("hello {0}", i));
      await Console.Out.WriteLineAsync("Done");
      return name;

In practice there is a small performance hit compared to the C# version, because the C# compiler compiles each async method to a specialized state machine class, while TaskBuilder uses a general-purpose state machine and must chain together continuation functions to represent the computation. However, TaskBuilder should still be faster than using Task.ContinueWith or Async.StartAsTask.


This is public domain code. I encourage you to simply copy TaskBuilder.fs into your own project and use it as you see fit. It is not necessary to credit me or include any legal notice with your copy of the code.

The other files are tests which you do not need to copy (but again, you are free to do so).

Note that by default, if you open FSharp.Control.Tasks, you'll get a task { ... } builder that behaves as closely to C#'s async methods as possible.

However, I have also included a version of the task { ... } builder under FSharp.Control.Tasks.ContextInsensitive which makes one minor change: it will automatically call task.ConfigureAwait(false) on every task you await.

This can improve performance if you're writing library code or server-side code and don't need to interact with thread-unsafe things like Windows forms controls. If you're not sure whether you want to use this version of the builder, reading this MSDN article may help.

What you can bind with let!

As of 7a04419, you should be able to bind anything "awaitable" with let!.

This basically means any type that has:

  • task.GetAwaiter()
  • task.GetAwaiter().GetResult()
  • task.GetAwaiter().IsCompleted

When using FSharp.Control.Tasks.ContextInsensitive, you can also bind any type that has a task.ConfigureAwait(false) returning an "awaitable" type.

Tail calls are not optimized

In F# it is idiomatic to use tail recursion to implement loops more complex than a simple for or while.

This works with some computation expressions (like the built-in F# async builder), but not with TaskBuilder.fs. As far as I know it is not possible to make this work with TPL tasks. C# async/await function are not tail-call optimized either, so at least this is consistent.

To implement a loop that may iterate many times (or indefinitely), use a while loop instead of tail recursion.

For example:

DO ✓

let runPendingJobs() =
    task {
        let mutable anyPending = true
        while anyPending do
            let! jobToRun = checkForJob()
            match jobToRun with
            | None ->
                anyPending <- false
            | Some pendingJob ->
                do! pendingJob()


let rec runPendingJobs() =
    task {
        let! jobToRun = checkForJob()
        match jobToRun with
        | None ->
            return ()
        | Some pendingJob ->
            do! pendingJob()
            return! runPendingJobs()
Product Versions
.NET net5.0 net5.0-windows net6.0 net6.0-android net6.0-ios net6.0-maccatalyst net6.0-macos net6.0-tvos net6.0-windows
.NET Core netcoreapp1.0 netcoreapp1.1 netcoreapp2.0 netcoreapp2.1 netcoreapp2.2 netcoreapp3.0 netcoreapp3.1
.NET Standard netstandard1.6 netstandard2.0 netstandard2.1
.NET Framework net45 net451 net452 net46 net461 net462 net463 net47 net471 net472 net48
MonoAndroid monoandroid
MonoMac monomac
MonoTouch monotouch
Tizen tizen30 tizen40 tizen60
Xamarin.iOS xamarinios
Xamarin.Mac xamarinmac
Xamarin.TVOS xamarintvos
Xamarin.WatchOS xamarinwatchos
Compatible target framework(s)
Additional computed target framework(s)
Learn more about Target Frameworks and .NET Standard.

NuGet packages (38)

Showing the top 5 NuGet packages that depend on TaskBuilder.fs:

Package Downloads

Chauffeur is a tool for helping with delivering changes to an Umbraco instance.


This library Chia contains utils for internal Danpower Reporting and is used by serveral reports.


A library that provides an idiomatic F# API for Microsoft Azure services.


Simple library(100% written in F#) to use leader election functionality based on Consul.


A driver for the Postgresql fork Agensgraph

GitHub repositories (5)

Showing the top 5 popular GitHub repositories that depend on TaskBuilder.fs:

Repository Stars
Azure WebJobs SDK
GRPC bindings for protobuf-net and grpc-dotnet
Durable Task Framework extension for Azure Functions
Connecting a Redux state store with Blazor.
Lightweight HTML processor
Version Downloads Last updated
2.2.0-alpha 11,739 12/5/2019
2.1.0 1,580,872 9/16/2018
2.0.0 24,704 5/27/2018
1.2.0-rc 1,968 2/26/2018
1.1.1 25,395 2/19/2018
1.1.0 886 2/17/2018
1.0.1 64,373 2/9/2018
1.0.0 11,036 10/7/2017