AsyncEnumerator 4.0.2

.NET Standard 1.4 .NET Framework 4.5
dotnet add package AsyncEnumerator --version 4.0.2
NuGet\Install-Package AsyncEnumerator -Version 4.0.2
This command is intended to be used within the Package Manager Console in Visual Studio, as it uses the NuGet module's version of Install-Package.
<PackageReference Include="AsyncEnumerator" Version="4.0.2" />
For projects that support PackageReference, copy this XML node into the project file to reference the package.
paket add AsyncEnumerator --version 4.0.2
#r "nuget: AsyncEnumerator, 4.0.2"
#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 AsyncEnumerator as a Cake Addin
#addin nuget:?package=AsyncEnumerator&version=4.0.2

// Install AsyncEnumerator as a Cake Tool
#tool nuget:?package=AsyncEnumerator&version=4.0.2

1: How to use this library?

See examples above. You can the core code and lots of useful extension methods in the Dasync.Collections namespace.

2: Using CancellationToken

    using Dasync.Collections;
    IAsyncEnumerable<int> ProduceNumbers()
      return new AsyncEnumerable<int>(async yield => {

        await FooAsync(yield.CancellationToken);

3: Always remember about ConfigureAwait(false)

To avoid performance degradation and possible dead-locks in ASP.NET or WPF applications (or any SynchronizationContext-dependent environment), you should always put ConfigureAwait(false) in your await statements:

    using Dasync.Collections;
    IAsyncEnumerable<int> GetValues()
      return new AsyncEnumerable<int>(async yield =>
        await FooAsync().ConfigureAwait(false);

        // Yes, it's even needed for 'yield.ReturnAsync'
        await yield.ReturnAsync(123).ConfigureAwait(false);

4: Clean-up on incomplete enumeration

Imagine such situation:

    using Dasync.Collections;

    IAsyncEnumerable<int> ReadValuesFromQueue()
      return new AsyncEnumerable<int>(async yield =>
        using (var queueClient = CreateQueueClient())
          while (true)
            var message = queueClient.DequeueMessageAsync();
            if (message == null)
            await yield.ReturnAsync(message.Value);

    Task<int> ReadFirstValueOrDefaultAsync()
      return ReadValuesFromQueue().FirstOrDefaultAsync();

The FirstOrDefaultAsync method will try to read first value from the IAsyncEnumerator, and then will just dispose it. However, disposing AsyncEnumerator does not mean that the queueClient in the lambda function will be disposed automatically as well, because async methods are just state machines which need somehow to go to a particular state to do the clean-up. To provide such behavior, when you dispose an AsyncEnumerator before you reach the end of enumeration, it will tell to resume your async lambda function (at await yield.ReturnAsync()) with the AsyncEnumerationCanceledException (derives from OperationCanceledException). Having such exception in your lambda method will break normal flow of enumeration and will go to terminal state of the underlying state machine, what will do the clean-up, i.e. dispose the queueClient in this case. You don't need (and shouldn't) catch that exception type, because it's handled internally by AsyncEnumerator. The same exception is thrown when you call yield.Break().

There is another option to do the cleanup on Dispose:

    using Dasync.Collections;

    IAsyncEnumerator<int> GetQueueEnumerator()
      var queueClient = CreateQueueClient();

      return new AsyncEnumerable<int>(async yield =>
        while (true)
          var message = queueClient.DequeueMessageAsync();
          if (message == null)
          await yield.ReturnAsync(message.Value);
      onDispose: () => queueClient.Dispose());

5: Why is GetAsyncEnumeratorAsync async?

The IAsyncEnumerable.GetAsyncEnumeratorAsync() method is async and returns a Task<IAsyncEnumerator>, where the current implementation of AsyncEnumerable always runs that method synchronously and just returns an instance of AsyncEnumerator. Having interfaces allows you to do your own implementation, where classes mentioned above are just helpers. The initial idea was to be able to support database-like scenarios, where GetAsyncEnumeratorAsync() executes a query first (what internally returns a pointer), and the MoveNextAsync() enumerates through rows (by using that pointer).

6: Returning IAsyncEnumerable vs IAsyncEnumerator

When you implement a method that returns an async-enumerable collection you have a choice to return either IAsyncEnumerable or IAsyncEnumerator - the constructors of the helper classes AsyncEnumerable and AsyncEnumerator are absolutely identical. Both interfaces have same set of useful extension methods, like ForEachAsync.

When you create an 'enumerable', you create a factory that produces 'enumerators', i.e. you can enumerate through a collection many times. On the other hand, creating an 'enumerator' is needed when you can through a collection only once.

7: Where is Reset or ResetAsync?

The Reset method must not be on the IEnumerator interface, and should be considered as deprecated. Create a new enumerator instead. This is the reason why the 'oneTimeUse' flag was removed in version 2 of this library.

8: How can I do synchronous for-each enumeration through IAsyncEnumerable?

You can use extension methods like IAsyncEnumerable.ToEnumerable() to use built-in foreach enumeration, BUT you should never do that! The general idea of this library is to avoid thread-blocking calls on worker threads, where converting an IAsyncEnumerable to IEnumerable will just defeat the whole purpose of this library. This is the reason why such synchronous extension methods are marked with [Obsolete] attribute.

9: What's the difference between ForEachAsync and ParallelForEachAsync?

The ForEachAsync allows you to go through a collection and perform an action on every single item in sequential manner. On the other hand, ParallelForEachAsync allows you to run the action on multiple items at the same time where the sequential order of completion is not guaranteed. For the latter, the degree of the parallelism is controlled by the maxDegreeOfParalellism argument, however it does not guarantee to spin up the exact amount of threads, because it depends on the thread pool size and its occupancy at a moment of time. Such parallel approach is much better than trying to create a task for an action for every single item on the collection and then awaiting on all of them with Task.WhenAll, because it adds less overhead to the runtime, better with memory usage, and helps with throttling-sensitive scenarios.

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 net7.0 net7.0-android net7.0-ios net7.0-maccatalyst net7.0-macos net7.0-tvos net7.0-windows
.NET Core netcoreapp1.0 netcoreapp1.1 netcoreapp2.0 netcoreapp2.1 netcoreapp2.2 netcoreapp3.0 netcoreapp3.1
.NET Standard netstandard1.4 netstandard1.5 netstandard1.6 netstandard2.0 netstandard2.1
.NET Framework net45 net451 net452 net46 net461 net462 net463 net47 net471 net472 net48 net481
MonoAndroid monoandroid
MonoMac monomac
MonoTouch monotouch
Tizen tizen30 tizen40 tizen60
Universal Windows Platform uap uap10.0
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 (45)

Showing the top 5 NuGet packages that depend on AsyncEnumerator:

Package Downloads

Package Description


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GitHub repositories (11)

Showing the top 5 popular GitHub repositories that depend on AsyncEnumerator:

Repository Stars
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Version Downloads Last updated
4.0.2 7,461,276 12/4/2019
4.0.1 108,219 10/22/2019
4.0.0 28,916 10/18/2019
3.1.0 92,277 9/23/2019
2.2.2 1,767,534 1/27/2019
2.2.1 1,339,316 5/29/2018
2.2.0 31,330 5/18/2018
2.1.1 246,843 1/20/2018
2.1.0 299,834 5/22/2017
2.0.1 52,321 2/13/2017
1.5.0 4,115 2/12/2017
1.4.2 1,758 2/6/2017
1.3.0 2,012 1/20/2017
1.2.3 7,082 1/6/2017
1.2.2 2,421 12/11/2016
1.2.1 997 12/10/2016
1.2.0 19,623 11/29/2016
1.1.3 1,091 11/28/2016
1.1.2 46,908 8/29/2016
1.0.3 4,199 4/28/2016

4.0.0: Use interfaces from Microsoft.Bcl.AsyncInterfaces package in NET Standard 2.0.
3.1.0: Add support for NET Standard 2.1, consolidate interface with Microsoft's implementation.
2.2.0: New extension methods: SelectMany, Append, Prepend, OfType, Concat, Distinct, ToDictionaryAsync, ToLookupAsync, AggregateAsync.
2.1.0: New extension methods: Batch, UnionAll, Single, SingleOrDefault, DefaultIfEmpty, Cast.
2.0.0: Revise design of the library: same features, but slight paradigm shift and interface breaking changes.
1.5.0: Add support for .NET Standard, minor improvements.
1.4.2: Add finalizer to AsyncEnumerator and call Dispose in ForEachAsync and ParallelForEachAsync extension methods.
1.4.0: Add new generic type AsyncEnumeratorWithState for performance optimization.
      Now IAsyncEnumerator<T> is covariant.
      Add ForEachAsync, ParallelForeachAsync, and LINQ-style extension methods for IAsyncEnumerator.
1.2.1: New Linq-style extension methods in System.Collections.Async namespace.
1.1.0: Add ParallelForEachAsync extension methods for IEnumerable<T> and IAsyncEnumerable<T> in System.Collections.Async namespace.