This blog is one of The December 1st entries on the 2019 C# Advent Calendar. Thanks for having me again Matt!

My favorite new feature in C# 8 has got to be Asynchronous Streams, a.k.a. Asynchronous Enumerables. However, I think there may some confusion as to what they do, when to use them, and even if they can be used in a particular project. This post will hopefully clarify some of these points.

TL;DR

You can use IAsyncEnumerable<T> and the related C# 8 features in .NET Core 2.x or .NET Framework 4.6.1, not just .NET Core 3.0!

Why use IAsyncEnumerable<T> ?`

When writing efficient applications intended to handle large numbers of simultaneous operations, such as web applications, blocking operations are the enemy. Any time an application is waiting on some kind of I/O-bound operation, such as a network response or a hard disk read, it is always best to relinquish the thread back to the thread pool. This allows the CPU to work on other operations while the method is waiting, and then continue the work once there is more to be done, without using up all of the threads in the thread pool. Lots of details about why are available here.

In many cases, it’s efficient enough to simply return a regular IEnumerable<T> asynchronously, like so:

public async Task DoWork () { var query = BuildMyQuery (); IEnumerable < XYZ > queryResult = await query . ExecuteAsync (); foreach ( var item in queryResult ) { // Do work here } }

However, this is may not be the most efficient approach. There are two potential limitations, depending on the backing implementation of ExecuteAsync in the example above.

The implementation of ExecuteAsync may return after it gets the first part of the data, or even none. If it is enumerated faster than the data is arriving it will block waiting for more items to arrive. This will block the executing thread. The implementation of ExecuteAsync may wait until it has all of the data before returning. This delays the point where DoWork may begin processing data.

IAsyncEnumerable<T> and its sibling IAsyncEnumerator<T> , on the other hand, return a Task as the stream is iterated. This allows methods such as ExecuteAsync to return early and then wait for more data as it is iterated, without blocking the thread.

When Not To Use IAsyncEnumerable

Don’t use IAsyncEnumerable<T> for any collection which is inherently synchronous and CPU-bound. For those cases, continuing using IEnumerable<T> . The extra overhead of handling asynchronous tasks will usually be less performant in these scenarios.

Knowing which type to use as the return type on an interface method is a bit tricker. The interface can have different backing implementations which may or may not be synchronous. In this case, use the pattern for the most likely scenario. If in doubt, lean towards IAsyncEnumerable<T> because it can be used for either scenario and is therefore more flexible.

Returning an IAsyncEnumerable in C# 8

For simple use cases, returning an IAsyncEnumerable<T> using an iterator function is as easy as an IEnumerable<T> .

public async IAsyncEnumerable < XYZ > GetXYZAsync () { for ( var i = 0 ; i < 20 ; i ++) { var item = await GetXYZById ( i ); yield return item ; } }

It is also possible to write an iterator method which returns IAsyncEnumerator<T> .

If the method supports cancellation, the CancellationToken should to be decorated with the EnumeratorCancellation attribute:

public async IAsyncEnumerable < XYZ > GetXYZAsync ([ EnumeratorCancellation ] CancellationToken cancellationToken = default ) { for ( var i = 0 ; i < 20 ; i ++) { var item = await GetXYZById ( i , cancellationToken ); yield return item ; } }

Consuming an IAsyncEnumerable in C# 8

To consume an IAsyncEnumerable, simply use the new await foreach statement within an asynchronous method.

await foreach ( var item in GetXYZAsync ()) { // do things here }

To control the synchronization context, ConfigureAwait is available, just like on Task .

await foreach ( var item in GetXYZAsync (). ConfigureAwait ( false )) { // do things here }

To pass a cancellation token:

await foreach ( var item in GetXYZAsync (). WithCancellation ( cancellationToken )) { // do things here }

Special Compatibility Concerns

Depending on the type of project and the version of .NET being targeted, there may be concerns about compatibility. One myth is that these features can only be used with .NET Core 3.0 and C# 8.

Can I Use IAsyncEnumerable When Targeting .NET Core 2.x?

Short answer: Yes

To gain access to IAsyncEnumerable, install the compatibility NuGet package Microsoft.Bcl.AsyncInterfaces. This provides the types that are missing in .NET Standard 2.0.

However, producing and consuming IAsyncEnumerables is a bit more difficult. Here’s an example consumer:

var enumerable = GetXYZAsync (); var enumerator = enumerable . GetAsyncEnumerator (); try { while ( await enumerator . MoveNextAsync ()) { var item = enumerator . Current ; // Do things here } } finally { await enumerator . DisposeAsync (); }

To get around this limitation, there are ways to access C# 8 language features, like asynchronous streams, even from .NET Core 2.x. The requirements are:

Use .NET Core SDK 3.0 or MSBuild Tools 2019 as the compiler.

Use Visual Studio 2019 or VSCode as the IDE.

Add a <LangVersion>8</LangVersion> property to the project file.

<Project Sdk= "Microsoft.NET.Sdk" > <PropertyGroup> <OutputType> Exe </OutputType> <TargetFramework> netcoreapp2.0 </TargetFramework> <LangVersion> 8 </LangVersion> </PropertyGroup> <ItemGroup> <PackageReference Include= "Microsoft.Bcl.AsyncInterfaces" Version= "1.0.0" /> </ItemGroup> </Project>

Can I Use IAsyncEnumerable When Targeting .NET Framework

Short answer: Yes, 4.6.1 or later

.NET Framework 4.6.1 is compatible with .NET Standard 2.0. Therefore, so long as the project is targeting .NET Framework 4.6.1 or later, the same rules apply as for .NET Core 2.x. Just follow the same steps as above.

What About NuGet Packages?

Short answer: Yes, targeting .NET Standard 2.0 or later

When creating a NuGet package, maintaining backwards compatibility is key. This can make it difficult to use new features like asynchronous streams. The good news is that there is a relatively easy path.

Dual target .NET Standard 2.0 and 2.1.

Consider targeting .NET 4.6.1 as well, see Microsoft’s guidance on cross platform targeting.

Include Microsoft.Bcl.AsyncInterfaces for the lower version targets only.

Add a <LangVersion>8</LangVersion> property to the project file.

<Project Sdk= "Microsoft.NET.Sdk" > <PropertyGroup> <TargetFrameworks> net461;netstandard2.0;netstandard2.1 </TargetFrameworks> <LangVersion> 8 </LangVersion> </PropertyGroup> <ItemGroup Condition= " '$(TargetFramework)' == 'net461' Or '$(TargetFramework)' == 'netstandard2.0' " > <PackageReference Include= "Microsoft.Bcl.AsyncInterfaces" Version= "1.0.0" /> </ItemGroup> </Project>

However, this approach has problems if the package needs to target versions of .NET Core before 2.0, .NET Standard before 2.0, or .NET Framework before 4.6.1. The Microsoft.Bcl.AsyncInterfaces package isn’t compatible with frameworks prior these versions. This can still be addressed by using preprocessor conditionals within the codebase to exclude IAsyncEnumerable<T> support, but is much more cumbersome and outside the scope of this post.

What about LINQ?

Most .NET developers love to use LINQ, either using the query syntax or using the functional syntax like .Where(p => p != null) or .Select(p => p.Property) . Is it possible to use LINQ with IAsyncEnumerable<T> ?

To get support for the functional LINQ syntax, such as .Where(p => p != null) or .Select(p => p.Property) , install System.Linq.Async. This package is brought to us by the group that makes ReactiveX. Be sure to use at least version 4.0.0.

Conclusion