Tuesday, January 14, 2020

Asynchronous streams - Using IAsyncEnumerable in .NET 4.7

Although IAsyncEnumerable is a part of the C# 8 release and C# 8.0 is supported on .NET Core 3.x and .NET Standard 2.1, this doesn’t have to mean that you cannot use this feature in .NET Core 2.x or the full .NET Framework.

We’ll start with the following (failing to compile) code in a .NET 4.7 project and try to make it work:

A .NET Standard 2.0 project doesn’t know the IAsyncEnumerable interface. So the first thing we need to do is to install the compatibility NuGet package Microsoft.Bcl.AsyncInterfaces.

Now the compiler finds the IAsyncEnumerable interface but Visual Studio still complains because we are targeting C# 7.3 and asynchronous streams is a C# 8 feature.

We can fix this but this requires that the following things are installed on our computer:

  • .NET Core SDK 3.0 or MSBuild Tools 2019
  • Visual Studio 2019 or VSCode

If these requirements are met we can update our project file and tell the compiler to use C# 8 as the language version:

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