Detailed explanation of compatibility between C# 8 and .NET Framework

Contrary to the screenshot in the question, C# 8 is actually compatible with the .NET Framework. To use C# 8 with .NET Framework, simply set the language version to 8.0 in the .csproj file.

Supported features

Most C# 8 functions, including static local functions, using declarations, and switch expressions, work properly in the target framework.

Some supported features

Async streams and indexes/ranges require new types not available in the .NET Framework. However, these features can be enabled using the "polyfill" NuGet package or a custom code file.

Unsupported features

Default interface members are not supported and are never compatible with the .NET Framework because runtime changes are required in the CLR.

Visual Studio 2019

In Visual Studio 2019 16.3 and later, the language selection drop-down menu has been disabled and C# 8 must be enabled explicitly in the .csproj file.

Precautions

While C# 8 is technically compatible with the .NET Framework, some features may be supported in limited support or additional workarounds are required. It is recommended to be cautious and thoroughly test any C# 8 code for the .NET Framework.