Top 3 C# Polyfill Projects

Nullable

2 179 4.2 C#

A source code only package which allows you to use .NET's nullable attributes in older target frameworks like .NET Standard 2.0 or the "old" .NET Framework.

Project mention: Why would you use Windows containers ? | /r/programming | 2023-05-08

https://github.com/manuelroemer/Nullable (backport nullable attributes so you can use things like [NotNullWhen]). Alternatively, https://github.com/tunnelvisionlabs/ReferenceAssemblyAnnotator/ is even niftier, as it gives you .NET 5's annotations as well. So the compiler tells you whether a Dictionary.TryGetValue result is nullable or not based on annotations that didn't exist in .NET Standard 2.0.

Theraot

4 158 6.7 C#

Backporting .NET and more: LINQ expressions in .net 2.0 - nuget Theraot.Core available.
WorkOS

workos.com sponsored

The modern identity platform for B2B SaaS. The APIs are flexible and easy-to-use, supporting authentication, user identity, and complex enterprise features like SSO and SCIM provisioning.
IsExternalInit

7 94 0.0 C#

A source code only package which allows you to use C# 9's init and record features in older target frameworks like .NET Standard 2.0 or the "old" .NET Framework by providing a polyfill for the IsExternalInit class.

Project mention: Multi-targeted library, with a recent Language Version | /r/dotnet | 2023-05-09

You can also just reference the https://github.com/manuelroemer/IsExternalInit NuGet package.

NOTE: The open source projects on this list are ordered by number of github stars. The number of mentions indicates repo mentiontions in the last 12 Months or since we started tracking (Dec 2020).