This change updates the target frameworks for various C# projects within Protobuf to .NET 8.0, while also maintaining netstandard2.0 for the core library. Support for older frameworks like net45, net50, and netstandard1.1 has been removed.
This updates our project to match our support policy in https://github.com/google/oss-policies-info/blob/main/foundational-dotnet-support-matrix.md which actually dropped support for net45, net50, etc long ago.
PiperOrigin-RevId: 919111969
This has one important packaging change: the netstandard version now
depends (implicitly) on netstandard1.6.1 rather than on individual
packages. This is the preferred style of dependency, and shouldn't
affect any users - see http://stackoverflow.com/questions/42946951
for details.
The tests are still NUnit, but NUnit doesn't support "dotnet test"
yet; the test project is now an executable using NUnitLite. (When
NUnit supports dotnet test, we can adapt to it.)
Note that the project will now only work in Visual Studio 2017 (and
Visual Studio Code, and from the command line with the .NET Core
1.0.0 SDK); Visual Studio 2015 does *not* support this project file
format.
