With the recent release of .NET Core, you can now use C# to write cross-platform codes.
This contains the 1.0 final release of the new MSBuild based .NET Core Tools.
The tools are supported on Windows, macOS and Linux. You can use them at the command-line, with VS Code or as part of Visual Studio 2017.
Using Visual Studio 2017, you can create .NET Standard libraries, .NET Core apps/libraries and ASP.NET web projects that use the new csproj format.
Get started, check out the following video.