The whole point of .NET is that you can write in any compliant language, including C#, C++/CLI, VB.NET, and F#
List of CLI languages
https://en.wikipedia.org/wiki/List_of_CLI_languages
The C# compiler is csc.exe and is part of the .NET framework which can be found in:
C:\Windows\Microsoft.NET\Framework\v4.0.30319
And also, on 64-but systems:
C:\Windows\Microsoft.NET\Framework64\v4.0.30319
(version 4.0 build 30319 is the original release of .NET 4.0, but the version found in the folder should be rather newer.)
You can run csc.exe from the command line easily enough, which I find handy when I'm writing small apps. It's especially handy if you need to write a utility to check something on a Windows PC where there are no (other) build tools.
Command-line build with csc.exe
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/command-line-building-with-csc-exe
(For VB.NET code you can so the same thing with vbc.exe, and JavaScript with jsc.exe)
Microsoft's .NET frameworks are written at least predominantly in C# -- C#, .NET, the Common Language Runtime (CLR) and Common Intermediate Language (CIL) all co-evolved after all. If you're interested, check out:
.NET Platform -- Home of the open source .NET platform
https://github.com/dotnet
Andy