"The .NET Framework (pronounced as 'dot net') is a proprietary software framework developed by Microsoft that runs primarily on Microsoft Windows".
The official documentation for both C# [1] and .NET Framework [2] live on Microsoft.com.
I would say the above constitutes a pretty Windows-centric existence relative to some other vendor-agnostic languages, but maybe we're saying different things. What did you mean?
Refs
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0: https://en.wikipedia.org/wiki/.NET_Framework
1: https://docs.microsoft.com/en-us/dotnet/csharp/
2: https://dotnet.microsoft.com/en-us/download/dotnet-framework