Source: so old it’s lost
Also C# (or should I say the .net framework) is now cross platform, which wasn’t really the case when I first saw this meme.
This joke made sense when instead of .net you could only use Mono with C# on other platforms, which wasn’t very good at the time.
I hosted my personal site using Mono over 10 years ago now and it mostly worked well. I contributed some code to Mono to fix a few edge cases where their behaviour deviated slightly from Microsoft’s.
Of course, I couldn’t actually look at Microsoft’s shared source code when doing that, so I had to just observe its outputs. At the time, Mono code had to all be clean-room implementations, since Microsoft’s shared source program, where they released parts of the .NET Framework 4.x source code publicly, had a very restrictive license that didn’t permit reuse (it wasn’t open-source). Even just looking at the code meant you couldn’t contribute to Mono.
I was very happy when .NET Core was announced and switched to a beta of 1.0 as soon as I could.