I think MichaelGG is thinking of the Rotor "shared source" implementation for BSD, which was released around the same time as .NET for Windows as a proof of concept for implementing the .NET standards on other platforms. (These standards included CIL and the Common Type System, not just C#.) But as Michael says though it fell way short of full .NET, lacking many libraries and a lot of the optimisations of the Windows .NET implementation; and it wasn't open source in that you couldn't fork it, you could only look at the code 'under glass.'
The .NET design has always been multiplatform. MS has released code over a decade ago to demonstrate this: https://msdn.microsoft.com/en-us/library/cc749640.aspx
https://en.wikipedia.org/wiki/.NET_Framework