A pull request was opened during the weekend for initial RISC-V build support for the .NET runtime. The code has been successfully tested for cross-building for RISC-V and even running a simple "hello world" type application on RISC-V.
With Microsoft having not brought Windows to RISC-V, you may be curious who is doing this work... It's coming from a Samsung engineer, Dong-Heon Jung, who is involved with the .NET platform team and works on it as part of his role at Samsung.
See this pull request , which was merged overnight, for the initial bits of the Linux RISC-V support for the open-source Microsoft .NET run-time.