RMS: You shouldn't write software to use .NET. No exceptions.
The basic point is that Microsoft has patents over features in .NET, and its patent promise regarding free software implementations of those is inadequate. It may someday attack the free implementations of these features.
[…] If you already have a program in C#, by all means use a free platform to run it. But don't increase your exposure to the danger – don't write additional code in C#, and don't encourage people to make more use of C# programs.