Comment by pbgodwin

Windows Forms is only a subset of the framework. I don't think exposing Windows Forms as an open source GUI stack would be worth while. It's old and clunky compared to WPF and XAML.

Even without a open source GUI stack the open source .Net Runtime and Framework provide value to .Net developers like myself who enjoy C# (and the F# lovers too!) to develop on Linux, FreeBSD, and OS X with much less pain than in the past. Not fitting each and every scenario doesn't diminish its significance.