I recently released new phoenix channels library for .NET. It supports all .NET implementations, Framework, Core, and mono. Works in Xamarin projects (mobile) and Unity3D. Comparing to other libs written in .NET, this one is TPL based so API is a slightly different. Currently it do not contain presence helper class, but we are planning to implement it in one of next versions.
First production use will be as soon as we finish development of our mobile apps.