There is a project called Raspberry♯ (RaspberrySharp)
At the site you find a sub project called
raspberry-sharp-io which contians examples on how to use the SPI, I2C, GPIO and some other general purpose things.
You can get the GPIO using Nuget
but to use the SPI you will need to compile or add the project and then reference
It is only very new this project so you might find some issues but it seems like it works. You could also look into the source and see how the wrapped everything to call SPI and that could give you some insight for your own library.