LastSharp is a project mainly written in C#, it's free.
Open Source Last.fm Library written in C#
.NET C# Client for the Last.fm API
Example usage:
LastfmApi _lastfm = new LastfmApi("b25b959554ed76058ac220b7b2e0a026");
var artist = _lastfm.artistGetInfo("Blur");
Console.WriteLine(artist.bio);