OppaiSharp is a C# port of oppai-ng by Francesco149, based on its Java port Koohii. It allows you to calculate star ratings and PP values for any osu!standard map, just like oppai(-ng) does.
It is "licensed" under the Unlicense, so you're free to do whatever you want with it. But giving me or Francesco149 a bit of credit doesn't hurt, and I would appreciate it if you didn't claim it as your own :)
Although it uses the same pp calc math as osu!, result will differ because of different slider length calculation! If you need values that are exactly the same as in osu! use official performance calculator
//create a StreamReader for your beatmap
byte[] data = new WebClient().DownloadData("https://osu.ppy.sh/osu/774965");
var stream = new MemoryStream(data, false);
var reader = new StreamReader(stream);
//read a beatmap
var beatmap = Beatmap.Read(reader);
//calculate star ratings for HDDT
Mods mods = Mods.Hidden | Mods.DoubleTime;
var diff = new DiffCalc().Calc(beatmap, mods);
Console.WriteLine(string.Format("Star rating: {0:F2} (aim stars: {1:F2}, speed stars: {2:F2})",
diff.Total, diff.Aim, diff.Speed));
//calculate the PP for a play on this map
//the play has no misses so we don't specify them
//accuracy should be in 0..1 range
var pp = new PPv2(new PPv2Parameters(beatmap, diff, accuracy: 98.5 / 100, mods: mods));
Console.WriteLine(string.Format("Play is worth {0:F2}pp ({1:F2} aim pp, {2:F2} acc pp, {3:F2} speed pp) ",
pp.Total, pp.Aim, pp.Acc, pp.Speed));