Getting the relative uri from the absolute uri

What you are looking for is either Uri.AbsolutePath or Uri.PathAndQuery. PathAndQuery will include the querystring (which you don't have in your question), while AbsolutePath will give you just the path.

Console.WriteLine(uri.PathAndQuery);
// or 
Console.WriteLine(uri.AbsolutePath);

...outputs the following results for both...

/foo/bar