You need Environment.GetFolderPath
.
string myDocuments = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
string desktop = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);
Don't concatenate strings to create path, use Path.Combine
instead. So when you need subfolder of desktop you'll use
string subFolder = Path.Combine(desktop,"MySubFolderName");
So in your case
StreamReader fileitem = new StreamReader(Path.Combine(desktop,"dataset.txt"));