var FD = new System.Windows.Forms.OpenFileDialog();
if (FD.ShowDialog() == System.Windows.Forms.DialogResult.OK) {
string fileToOpen = FD.FileName;
System.IO.FileInfo File = new System.IO.FileInfo(FD.FileName);
//OR
System.IO.StreamReader reader = new System.IO.StreamReader(fileToOpen);
//etc
}