Hi Esmond,
Well, first I would like to let you know that if you're using OpenFileDialog to select a file, it returns the full path of selected file or files, So you won't need at all to indicate the drive to search a file. Just pass the complete file name which is absolute. If you're using any other mean to select a file then please specify that in your question using "Improve question" link in your post so that we can get a better picture of your problem.
You can parse contents from a text file like this:
string DataInTextFile = "";
OpenFileDialog ofd = new OpenFileDialog();
ofd.Filter = "Text Files|*.txt";
DialogResult dr = ofd.ShowDialog();
if (dr == DialogResult.OK)
{
StreamReader SR = new StreamReader(ofd.FileName);
DataInTextFile = SR.ReadToEnd();
SR.Close();
SR.Dispose();
}
MessageBox.Show(DataInTextFile);
Go ahead with the manipulation on text file contents using "DataInTextFile" string object.
Hope this helps.
Happy Coding :)