System.IO.StreamReader file = new System.IO.StreamReader(textBoxIdModele.Text);
string Text = file.ReadToEnd();
file.Close();

List<string> lines = new List<string>();
 
string line;
System.IO.StreamReader file = new System.IO.StreamReader(@"c:\test.txt");
while ((line = file.ReadLine()) != null)
{
   lines.Add(line);
}