Code:

string fullPathname = openFileDialog.FileName;
            FileInfo src = new FileInfo(fullPathname);
            filename.Text = src.Name;          
            
            source.Text = "";

            
            TextReader reader = src.OpenText();

            
            string line = reader.ReadLine();

            while (line != null) 
            {
                source.Text += line + '\n';                
                line = reader.ReadLine();
            }            
            reader.Close();