-->

var openTextPath = 「開きたいテキストファイルのパス」;
ProcessStartInfo pInfo = new ProcessStartInfo();
pInfo.FileName = "notepad";
pInfo.Arguments = openTextPath;
pInfo.ErrorDialog = true;
pInfo.UseShellExecute = true;
Process.Start(pInfo);

Process[] ps = Process.GetProcessesByName("notepad");
foreach (Process p in ps)
{
p.CloseMainWindow();
p.Close();
}