insert里的字符串参数变量应该怎么写?

using (SqlConnection conn = DB.crteatConn())
  {
  using (SqlCommand cmd = new SqlCommand())
  {
  cmd.Parameters.Add("@userName", SqlDbType.NVarChar, 20).Value = HiddenField1.Value;
  cmd.Parameters.Add("@DateTime", SqlDbType.NVarChar, 8).Value = DateTime.Now.ToString();
  cmd.Parameters.Add("@Message", SqlDbType.NVarChar, 150).Value = txtMessage.Text;
  cmd.CommandText = "insert into ChatRoom values(@userName,@DateTime,@Message)";
  cmd.Connection = conn;
  conn.Open();
  cmd.ExecuteNonQuery();
  }