检查数据库中是否已存在用户名和电子邮件

I am trying to check if the username and email have already been used before. When the user tries to enter a username that has been used before the error message displays. Same with email. However when a user tries to sign-up with a different username and email an error appears saying myCommand.ExecuteNonQuery must be outside rdr close but I can't put it there as it will then not be recognised.I am trying to check if the username and email