Invalid attempt to read when no data is present.

        public void GetAllProgramFromOneUser(string UserName)
        {
            Guid UserID = GetUserID(UserName);
            SqlCommand SQLCom = new SqlCommand(@"SELECT Name, ProgramFileName, SourceCodeFileName " +
            "FROM Upload_program " +
            "WHERE UserID ='" + UserID + "'");
            SQLCom.Connection = DatabaseConnection;
            SQLCom.Connection.Open();
            string ReturnValues;
            SqlDataReader rdr = SQLCom.ExecuteReader();
            ReturnValues = rdr[0].ToString();
            SQLCom.Clone();
        }

        private Guid GetUserID(string UserName)
        {
            //SQL command
            SqlCommand SQLCom = new SqlCommand("SELECT UserId " +
            "FROM aspnet_Users " +
            "WHERE (UserName = '" + UserName + "')");
            //get the Database connection.
            SQLCom.Connection = DatabaseConnection;
            SQLCom.Connection.Open();
            //Get the data.
            Guid UserID = new Guid();
            UserID = Guid.Parse(Convert.ToString(SQLCom.ExecuteScalar()));
            SQLCom.Connection.Close();
            return UserID;
        }

SqlDataReader rdr = SQLCom.ExecuteReader();
if (rdr.Read())  // Kolla att det finns data (false if not), och stega fram readern dit.
{
            ReturnValues = rdr[0].ToString();
}

Guid UserID = new Guid();
UserID = Guid.Parse(Convert.ToString(SQLCom.ExecuteScalar()));