求帮助，急。未实例化的问题。

public static double AvgFromDB(int id, DateTime time, int userID)
        {
            DateTime startTime = DateTime.Parse( time.Year.ToString() + "-" + time.Month.ToString() + "-01 00:00:00");
            string sqlString = "select average from HourAssessmentMonthScore where ownerId ='" + id + "'and workgroup = '" + userID + "' and date = '" + startTime + "'";
            string returnValue = runSQL(sqlString);
            if (returnValue == null || returnValue.Length==0)
                //throw new Exception("error no data!");
                return Convert.ToDouble(0);
            else
                return Convert.ToDouble(returnValue);
        }

这段代码怎么改始终报object reference not set to an instance of an boject。由于某些原因暂时无法调试，请高手帮忙看看问题到底出在哪。谢谢，在线等。
[解决办法]
是不是runSQL(sqlString); 这个方法中报的错？
[解决办法]
问题应该在runSQL(sqlString)， 把这个方法粘出来看看
[解决办法]
object objValue = dbCommand.ExecuteScalar();
if(objValue==null)return string.Empty;
return objValue.ToString();