C# 数据库操作，

方法如下，执行插入数据库操作，一两个客户端没事，多个客户端时，偶尔出现，“未将对象引用设置到对象的实例”哪位大侠帮看看什么原因？
       
 public static bool EditDatabase(string sql)
        {
            lock (typeof(SQLServerDAL.SQLHelper))
            {
                long lcount = 0;
                SqlTransaction myTrans = null;                try
                {
                    myTrans = myConnection.BeginTransaction();                    SqlCommand command = new SqlCommand(sql, myConnection);                    command.Transaction = myTrans;
                    command.ExecuteNonQuery();                    if (myTrans != null)
                    {
                        myTrans.Commit();                        lcount = 1;
                    }                    if (command != null)
                    {
                        command.Dispose();
                    }                    if (lcount == 0)
                    {
                        SystemError.CH_Wms_WriteXmlLog("DB-EditDatabase", "执行失败", 1);                        return false;
                    }
                    else
                    {
                        return true;
                    }
                }
                catch
                {
                    myTrans.Rollback();                                        return false;
                }
            }
        }