我在写一个程序,其中有涉及到对数据的修改,可是怎么也不能修改,系统老报告错误:"未将对象引用设置到对象实列" 
这是代码    


                try
         {
             int a=this.dataGrid1.CurrentCell.RowNumber;
                    string strCon="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=d:\\book.mdb";
                    OleDbConnection myConn=new OleDbConnection(strCon);
                    myConn.Open();
             string  strCom1="UPDATE book set CSharp='"+this.myDataSet.Tables["book"].Rows[a][2]+"',数学='"+this.myDataSet.Tables["book"].Rows[a][3]+"',英语="+this.myDataSet.Tables["book"].Rows[a][4]+",WINDOWS2K="+this.myDataSet.Tables["book"].Rows[a][5]+" ,共选课='"+this.myDataSet.Tables["book"].Rows[a][6]+",电路分析='"+this.myDataSet.Tables["book"].Rows[a][7]+",PHOTOSHOP='"+this.myDataSet.Tables["book"].Rows[a][8]+",平均分='"+this.myDataSet.Tables["book"].Rows[a][9]+"where 学号="+this.myDataSet.Tables["book"].Rows[a][1];
             
                    OleDbCommand inst=new OleDbCommand(strCom1,myConn);
                    System.Windows.Forms.MessageBox.Show(strCom1);
             inst.ExecuteNonQuery();
             OleDbDataAdapter myCommand2=new OleDbDataAdapter("SELECT * FROM sj1 order by 学号",myConn);
             this.myDataSet.Clear();
             myCommand2.Fill(myDataSet,"sj1");
             myConn.Close();
         }
         catch(Exception x)
         {MessageBox.Show("错误"+x.ToString(),"error");}