How to check data in different rows in gridview

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C#

ASP.NET

I am currently getting data from database, then if i checked the checkbox more than 2 rows, it will sum up the total of SENDQTY and send the details from GridView3 to GridView4.

What I have tried:

Now how can i check if the STOCKCODE from the 2 rows is different, it will prompt error message?

protected void showGridView()
        {
            int match = 0;
            int total = 0;
          
            DataTable dt = new DataTable();
            dt.Columns.AddRange(new DataColumn[9] { new DataColumn("DODetailID"), new DataColumn("SALESORDERNO"), new DataColumn("STOCKCODE"), new DataColumn("SENDQTY"), new DataColumn("scanflag"), new DataColumn("REJECT"), new DataColumn("DispatchOrderID"), new DataColumn("DispatchDATE"), new DataColumn("DELORDERNO") });
            foreach (GridViewRow row in GridView3.Rows)
            {
                if (row.RowType == DataControlRowType.DataRow)
                {
                    CheckBox CheckBox1 = (row.Cells[0].FindControl("CheckBox1") as CheckBox);

                    //int qty = Convert.ToInt32(row.Cells[4].Text);

                    if (CheckBox1.Checked)
                    {
                        string DODetailID = (row.Cells[1].FindControl("DODetailID") as Label).Text;
                        string SALESORDERNO = (row.Cells[2].FindControl("SALESORDERNO") as Label).Text;
                        string STOCKCODE = (row.Cells[3].FindControl("STOCKCODE") as Label).Text;                 
                        int SENDQTY = Convert.ToInt32((row.Cells[4].FindControl("SENDQTY") as Label).Text);
                        CheckBox scanflag = (row.Cells[5].FindControl("scanflag") as CheckBox);
                        string REJECT = (row.Cells[6].FindControl("REJECT") as Label).Text;
                        string DispatchOrderID = (row.Cells[7].FindControl("DispatchOrderID") as Label).Text;
                        string DispatchDATE = (row.Cells[8].FindControl("DispatchDATE") as Label).Text;
                        string DELORDERNO = (row.Cells[9].FindControl("DELORDERNO") as Label).Text;
                        Session["STOCKCODE"] = STOCKCODE.ToString();
                       
                            dt.Rows.Add(DODetailID, SALESORDERNO, STOCKCODE, SENDQTY, scanflag, REJECT, DispatchOrderID, DispatchDATE, DELORDERNO);
                            match++;
                       
                            total = total + SENDQTY;
                            GridView4.DataSource = dt;
                            GridView4.DataBind();
                        
                       
                    }
                   
                }
            }
       

        }

   protected void btnContinue_Click(object sender, EventArgs e)
        {  
               showGridView();

                mp3.Show(); // this is showing GridView4
        }