How can i raise text_changed() from item template in gridview

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ASP.NET

Hello buddy

This is helpful for me, but i had other problem in below code

TextBox txt = (TextBox)sender;
        GridViewRow gvr = (GridViewRow)txt.Parent.Parent;
        int i = gvr.RowIndex;
       
        txt = (TextBox)GridView1.Rows[i].FindControl("txtqty");
        str7 = txt.Text;
        if (i <= 0)
        {
            lbltotcost.Text =(Convert.ToInt32(lblpurchasecost.Text) * Convert.ToInt32(str7)).ToString();
            str3 = lbltotcost.Text;
        }
        else
        {
            lbltotcost.Text = (Convert.ToInt32(lblpurchasecost.Text) * Convert.ToInt32(str7)).ToString();
            str4=lbltotcost.Text;
        }
        str5 = str3;
        str6 = str4;

Here i want to store first value of textbox and go to have a loop for the second value.

My problem is first is not stored and becoming null so please can you help me for this.