gridview binds datatable

public partial class Default16 : System.Web.UI.Page
{
    int result1, result2, result3;
    private void SetInitialRow()
    {
        DataTable dt = new DataTable();
        DataRow dr = null;

        dt.Columns.Add(new DataColumn("RowNumber", typeof(string)));
        dt.Columns.Add(new DataColumn("Column1", typeof(string)));
        dt.Columns.Add(new DataColumn("Column2", typeof(string)));
        dt.Columns.Add(new DataColumn("Column3", typeof(string)));
        dt.Columns.Add(new DataColumn("Column4", typeof(string)));
        dt.Columns.Add(new DataColumn("Column5", typeof(string)));
        dt.Columns.Add(new DataColumn("Column6", typeof(string)));
        dr = dt.NewRow();

        dr["RowNumber"] = 1;
        dr["Column1"] = string.Empty;
        dr["Column2"] = string.Empty;
        dr["Column3"] = string.Empty;
        dr["Column4"] = string.Empty;
        dr["Column5"] = string.Empty;
        dr["Column6"] = string.Empty;

        dt.Rows.Add(dr);

        ViewState["CurrentTable"] = dt;

        Gridview1.DataSource = dt;
        Gridview1.DataBind();

        DropDownList ddl1 = (DropDownList)Gridview1.Rows[0].Cells[2].FindControl("DropDownList1");
        FillDropDownList(ddl1);
        DropDownList ddl2 = (DropDownList)Gridview1.Rows[0].Cells[3].FindControl("DropDownList2");
        FillDropDownList1(ddl2);

    }
    private void AddNewRowToGrid()
    {
        int rowIndex = 0;

        if (ViewState["CurrentTable"] != null)
        {
            DataTable dtCurrentTable = (DataTable)ViewState["CurrentTable"];
            DataRow drCurrentRow = null;
            int icount, count;
            icount = Gridview1.Rows.Count;
            count = Gridview1.Rows.Count;
            if (icount==1)
            {
                icount = 0;
            }
                   
            if (dtCurrentTable.Rows.Count > 0)
            {
                for (int i = icount; i <= count; i++)
                {
                    if (i==0)
                    {
                        dtCurrentTable.Rows[0]["RowNumber"] = i+1;
                        dtCurrentTable.Rows[0]["Column1"] = string.Empty;
                        dtCurrentTable.Rows[0]["Column2"] = string.Empty;
                        dtCurrentTable.Rows[0]["Column3"] = string.Empty;
                        dtCurrentTable.Rows[0]["Column4"] = string.Empty;
                        dtCurrentTable.Rows[0]["Column5"] = string.Empty;
                        dtCurrentTable.Rows[0]["Column6"] = string.Empty;
                        
                    }
                    else
                    {
                        TextBox box1 = (TextBox)Gridview1.Rows[rowIndex].Cells[1].FindControl("TextBox1");
                        TextBox box2 = (TextBox)Gridview1.Rows[rowIndex].Cells[4].FindControl("TextBox2");
                        TextBox box3 = (TextBox)Gridview1.Rows[rowIndex].Cells[5].FindControl("TextBox3");
                        TextBox box4 = (TextBox)Gridview1.Rows[rowIndex].Cells[6].FindControl("TextBox4");
                        DropDownList ddl1 = (DropDownList)Gridview1.Rows[rowIndex].Cells[2].FindControl("DropDownList1");
                        DropDownList ddl2 = (DropDownList)Gridview1.Rows[rowIndex].Cells[3].FindControl("DropDownList2"); 
                       
                        drCurrentRow = dtCurrentTable.NewRow();
                        drCurrentRow["RowNumber"] = i + 1;
                        dtCurrentTable.Rows.Add(drCurrentRow);
                        dtCurrentTable.Rows[i]["Column1"] = box1.Text;
                        dtCurrentTable.Rows[i]["Column2"] = ddl1.SelectedItem.Text;
                        dtCurrentTable.Rows[i]["Column3"] = ddl2.SelectedItem.Text;
                        dtCurrentTable.Rows[i]["Column4"] = box2.Text;
                        dtCurrentTable.Rows[i]["Column5"] = box3.Text;
                        dtCurrentTable.Rows[i]["Column6"] = box4.Text;
                        rowIndex++;
                    }
                }
                ViewState["CurrentTable"] = dtCurrentTable;
                Gridview1.DataSource = dtCurrentTable;
                Gridview1.DataBind();
            }
        }
        else
        {
            Response.Write("ViewState is null");
        }
        SetPreviousData();
    }

    private void SetPreviousData()
    {
        int rowIndex = 0;
        if (ViewState["CurrentTable"] != null)
        {
            DataTable dt = (DataTable)ViewState["CurrentTable"];
            if (dt.Rows.Count > 0)
            {
                for (int i = 0; i < dt.Rows.Count; i++)
                {
                    TextBox box1 = (TextBox)Gridview1.Rows[rowIndex].Cells[1].FindControl("TextBox1");
                    TextBox box2 = (TextBox)Gridview1.Rows[rowIndex].Cells[4].FindControl("TextBox2");
                    TextBox box3 = (TextBox)Gridview1.Rows[rowIndex].Cells[5].FindControl("TextBox3");
                    TextBox box4 = (TextBox)Gridview1.Rows[rowIndex].Cells[6].FindControl("TextBox4");
                    DropDownList ddl1 = (DropDownList)Gridview1.Rows[rowIndex].Cells[2].FindControl("DropDownList1");
                    DropDownList ddl2 = (DropDownList)Gridview1.Rows[rowIndex].Cells[3].FindControl("DropDownList2");

                    FillDropDownList(ddl1);
                    FillDropDownList1(ddl2); 

                    box1.Text = dt.Rows[i]["Column1"].ToString();

                    ddl1.ClearSelection();

                    if (ddl1.Items.FindByText(dt.Rows[i]["Column2"].ToString()) != null)
                    {
                        ddl1.Items.FindByText(dt.Rows[i]["Column2"].ToString()).Selected = true;
                    }
                    if (ddl2.Items.FindByText(dt.Rows[i]["Column3"].ToString()) != null)
                    {
                        ddl2.Items.FindByText(dt.Rows[i]["Column3"].ToString()).Selected = true;
                    }
                    box2.Text = dt.Rows[i]["Column4"].ToString();
                    box3.Text = dt.Rows[i]["Column5"].ToString();
                    box4.Text = dt.Rows[i]["Column6"].ToString();
                    Button b2 = (Button)Gridview1.Rows[rowIndex].Cells[7].FindControl("ButtonAdd123");

                    if (i == 0)
                    {
                        b2.Visible = true;
                        box1.ReadOnly = false ;
                        box2.ReadOnly = false;
                        box3.ReadOnly = false;
                        box4.ReadOnly = false;
                        ddl1.Enabled = true ;
                        ddl2.Enabled = true;
                    }
                    else
                    {
                        b2.Visible = false;
                        box1.ReadOnly = true;
                        box2.ReadOnly = true;
                        box3.ReadOnly = true;
                        box4.ReadOnly = true;
                        ddl1.Enabled = false;
                        ddl2.Enabled = false;
                    }
                    rowIndex++;
                }
            }
        }
    }


    private ArrayList GetDummyData()
    {
        ArrayList arr = new ArrayList();
        arr.Add(new ListItem("Item1","1"));
        arr.Add(new ListItem("Item2","2"));
        arr.Add(new ListItem("Item3","3"));
        arr.Add(new ListItem("Item4","4"));
        arr.Add(new ListItem("Item5","5"));
        return arr;
    }

    private void FillDropDownList(DropDownList ddl)
    {
        ArrayList arr = GetDummyData();

        foreach (ListItem item in arr)
        {
            ddl.Items.Add(item);
        }
    }

    private ArrayList GetDummyData1()
    {
        ArrayList arr = new ArrayList();
        arr.Add(new ListItem("KGS","1"));
        arr.Add(new ListItem("NOS","2"));
        arr.Add(new ListItem("LITRE","3"));
        arr.Add(new ListItem("PACKS","4"));
        arr.Add(new ListItem("BALE","5"));
        return arr;
    }

    private void FillDropDownList1(DropDownList ddl)
    {
        ArrayList arr = GetDummyData1();
        foreach (ListItem item in arr)
        {
            ddl.Items.Add(item);
        }
    } 

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            SetInitialRow();
        }
    }

   
    protected void ButtonAdd123_Click(object sender, EventArgs e)
    {
        AddNewRowToGrid();     
    }
    
    protected void Gridview1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            TextBox TextBox2 = (TextBox)e.Row.FindControl("TextBox2");
            TextBox TextBox3 = (TextBox)e.Row.FindControl("TextBox3");
            TextBox TextBox4 = (TextBox)e.Row.FindControl("TextBox4");
        }
    }



    protected void TextBox3_TextChanged(object sender, EventArgs e)
    {
        try
        {
            int value;
            for (int i = 0; i < Gridview1.Rows.Count; i++)
            {
                GridViewRow row = Gridview1.Rows[i];
                TextBox TextBox2 = (TextBox)row.FindControl("TextBox2");
                TextBox TextBox3 = (TextBox)row.FindControl("TextBox3");
                TextBox TextBox4 = (TextBox)row.FindControl("TextBox4");
                value = Convert.ToInt32(TextBox2.Text) * Convert.ToInt32(TextBox3.Text);
                TextBox4.Text = Convert.ToString(value);
            }
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }

}