Gridview and Listbox

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Data;

public partial class About : System.Web.UI.Page
{
    private String strConnection = "Data Source=.\\SQLEXPRESS;Initial Catalog=INDHS;Integrated Security=True";

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

    private void SetInitialRow()
    {
        DataTable dt = new DataTable("PatAllergy");
        DataRow dr = null;
       
        dt.Columns.Add(new DataColumn("AllegType", typeof(string)));
        dt.Columns.Add(new DataColumn("AllgName", typeof(string)));
       
        dr = dt.NewRow();
    
        dr["AllegType"] = string.Empty;
        dr["AllgName"] = string.Empty;
      
        dt.Rows.Add(dr);

        //Store the DataTable in ViewState
        ViewState["PatAllergy"] = dt;

        

        Gridview1.DataSourceID = null;
        Gridview1.DataSource = dt;
    }
    private void AddNewRowToGrid()
    {

        int rowIndex = 0;
        if (Gridview1.Rows.Count > 0)
        {
           
            DataTable dt1 = new DataTable("PatAllergy");
            DataTable dt = (DataTable)ViewState["PatAllergy"];
            DataRow dr = null;
            if (dt.Rows.Count > 0)
            {
                for (int i = 1; i <= dt.Rows.Count; i++)
                {
                     dr = dt.NewRow();
                 
                    dr["AllegType"] = Gridview1.Rows[rowIndex].Cells[0].FindControl("Allergy Type");
                    dr["AllgName"] = Gridview1.Rows[rowIndex].Cells[1].FindControl("Allergy Name");
                 

                    rowIndex++;
                }

                //add new row to DataTable
                dt.Rows.Add(dr);
                //Store the current data to ViewState
                ViewState["PatAllergy"] = dt;

                //Rebind the Grid with the current data
                Gridview1.DataSource = dt;
                Gridview1.DataBind();
            }
        }
        else
        {
            Response.Write("ViewState is null");
        }

        //Set Previous Data on Postbacks
        SetPreviousData();
    }
    private void SetPreviousData()
    {
        int rowIndex = 0;
        if (ViewState["PatAllergy"] != null)
        {
            DataTable dtCurrentTable = (DataTable)ViewState["PatAllergy"];
            DataRow drPrevRow;
            DataTable dt = (DataTable)ViewState["PatAllergy"];
            if (dt.Rows.Count > 0)
            {
                for (int i = 1; i < dt.Rows.Count; i++)
                {
                    drPrevRow = dtCurrentTable.NewRow();
                    drPrevRow["AllegType"] = Gridview1.Rows[rowIndex].Cells[0].FindControl("AllegType");
                    drPrevRow["AllgName"] = Gridview1.Rows[rowIndex].Cells[1].FindControl("AllgName");
                  

                    drPrevRow["AllegType"] = dt.Rows[i]["AllegType"].ToString();
                    drPrevRow["AllgName"] = dt.Rows[i]["AllgName"].ToString();
                 

                    rowIndex++;

                }
            }
        }
    }

    protected void Button1_Click(object sender, EventArgs e)
    {      
        AddNewRowToGrid();
    }
   
    
    protected void lstAllergy_SelectedIndexChanged(object sender, EventArgs e)
    {
        // int CountryID = Convert.ToInt32(ddlCountry.SelectedValue);
        SqlConnection con = new SqlConnection(strConnection);
        con.Open();
        SqlCommand cmd = new SqlCommand("select AllgName from PatAllergy where AllegType='" + lstAllergy.SelectedItem.Text + "'", con);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
        con.Close();
        lstAllgyItem.DataSource = ds;
        lstAllgyItem.DataTextField = "AllgName";
        lstAllgyItem.DataBind();
    }
}

Can you share your code?