How to fire Gridview dropdownlist selected index changed

See more:

C#

ASP.NET

In my Gridview has following

Two dropdown list,
Three TextBoxes.

When I Select the Dropdown list item,

The Gridview First textbox will be changed the value

For Example,

In my Gridview the Drop down list has Itemname

When I selectedChanged in the dropdownlist, The Rate TextBox in the Gridview display the rate of the textbox. I give the following coding, But the Dropdown list get null value,

protected void CMBItem_SelectedIndexChanged(object sender, EventArgs e)
   {
       DropDownList lb = (DropDownList)sender;
       GridViewRow gvRow = (GridViewRow)lb.NamingContainer;
         if (ViewState["CurrentTable"] != null)
       {
           DataTable dt = (DataTable)ViewState["CurrentTable"];
           string Iname = dt
           connect.cn();
           SqlCommand sqlcmd = new SqlCommand("Get_Item_PurchasePrice", connect.cn());
             sqlcmd.Parameters.AddWithValue("@ITEMCODE", lb.SelectedValue);
           sqlcmd.CommandType = CommandType.StoredProcedure;
           SqlDataAdapter da = new SqlDataAdapter(sqlcmd);
           DataTable dt1 = new DataTable();
           da.Fill(dt1);
           if (dt1.Rows.Count > 0)
           {
               gvRow.Cells[4].Text = dt1.Rows[0][0].ToString();
               gvRow.Cells[5].Text = dt1.Rows[0][1].ToString();
           }
           //Store the current data in ViewState for future reference
           //ViewState["CurrentTable"] = dt;
           ////Re bind the GridView for the updated data
           //Gridview2.DataSource = dt;
           //Gridview2.DataBind();
       }
       //SetPreviousData();
   }

Please help me for this problem