asp.net dropDownList选定的值null

Question

I am making a school application about articles publication. In a dropdownList I want to display the value that exists in database as default pre-selected value. DropdownList conatins "emertimi" from table "kategorite". When user selects a value it saves id of "kategoria_id" in table "artikulli". Here is my code behind

if (e.Item.ItemType == ListItemType.EditItem)
            {
                DropDownList drpdKategoria = e.Item.FindControl("drpdKategoria") as DropDownList;
                SqlConnection con = new SqlConnection(connection);
                string Qry = "select * from kategoria";
                string id = Request.QueryString["id"];
                SqlDataAdapter da = new SqlDataAdapter(Qry, con);
                DataSet ds = new DataSet();
                DataSet ds2 = new DataSet();
                DataSet ds3 = new DataSet();
                con.Open();
                da.Fill(ds);
                string kategoria_id = "select kategoria_id from artikulli where id='" + id + "'";
                SqlDataAdapter dk = new SqlDataAdapter(kategoria_id, con);
                dk.Fill(ds2);
                var kategoria_id_result = Convert.ToInt32(ds.Tables[0].Rows[0][0]);
                string emertimi = "select emertimi from kategoria where id='" + kategoria_id_result + "'";
                SqlDataAdapter de = new SqlDataAdapter(emertimi, con);
                de.Fill(ds3);

                drpdKategoria.DataSource = ds;
                drpdKategoria.DataValueField = "id";
                drpdKategoria.DataTextField = "emertimi";              
                drpdKategoria.DataBind();
                drpdKategoria.SelectedValue = drpdKategoria.Items.FindByText(emertimi).Value;
                con.Close();
                con.Dispose();
                ds.Dispose();
                ds2.Dispose();
                ds3.Dispose();
                da.Dispose();
                dk.Dispose();
                de.Dispose();
            }
        }

But it's showing this error: Object reference not set to an instance of an object . at this line: drpdKategoria.SelectedValue = drpdKategoria.Items.FindByText(emertimi).Value;

Answer 1

I guess emertimi contains a value that cannot be found in the dropdown list. Therefore, the FindByText will return null and getting the Value will result in this exception.

Test the result before you try to get the Value .

Also, instead of the separate Dispose calls, use using statements. And be aware of the SQL injection risk: use parameters instead.

Answer 2

That's because the item you're looking for has not been found. You should troubleshoot your code and figure out what emertimi is set to at run time.