dropdownlist不从OnSelectedIndexChanged的表中获取值

Question

am trying to fetch values from database table on dropdownlist value change and display them in textbox. While selecting any value from the dropdownlist the page is refreshing but no values are displaying in the textbox and following are the codes:

Default.aspx

<asp:DropDownList ID="DropDownList1" runat="server" AppendDataBoundItems="true" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" AutoPostBack="true">
    <asp:ListItem Text="<Select Subject>" Value="0" />
</asp:DropDownList>

Default.aspx.cs

 protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        string ddl2value = DropDownList1.SelectedValue.ToString();
        // fillDropdown3(ddl3, ddl2value);
        SqlConnection objConn2 = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
        SqlCommand objCmd2;
        SqlDataReader objRdr2;
        //  String strCmd2;
        objConn2.Open();




        objCmd2 = new SqlCommand("SELECT code, rank, address FROM agen_mast WHERE name = " +
        "'" + ddl2value.ToString() + "'", objConn2);
        objRdr2 = objCmd2.ExecuteReader();



        while (objRdr2.Read())
        {

            TextBox9.Text = (string)objRdr2["code"].ToString();
            TextBox8.Text = (string)objRdr2["address"].ToString().ToUpper();
            TextBox10.Text = (string)objRdr2["rank"].ToString().ToUpper();
        }

        objRdr2.Close();
        objConn2.Close();
     //   Response.Write(ddl2value.ToString());
    }

Answer 1

You could try something like this:

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        if(DropDownList1.SelectedValue !="-1"){
        string ddl2value = DropDownList1.SelectedValue.ToString();
        SqlConnection objConn2 = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
        SqlCommand objCmd2;
        SqlDataReader objRdr2;
        objConn2.Open();

        objCmd2 = new SqlCommand("SELECT code, rank, address FROM agen_mast WHERE name = " +
        "'" + ddl2value + "'", objConn2);
        objRdr2 = objCmd2.ExecuteReader();

        while (objRdr2.Read())
        {

            TextBox9.Text = (string)objRdr2["code"].ToString();
            TextBox8.Text = (string)objRdr2["address"].ToString().ToUpper();
            TextBox10.Text = (string)objRdr2["rank"].ToString().ToUpper();
        }

        objRdr2.Close();
        objConn2.Close();
        }
    }

And add a dummy ListItem with Value -1 as the first item in the DropDownList1 in the .aspx side. By the way, make sure you are sending the correct parameter to SqlCommand . Right now you are looking for a record with Name = 0 . Also, ddl2Value is already of type string so you don't need to call ToString() inside SqlCommand