Using a DropDownList with Postback
Prerequisites
Description of Video
This lecture goes over how to create and use a dropdownlist and have it trigger the contents of a second dropdownlist. In addition it demonstrates how to create a DataTable in Code. Code for this example is in C#
Video
Reference Materials
Default.aspx.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace ComboBoxExample
{
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
ddlStates.AutoPostBack = true;
ddlStates.DataSource = ResourceCommon.DataTableStates();
ddlStates.DataTextField = ResourceCommon.DataTable_States_StateName;
ddlStates.DataValueField = ResourceCommon.DataTable_States_StateAbbreviation;
ddlStates.DataBind();
}
}
protected void ddlStates_SelectedIndexChanged(object sender, EventArgs e)
{
ddlCities.Visible = true;
ddlCities.DataSource = ResourceCommon.DataTableCities(ddlStates.SelectedValue);
ddlCities.DataTextField = "CityName";
ddlCities.DataValueField = "CityName";
ddlCities.DataBind();
}
}
}
Default.aspx
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="ComboBoxExample._Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:DropDownList runat="server" ID = "ddlStates"
onselectedindexchanged="ddlStates_SelectedIndexChanged" ></asp:DropDownList><br />
<asp:DropDownList runat="server" ID = "ddlCities" visible="false"></asp:DropDownList><br />
</div>
</form></body></html>
ResourceCommon.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data;
namespace ComboBoxExample
{
public class ResourceCommon
{
public const String DataTable_States_StateName = "StateName";
public const String DataTable_States_StateAbbreviation = "StateAbbreviation";
public static DataTable DataTableStates()
{
DataTable dt = new DataTable("States");
dt.Columns.Add("StateName", Type.GetType("System.String"));
dt.Columns.Add("StateAbbreviation", Type.GetType("System.String"));
FillRowState(dt, "Florida", "FL");
FillRowState(dt, "Georgia", "GA");
FillRowState(dt, "Alabama", "AL");
return dt;
}
private static void FillRowState(DataTable dt, String StateName, String StateAbbreviation)
{
DataRow dr = dt.NewRow();
dr["StateName"] = StateName;
dr["StateAbbreviation"] = StateAbbreviation;
dt.Rows.Add(dr);
}
public static DataTable DataTableCities(String StateAbbreviation)
{
DataTable dt = new DataTable("Cities");
dt.Columns.Add("CityName", Type.GetType("System.String"));
switch (StateAbbreviation)
{
case "FL":
FillRowCities(dt, "Jacksonville");
FillRowCities(dt, "Orlando");
FillRowCities(dt, "Miami");
FillRowCities(dt, "Tampa");
break;
case "GA":
FillRowCities(dt, "Atlanta");
FillRowCities(dt, "Savannah");
break;
case "AL":
FillRowCities(dt, "Birminingham");
FillRowCities(dt, "Huntsville");
break;
}
return dt;
}
private static void FillRowCities(DataTable dt, String CityName)
{
DataRow dr = dt.NewRow();
dr["CityName"] = CityName;
dt.Rows.Add(dr);
}
}
}
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