dynamic dropdown lists 1. objectives you will be able to use dropdown lists to solicit multiple...
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Dynamic Dropdown Lists
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Objectives
You will be able to Use Dropdown Lists to solicit multiple choice user input in an ASPX web page. Populate a Dropdown List at run time using data from a file.
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DropDownList
The ASPX DropDownList generates an HTML Select control.
A good way to get a user input from an extensive set of discrete choices.
The list should not be too long. If there are many choices divide it into
subsets. Use one DropDownList to select the subset. Use a second DropDownList to select from the
subset. Second DropDownList must be dynamically
populated based on choice from the first.3
Example
Select a city from a list of 275 largest US cities and display its population.
Let first DropDownList select a state. Second DropDownList includes just the
cities in that state.
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Largest US Cities
http://en.wikipedia.org/wiki/List_of_United_States_cities_by_population
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Data File
Download to desktop: http://www.cse.usf.edu/~turnerr/Web_Application_Design/Downl
oads/
File cities.csv
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Dropdown List Demo Create a new C# ASP.NET empty web site.
Name: Dropdown_List_Demo
Add Default.aspx
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Add Default.aspx
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Add New Folder
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New Folder for the Data File In Visual Studio
Rename the new folder App_Data. Note: This name is significant.
Open the App_Data folder in the website folder and copy the downloaded file cities.csv into it.
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The App_Data Folder
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Add Data File to the Project
Back in Visual Studio, right click on App_Data and add the file cities.csv to the project.
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Add cities.csv to the Project
Select “All Files”
cities.csv in the Project
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Class City
Let’s create a class to hold information about a city.
An Entity class Frequently used to create objects corresponding to
information in a database.
Object-Relational Mapping http://en.wikipedia.org/wiki/Object-relational_mapping
Automated tools are available Best to do manually until you have a thorough understanding.
(This is an especially simple example.)
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Class City
Add Class City
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Add Class City
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Class Cityusing System;
public class City : IComparable<City>
{
public int rank;
public String name;
public String state;
public int population;
public City(string line)
{
string[] info;
info = line.Split(',');
rank = int.Parse(info[0]);
name = info[1];
state = info[2];
population = int.Parse(info[3]);
}
public int CompareTo(City other)
{
int result = string.Compare(this.state, other.state);
if (result == 0)
{
result = string.Compare(this.name, other.name);
}
return result;
}
}
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In order for us to use the sort method for List<City>, the City class must implement the IComparable<City> interface and provide a CompareTo method.
Design the Page
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ddlState ddlCity
Read the Data
On Page_Load Read the file cities.csv Create a City object from each line. Add the City objects to a list. Sort the list by state name then city
name. Save the list as an Application variable. Populate the State Dropdown List.
It will never change.
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Default.aspx.cs
using System;
using System.IO;
using System.Collections.Generic;
using System.Web.UI.WebControls;
public partial class _Default : System.Web.UI.Page
{
List<City> cities = new List<City>();
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Default.aspx.cs
protected void Page_Load(object sender, EventArgs e)
{
if (Application["cities"] == null)
{
string path = Server.MapPath("App_Data/cities.csv");
using (StreamReader readFile = new StreamReader(path))
{
string line;
while ((line = readFile.ReadLine()) != null)
{
City city = new City(line);
cities.Add(city);
}
}
cities.Sort();
Application["cities"] = cities;
}
else
{
cities = (List<City>)Application["cities"];
}24
Default.aspx.cs (continued)
if (IsPostBack)
{
return;
}
// Initial page load only
ListItem li = new ListItem("", "");
ddlState.Items.Add(li);
string prev_state = "";
foreach (City city in cities)
{
if (city.state != prev_state)
{
// Add state to dropdown list.
li = new ListItem(city.state, city.state);
ddlState.Items.Add(li);
prev_state = city.state;
}
}
25Build and run.
Dropdown_List_Demo in Chrome
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Note scrollbar on Dropdown List
Add Event Handler
When the user selects a state, populate the City list with cities in that state.
Need an event handler for Selected Index Changed.
Double click on the States Dropdown List in the designer. Visual Studio adds event handler to
Default.aspx.cs27
SelectedIndexChanged Event Handler
Start with a stub.
protected void ddlState_SelectedIndexChanged(object sender, EventArgs e)
{
string selected_state = ddlState.SelectedValue;
lblPopulation.Text = "You selected " + selected_state;
}
Try it!
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Page in Chrome
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State selected.
Nothing happens!
What’s wrong?
Nothing Happens
There is no postback when an item is selected from ddlState.
We need AutoPostback.
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Set AutoPostBack
31Try again
Works as Expected
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Add code to populate the City DropDownList.
Populate ddlCity
protected void ddlState_SelectedIndexChanged(object sender, EventArgs e)
{
string selected_state = ddlState.SelectedValue;
ddlCity.Items.Clear();
ListItem li = new ListItem("", "");
ddlCity.Items.Add(li);
foreach (City city in cities)
{
if (city.state == selected_state)
{
li = new ListItem(city.name, city.name);
ddlCity.Items.Add(li);
}
}
}
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ddlCity Populated
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Add a SelectedIndexChanged event handler for ddlCity and set AutoPostBack.
ddlCity Event Handler
protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
{
int population = 0;
foreach (City city in cities)
{
if (city.name == ddlCity.SelectedValue)
{
population = city.population;
break;
}
}
lblPopulation.Text =
"Population of " + ddlCity.SelectedValue + " is " +
population.ToString();
}
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Try it!
Select Tampa
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Result
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A Rough Edge
When we select a new state, the previous city population stays on the page until we select a new city.
Solution: Clear the label on
SelectedIndexChanged.
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A Minor Bug
What happens if user selects the blank entry at the beginning of the list? (after selecting a nonblank entry)
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A Minor Bug
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Solution
Check for this and ignore the event.protected void ddlState_SelectedIndexChanged(object sender, EventArgs e)
{
lblPopulation.Text = "";
if (ddlState.SelectedIndex == 0)
{
return;
}
...
protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
{
lblPopulation.Text = "";
if (ddlCity.SelectedIndex == 0)
{
return;
}
...
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Works as Expected
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Blank initial item selected.
Summary We can populate a DropDownList with
information determed at run time. Contents of a file. Information determined by user input.
If we need an immediate action when the user selects an item, set AutoPostBack to true.
End of Presentation
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