Cascading Drop down list in asp.net

Hi,

Using Ajax Control Toolkit, You can do it in a so easy way.

Here is .aspx source code

Add AjaxControlToolkit as Register directive

<![CDATA[<%@ Register
    Assembly="AjaxControlToolkit"
    Namespace="AjaxControlToolkit"
    TagPrefix="ajaxToolkit" %>]]>

Then Do code as showing below

  <ajaxtoolkit:toolkitscriptmanager enablepartialrendering="true" runat="server" id="ScriptManager1" xmlns:ajaxtoolkit="#unknown" />
    <div class="demoarea">
        <div class="demoheading">CascadingDropDown Demonstration</div>
        <table>
            <tr>
                <td>Make</td>
                <td><asp:dropdownlist id="DropDownList1" runat="server" width="170" xmlns:asp="#unknown" /></td>
            </tr>
            <tr>
                <td>Model</td>
                <td><asp:dropdownlist id="DropDownList2" runat="server" width="170" xmlns:asp="#unknown" /></td>
            </tr>
            <tr>
                <td>Color</td>
                <td><asp:dropdownlist id="DropDownList3" runat="server" width="170" autopostback="true" xmlns:asp="#unknown">
                    OnSelectedIndexChanged="DropDownList3_SelectedIndexChanged" /></asp:dropdownlist></td>
            </tr>
        </table>
        <br />
        
        <ajaxtoolkit:cascadingdropdown id="CascadingDropDown1" runat="server" targetcontrolid="DropDownList1" xmlns:ajaxtoolkit="#unknown">
            Category="Make"  PromptText="Please select a make"  LoadingText="[Loading makes...]"
            ServicePath="CarsService.asmx" ServiceMethod="GetDropDownContents" />
        <ajaxtoolkit:cascadingdropdown id="CascadingDropDown2" runat="server" targetcontrolid="DropDownList2">
            Category="Model" PromptText="Please select a model" LoadingText="[Loading models...]"
            ServiceMethod="GetDropDownContentsPageMethod" ParentControlID="DropDownList1" />
        <ajaxtoolkit:cascadingdropdown id="CascadingDropDown3" runat="server" targetcontrolid="DropDownList3">
            Category="Color" PromptText="Please select a color" LoadingText="[Loading colors...]"
            ServicePath="~/CascadingDropDown/CarsService.asmx" ServiceMethod="GetDropDownContents"
            ParentControlID="DropDownList2" />
      
        <asp:updatepanel id="UpdatePanel1" runat="server" updatemode="Conditional" rendermode="inline" xmlns:asp="#unknown">
            <contenttemplate>
                <asp:label id="Label1" runat="server" text="[No response provided yet]" />
            </contenttemplate>
            <triggers>
                <asp:asyncpostbacktrigger controlid="DropDownList3" eventname="SelectedIndexChanged" />
            </triggers>
        </asp:updatepanel>
    </ajaxtoolkit:cascadingdropdown></ajaxtoolkit:cascadingdropdown></ajaxtoolkit:cascadingdropdown></div>
    <div class="demobottom"></div>
    
    <asp:panel id="Description_HeaderPanel" runat="server" style="cursor: pointer;" xmlns:asp="#unknown">
        <div class="heading">
            <asp:imagebutton id="Description_ToggleImage" runat="server" imageurl="~/images/collapse.jpg" alternatetext="collapse" />
            CascadingDropDown Description
        </div>
    </asp:panel>
    <asp:panel id="Description_ContentPanel" runat="server" style="overflow:hidden;" xmlns:asp="#unknown">
        <p>
            CascadingDropDown is an ASP.NET AJAX extender that can be attached to an ASP.NET DropDownList
            control to get automatic population of a set of DropDownList controls.  Each time the selection
            of one the DropDownList controls changes, the CascadingDropDown makes a call to a specified web
            service to retrieve the list of values for the next DropDownList in the set.
        </p>
        <br />
        <p>
            CascadingDropDown enables a common scenario in which the contents of one list depends on the selection 
            of another list and does so without having to embed the entire data set in the page or transfer it to 
            the client at all.
        </p>
        <br />
        <p>    
            All the logic about the contents of the set of DropDownList controls lives on the server in a web service. 
            This web service can use any suitable method for storing and looking up the relevant data.
        </p>
        <br />
        <p>    
            The sample web service used here reads the data set from a simple hierarchical XML data file. The 
            sample data file shows that the DropDownList items can have distinct names and value (values are 
            optional in the sample). It also demonstrates that items can have an optional optionTitle attribute 
            which will be displayed as a tooltip over the item in some modern browsers. Finally, an item can be 
            selected by default. In the XML file, the Racing Blue option for the BMW is selected by default.  
        </p>
        <br />
        <p>    
        There is also a walkthrough showing how to use a 
            <a href="../Walkthrough/CCDWithDB.aspx">CascadingDropDown with a database</a>.        
        </p>
    </asp:panel>

    <asp:panel id="Properties_HeaderPanel" runat="server" style="cursor: pointer;" xmlns:asp="#unknown">
        <div class="heading">
            <asp:imagebutton id="Properties_ToggleImage" runat="server" imageurl="~/images/expand.jpg" alternatetext="expand" />
            CascadingDropDown Properties
        </div>
    </asp:panel>
    <asp:panel id="Properties_ContentPanel" runat="server" style="overflow:hidden;" height="0px" xmlns:asp="#unknown">
        <p>
            The control above is initialized with this code. The italic properties are optional:
        </p>
<pre><ajaxToolkit:CascadingDropDown ID="CDD1" runat="server"
    TargetControlID="DropDownList2"
    Category="Model"
    PromptText="Please select a model"
    LoadingText="[Loading models...]"
    ServicePath="CarsService.asmx"
    ServiceMethod="GetDropDownContents"
    ParentControlID="DropDownList1"
    SelectedValue="SomeValue" />

• TargetControlID - The ID of the DropDownList to populate.
• Category - The name of the category this DropDownList represents.
• PromptText - Optional text to display before the user has selected a value from the DropDownList.
• PromptValue - Optional value set when PromptText is displayed.
• EmptyText - Optional text to display when the DropDownList has no data to display.
• EmptyValue - Optional value set when EmptyText is displayed.
• LoadingText - Optional text to display while the data for the DropDownList
  is being loaded.
• ServicePath - Path to a web service that returns the data used to populate
  the DropDownList. This property should be left null if ServiceMethod refers to a page method.
  The web service should be decorated with the System.Web.Script.Services.ScriptService
  attribute.
• ServiceMethod - Web service method that returns the data used to populate
  the DropDownList. The signature of this method must match the following:
  

  [System.Web.Services.WebMethod]
  [System.Web.Script.Services.ScriptMethod]
  public CascadingDropDownNameValue[] GetDropDownContents(
         string knownCategoryValues, string category) { ... }

  
  Note that you can replace "GetDropDownContents" with a naming of your choice, but the return
  type and parameter name and type must exactly match, including case.
• ContextKey - User/page specific context provided to an optional overload of the
  web method described by ServiceMethod/ServicePath. If the context key is used, it should have the
  same signature with an additional parameter named contextKey of type string:
  

  [System.Web.Services.WebMethod]
  [System.Web.Script.Services.ScriptMethod]
  public CascadingDropDownNameValue[] GetDropDownContents(
         string knownCategoryValues, string category, string contextKey) { ... }

  
  Note that you can replace "GetDropDownContents" with a name of your choice, but the return type
  and parameter name and type must exactly match, including case.
• UseContextKey - Whether or not the ContextKey property should be used. This
  will be automatically enabled if the ContextKey property is ever set (on either the client or
  the server). If the context key is used, it should have the same signature with an additional
  parameter named contextKey of type string (as described above).
• ParentControlID - Optional ID of the parent DropDownList that controls the
  contents of this DropDownList.
• SelectedValue - Optional value to select by default. This needs to exactly
  match the string representation of a value in the DropDownList.
• UseHttpGet - Optional value which determines whether to use HttpGet or HttpPost
  method for the postback to the server.
• EnableAtLoading - Optional value which determines whether or not the dropdownlist
  control is disabled when the control is waiting to get data from the service.

<ajaxtoolkit:collapsiblepanelextender id="cpeDescription" runat="Server" xmlns:ajaxtoolkit="#unknown">
TargetControlID="Description_ContentPanel"
ExpandControlID="Description_HeaderPanel"
CollapseControlID="Description_HeaderPanel"
Collapsed="False"
ImageControlID="Description_ToggleImage" />
<ajaxtoolkit:collapsiblepanelextender id="cpeProperties" runat="Server">
TargetControlID="Properties_ContentPanel"
ExpandControlID="Properties_HeaderPanel"
CollapseControlID="Properties_HeaderPanel"
Collapsed="True"
ImageControlID="Properties_ToggleImage" />

.cs page code

protected void DropDownList3_SelectedIndexChanged(object sender, EventArgs e)
    {
        // Get selected values
        string make = DropDownList1.SelectedItem.Text;
        string model = DropDownList2.SelectedItem.Text;
        string color = DropDownList3.SelectedItem.Text;

        // Output result string based on which values are specified
        if (string.IsNullOrEmpty(make))
        {
            Label1.Text = "Please select a make.";
        }
        else if (string.IsNullOrEmpty(model))
        {
            Label1.Text = "Please select a model.";
        }
        else if (string.IsNullOrEmpty(color))
        {
            Label1.Text = "Please select a color.";
        }
        else
        {
            Label1.Text = string.Format("You have chosen a {0} {1} {2}. Nice car!", color, make, model);
        }
    }

    [WebMethod]
    [System.Web.Script.Services.ScriptMethod]
    public static CascadingDropDownNameValue[] GetDropDownContentsPageMethod(string knownCategoryValues, string category)
    {
        return new CarsService().GetDropDownContents(knownCategoryValues, category);
    }

Web Method for some Items in DropDownList

using System;
using System.Collections.Specialized;
using System.Text.RegularExpressions;
using System.Web;
using System.Web.Services;
using System.Xml;

/// <summary>
/// Helper web service for CascadingDropDown sample
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.Web.Script.Services.ScriptService]
public class CarsService : WebService
{
    // Member variables
    private static XmlDocument _document;
    private static Regex _inputValidationRegex;
    private static object _lock = new object();

    // we make these public statics just so we can call them from externally for the
    // page method call
    public static XmlDocument Document
    {
        get
        {
            lock (_lock)
            {
                if (_document == null)
                {
                    // Read XML data from disk
                    _document = new XmlDocument();
                    _document.Load(HttpContext.Current.Server.MapPath("~/App_Data/CarsService.xml"));
                }
            }
            return _document;
        }
    }

    public static string[] Hierarchy
    {
        get { return new string[] { "make", "model" }; }
    }

    public static Regex InputValidationRegex
    {
        get
        {
            lock (_lock)
            {
                if (null == _inputValidationRegex)
                {
                    _inputValidationRegex = new Regex("^[0-9a-zA-Z \\(\\)]*$");
                }
            }
            return _inputValidationRegex;
        }
    }

    /// <summary>
    /// Helper web service method
    /// </summary>
    /// <param name="knownCategoryValues">private storage format string</param>
    /// <param name="category">category of DropDownList to populate</param>
    /// <returns>list of content items</returns>
    [WebMethod]
    public AjaxControlToolkit.CascadingDropDownNameValue[] GetDropDownContents(string knownCategoryValues, string category)
    {
        // Get a dictionary of known category/value pairs
        StringDictionary knownCategoryValuesDictionary = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);

        // Perform a simple query against the data document
        return AjaxControlToolkit.CascadingDropDown.QuerySimpleCascadingDropDownDocument(Document, Hierarchy, knownCategoryValuesDictionary, category, InputValidationRegex);
    }
}

Implementation of MVC3 Cascading Dropdown using jQuery[^]