Steps to create a cascading drop-down list by using the Microsoft ASP.NET AJAX framework

This article shows how to build a cascading drop-down list by using the ASP.NET AJAX framework.

Original product version:   .NET Framework 3.5
Original KB number:   976156

Introduction

A cascading drop-down list is a series of dependent DropDownList controls in which one DropDownList control depends on the parent or previous DropDownList controls. The items in the DropDownList control are populated based on an item that is selected by the user from another DropDownList control. When the selection of a parent DropDownList control changes, the web page calls the AJAX Web service to retrieve the list of values for the child DropDownList control. The ASP.NET AJAX framework enables you to call Web services (.asmx files) from the browser by using client scripts. So when you call the Web service in the client-side onchange event of the parent DropDownList control, the Web page will be only partly refreshed to populate the items for the child DropDownList control.

Steps to create a cascading drop-down list

1. Create an ASP.NET Web Service application in Visual Studio.

   Note

   In the sample code that is provided, the project name of the C# sample project is KB_CascadingDDL_CS. The project name of the Visual Basic .NET sample project is KB_CascadingDDL_VB.

2. Add an AJAX Web Form to the project.

   Note

   In the sample code, the file name of the Web form is Default.aspx. The AJAX Web service name is WebService.

3. Open the WebServiceName.asmx file for the AJAX Web service, and then remove the comment mark for the class declaration as follows.

   Visual Basic

   <System.Web.Script.Services.ScriptService()>
   Public Class WebService
   

   Visual C#

   [System.Web.Script.Services.ScriptService]
   public class WebService : System.Web.Services.WebService
   

   Note

   This step enables the AJAX Web service to be accessed from the client-side scripts. An AJAX Web service class must have the ScriptServiceAttribute attribute applied, and the individual methods that will be called from client-side scripts must have the WebMethodAttribute attribute applied.

4. Open the WebFormName.aspx file of the Web form, and then change the <asp:ScriptManager> tag as follows:

   <asp:ScriptManager ID="ScriptManager1" runat="server">
       <Services>
           <asp:ServiceReference Path="~/WebServiceName.asmx" />
       </Services>
   </asp:ScriptManager>
   

   To enable an AJAX Web service to be called from the client-side scripts in an ASP.NET Web form, you must first add a ScriptManager control to the Web page. Then, reference the Web service by adding an <asp:ServiceReference> child element to the <asp:ScriptManager> tag. In the <asp:ScriptManager> tag, set the Path attribute to point to the Web service file.

   The ServiceReference object instructs ASP.NET 2.0 AJAX extensions to generate a JavaScript proxy class for calling the specified Web service from the client-side scripts in the Web page. The proxy class has methods that correspond to each Web Service method in the AJAX Web service. The Web page also contains JavaScript proxy classes that correspond to server data types that are used as input parameters or return values for the Web Service methods. These proxy classes enable you to write client-side scripts that initialize these parameters and pass them to the Web Service method call.

5. Set the EnableEventValidation property of the Web page to false in the source code of the WebFormName.aspx file, as shown in the following example.

   <%@ Page Language="vb" AutoEventWireup="false" CodeBehind="Default.aspx.vb" Inherits="KB_CascadingDDL_VB._Default"
    EnableEventValidation="false" %>
   

   Note

   This step is to avoid invalid postback when you use Web Service method calls.

6. Add the following code under the <asp:ScriptManager> tag to generate the required ASP.NET controls.

   <table>
       <tr>
           <td>
               Make:
           </td>
           <td>
               <asp:DropDownList ID="ddlMake" Width="200" runat="server" AutoPostBack="false" onchange="ddl_changed(this)" />
           </td>
       </tr>
       <tr>
           <td>
               Model:
           </td>
           <td>
               <asp:DropDownList ID="ddlModel" Width="200" runat="server" AutoPostBack="false" onchange="ddl_changed(this)" />
           </td>
       </tr>
       <tr>
           <td>
               Color:
           </td>
           <td>
               <asp:DropDownList ID="ddlColor" Width="200" runat="server" AutoPostBack="false" onchange="DisplayResult()" />
           </td>
       </tr>
       <tr>
           <td>
               <asp:Button ID="btnReset" runat="server" Text="Reset" />
           </td>
       </tr>
   </table>
   <a id="aResult" />
   

7. Create Web Service methods for the AJAX Web service. Copy and paste the following code into the WebServiceName.asmx file.

   Visual Basic

   Imports System.Web.Services
   Imports System.Web.Services.Protocols
   Imports System.ComponentModel
   
   ' To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
   <System.Web.Script.Services.ScriptService()> _
   <System.Web.Services.WebService(Namespace:="http://tempuri.org/")> _
   <System.Web.Services.WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)> _
   <ToolboxItem(False)> _
   Public Class WebService
       Inherits System.Web.Services.WebService
   
       <WebMethod()> _
       Public Function GetMakeValue() As String()
           Return New String() {"Please select", "Ford", "Dodge"}
       End Function
   
       <WebMethod()> _
       Public Function GetChildValue(ByVal parentID As String, ByVal parentValue As String) As String()
           If parentValue = "Please select" Then
               Return Nothing
           End If
   
           Dim values As String()
           If parentID = "ddlMake" Then
               If parentValue = "Ford" Then
                   values = New String() {"Please select", "Explorer", "F150", "Mustang"}
               ElseIf parentValue = "Dodge" Then
                   values = New String() {"Please select", "Durango", "Dakota", "Viper"}
               Else
                   Return New String() {"Invalid Make value!"}
               End If
           ElseIf parentID = "ddlModel" Then
               Select Case parentValue
                   Case "Explorer", "Dakota", "Viper"
                       values = New String() {"Please select", "Black", "Green", "Yellow"}
                   Case "Durango", "F150", "Mustang"
                       values = New String() {"Please select", "White", "Red", "Blue"}
                   Case Else
                       values = New String() {"Invalid Model value!"}
               End Select
           Else
               Return New String() {"Invalid Category value!"}
           End If
           Return values
       End Function
   End Class
   

   Visual C#

   using System;
   using System.Collections.Generic;
   using System.Linq;
   using System.Web;
   using System.Web.Services;
   
   namespace KB_CascadingDDL_CS
   {
       /// <summary>
       /// Summary description for WebService
       /// </summary>
       [WebService(Namespace = "http://tempuri.org/")]
       [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
       [System.ComponentModel.ToolboxItem(false)]
       // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
       [System.Web.Script.Services.ScriptService]
       public class WebService : System.Web.Services.WebService
       {
           [WebMethod()]
           public string[] GetMakeValue()
           {
               return new string[] { "Please select", "Ford", "Dodge" };
           }
   
           [WebMethod()]
           public string[] GetChildValue(string parentID, string parentValue)
           {
               if (parentValue == "Please select")
               {
                   return null;
               }
   
               string[] values;
               if (parentID == "ddlMake")
               {
                   if (parentValue == "Ford")
                   {
                       values = new string[] { "Please select", "Explorer", "F150", "Mustang" };
                   }
                   else if (parentValue == "Dodge")
                   {
                       values = new string[] { "Please select", "Durango", "Dakota", "Viper" };
                   }
                   else
                   {
                       return new string[] { "Invalid Make value!" };
                   }
               }
               else if (parentID == "ddlModel")
               {
                   switch (parentValue)
                   {
                       case "Explorer":
                       case "Dakota":
                       case "Viper":
                           values = new string[] { "Please select", "Black", "Green", "Yellow" };
                           break;
                       case "Durango":
                       case "F150":
                       case "Mustang":
                           values = new string[] { "Please select", "White", "Red", "Blue" };
                           break;
                       default:
                           values = new string[] { "Invalid Model value!" };
                           break;
                   }
               }
               else
               {
                   return new string[] { "Invalid Category value!" };
               }
               return values;
           }
       }
   }
   

8. Call the Web Service methods for the AJAX Web service from client-side scripts. Copy and paste the following code under the code that you added in step 6.

   <script type="text/javascript">
       function pageLoad()
       {
           // call a Web service method with no parameters and the user context.
           KB_CascadingDDL_VB.WebService.GetMakeValue(SucceededCallbackWithContext, FailedCallback, "fillMake");
       }
   
       // Event handler for ddlMake and ddlModel.
       // This function calls a Web service method
       // passing simple type parameters and the user context.
       function ddl_changed(sender)
       {
           // This initiates the call to the server-side method in your code-behind
           // The parameters are as follows:
           // 1st : Specify all the parameters expected by your code-behind method
           //      (in this case there are 2: parentControl's ID, parentControl's selected text)
           // 2nd : Specify a callback method when the call succeeds
           // 3rd : Specify a callback method when the call fails(optional)
           // 4th : Specify a user context object (option - not shown)
           //      (in this case we need to assign the parentControl's ID as the user context)
           KB_CascadingDDL_VB.WebService.GetChildValue(sender.id, sender[sender.selectedIndex].text,
           SucceededCallbackWithContext, FailedCallback, sender.id);
       }
   
       // This is the callback function called if the
       // Web service succeeded. It accepts the result
       // object, the user context, and the method name as
       // parameters.
       function SucceededCallbackWithContext(result, userContext) {
   
           if (userContext == "fillMake") {
               //fill the Make
               var ddl = $get('ddlMake');
           } else if (userContext == "ddlMake") {
               // fill the Model
               var ddl = $get('ddlModel');
               $get('ddlColor').options.length = 0;
           } else if (userContext == "ddlModel") {
               // fill the Color
               var ddl = $get('ddlColor');
           }
           // clear the options
           ddl.options.length = 0;
           if (result)
           {
               var i = 0;
               for (var item in result)
               {
                   // item is the key, result[item] is the value
                   ddl.options.add(new Option(result[item], item));
                   i++;
               }
           } else {
               alert("Invalid! Please reset the selection!");
               $get(userContext).focus();
           }
       }
   
       // This is the callback function called if the
       // Web service failed. It accepts the error object
       // as a parameter.
       function FailedCallback(error)
       {
           if (error)
           {
               alert(error.get_message());
           }
           else
               alert("An unexpeceted error occurred");
       }
   
       //This the function to show the three DropDownLists'selection
       function DisplayResult()
       {
           $get("aResult").innerHTML = "You have selected a " +
           $get("ddlMake")[$get("ddlMake").selectedIndex].text + "," +
           $get("ddlModel")[$get("ddlModel").selectedIndex].text + "," +
           $get("ddlColor")[$get("ddlColor").selectedIndex].text + " car!";
       }
   </script>
   

   Calling a Web service method from a script is asynchronous. To obtain a return value or to determine when the request has returned, you must provide a succeeded callback function. The callback function is invoked when the request has finished successfully, and it contains the return value from the Web Service Method call. You can also provide a failed callback function to handle errors. Additionally, you can pass user context information to use in the callback functions.

   You can call a Web Service method in an AJAX Web service in the following format:

   theServiceNameSpace.WebServiceName.ServiceMethodName(parameters, SucceededCallbackFunctionName, FailedCallbackFunctionName, userContextString);
   

   You can provide a single succeeded callback function that is invoked from multiple Web service method calls. To enable the function to differentiate different callers, you can pass the user context information to the function in the parameter.

9. Build the project.

References

ASP.NET Ajax : Enhanced Interactivity and Responsiveness