Joe Stevens' BlogEverything .NET and C#

In this post I’ll explain how to populate a select dropdownlist using jQuery and Ajax.  I am using an ASP.NET web application and page methods to perform the Ajax calls.  Using page methods means that you do not need a seperate web service, which is good if the functionality is specifically for the page.  The page methods must be declared public static and use the WebMethod attribute. 

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First of all here is the contents of my form with two select lists, one for gender and one for names.  Each list is given a unique ID to be able to reference them in jQuery.  The intention here is to display a list of genders in the first list, which when selected populates the second select with a list of names for that gender. 

<table>
    <tr>
        <th>
            Gender
        </th>
        <td>
            <select id="ddlGender">
            </select>
        </td>
    </tr>
    <tr>
        <th>
            Name
        </th>
        <td>
            <select id="ddlName">
            </select>
        </td>
    </tr>
</table>

Here is the page method that gets my list of genders: 

[WebMethod]
public static ArrayList GetGenders()
{
    return new ArrayList()
    {
        new { Value = 1, Display = "Male" },
        new { Value = 2, Display = "Female" }
    };
}

For the purpose of the example I am just returning an ArrayList of an anonymous type containing a value and display text which will be converted to JSON in jQuery and used to populate the select list.  You could easily replace this code to get some data from a database if required. 

Here is the method used to get the list of names based on the selected gender.  I will be passing the Value property from the anonymous gender type through to this method to filter the list of names: 

[WebMethod]
public static ArrayList GetNames(int genderID)
{
    if (genderID.Equals(1))
    {
        return new ArrayList()
        {
            new { Value = 1, Display = "Joe" },
            new { Value = 2, Display = "Tom" },
            new { Value = 3, Display = "Sylvain" }
        };
    }
    else if (genderID.Equals(2))
    {
        return new ArrayList()
        {
            new { Value = 4, Display = "Emily" },
            new { Value = 5, Display = "Lauri" },
        };
    }
    else
    {
        throw new ApplicationException("Invalid Gender ID");
    }
}

Again I am returning an ArrayList of an anonymous type containing a list of names depending on the genderID passed into the method. 

Now for the fun part, hooking it all up with jQuery.  First I need to populate the gender list when the page loads: 

   

$().ready(function() {
    $.ajax({
        type: "POST",
        url: "Default.aspx/GetGenders",
        data: "{}",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function(msg) {
            $("#ddlGender").get(0).options.length = 0;
            $("#ddlGender").get(0).options[0] = new Option("Select gender", "-1");   

            $.each(msg.d, function(index, item) {
                $("#ddlGender").get(0).options[$("#ddlGender").get(0).options.length] = new Option(item.Display, item.Value);
            });
        },
        error: function() {
            alert("Failed to load genders");
        }
    });
});   

Here I am using jQuery’s Ajax method to make a call to the GetGenders page method.  In the successful callback from the call I then do the following: 

Clear the select list’s current items: 

$("#ddlGender").get(0).options.length = 0;

Add an item at the top of the list prompting to select a gender with a value of -1, I’ll check this value later before making a call to GetNames: 

$("#ddlGender").get(0).options[0] = new Option("Select gender", "-1");

Loop through the results of the Ajax call and add the items to the list: 

$.each(msg.d, function(index, item) {
    $("#ddlGender").get(0).options[$("#ddlGender").get(0).options.length] = new Option(item.Display, item.Value);
});

Now the gender dropdown list is populated when the page loads. Next I need the names list to be populated when choosing a gender.  To do this I have created a JavaScript function called GetNames which accepts the genderID as a parameter: 

 

function GetNames(genderID) {
    if (genderID > 0) {
        $("#ddlName").get(0).options.length = 0;
        $("#ddlName").get(0).options[0] = new Option("Loading names", "-1"); 

        $.ajax({
            type: "POST",
            url: "Default.aspx/GetNames",
            data: "{genderID:" + genderID + "}",
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: function(msg) {
                $("#ddlName").get(0).options.length = 0;
                $("#ddlName").get(0).options[0] = new Option("Select name", "-1"); 

                $.each(msg.d, function(index, item) {
                    $("#ddlName").get(0).options[$("#ddlName").get(0).options.length] = new Option(item.Display, item.Value);
                });
            },
            error: function() {
                $("#ddlName").get(0).options.length = 0;
                alert("Failed to load names");
            }
        });
    }
    else {
        $("#ddlName").get(0).options.length = 0;
    }
} 

First I check if the genderID is greater than zero, if not (the ‘Select gender’ item has been chosen) then clear the names list.  If it is I clear the list and add an option with the text Loading names to the select list as I did with gender.  This will be displayed while the Ajax call is being made to let the user know something is happening.  Next I use the same approach as getting the list of gender to get the list of names but I pass a JSON string through as the data property containing the selected gender ID.  The successful callback works in the same way as populating the list of genders. 

Finally I need this method to be called when a gender is selected so I’ve added the following to my successful Ajax callback underneath the loop that populates the genders: 

$("#ddlGender").bind("change", function() {
    GetNames($(this).val());
});

This binds the onChange event to call the GetNames function passing through the selected gender ID.

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