Introduction
If you are not interested in using the ASP.NET AJAX library offered for ASP.NET, but would like to feature small amounts of AJAX functionality on your pages, you can do this easily with some JavaScript and a receptive page.
UPDATE: A much simpler way to add small doses of AJAX functionality to ASP.NET Web Forms can be achieved by using jQuery. Please see this article. I only use AJAX in small doses on web sites - mainly to improve the user-experience. Typical instances will involve retrieving a full record based on a users' selection in a drop down list. I don't want to perform a full page postback, so I get the JavaScript xmlhttpserver
object to do it for me behind the scenes. Here's an example that fetches the address details for a Northwind Traders customer. First, a generic function to instantiate an instance of xmlhttprequest
or xmlhttp
, depending on the browser:
function GetXmlHttpObject(handler)
{
var objXmlHttp=null
if (navigator.userAgent.indexOf("MSIE")>=0)
{
var strName="Msxml2.XMLHTTP"
if (navigator.appVersion.indexOf("MSIE 5.5")>=0)
{
strName="Microsoft.XMLHTTP"
}
try
{
objXmlHttp=new ActiveXObject(strName)
objXmlHttp.onreadystatechange=handler
return objXmlHttp
}
catch(e)
{
alert("Error. Scripting for ActiveX might be disabled")
return
}
}
if (navigator.userAgent.indexOf("Mozilla")>=0)
{
objXmlHttp=new XMLHttpRequest()
objXmlHttp.onload=handler
objXmlHttp.onerror=handler
return objXmlHttp
}
}
Now, a simple dropdownlist
populated by a SqlDataSource
control that fetches the CustomerID
and Customer Name. Note the empty div
, CustomerDetail
s below the dropdownlist
:
<div>
<asp:DropDownList ID="DropDownList1"
runat="server"
DataSourceID="SqlDataSource1"
DataTextField="CompanyName"
DataValueField="CustomerID">
<asp:SqlDataSource
ID="SqlDataSource1"
runat="server"
ConnectionString="Provider=Microsoft.Jet.OLEDB.4.0;
Data Source=|DataDirectory|Northwind.mdb"
DataSourceMode="DataReader"
ProviderName="System.Data.OleDb"
SelectCommand="SELECT [CustomerID], [CompanyName] FROM [Customers]">
</div>
<div id=""CustomerDetails""></div>
Two more JavaScript functions are needed. One to fire the GetXmlHttpObject
method, and denote the page to send a request to, and one to handle the response by checking the readyState
property of the object for a value of 4 or complete, then to write the response to the empty div
:
function GetCustomer(id)
{
var url="FetchCustomer.aspx?CustomerID=" + id ;
xmlHttp=GetXmlHttpObject(stateChanged);
xmlHttp.open("GET", url , true);
xmlHttp.send(null);
}
function stateChanged()
{
if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
{
document.getElementById('CustomerDetails').innerHTML=xmlHttp.responseText;
}
}
So now we have a page that gets called by xmlhttp
(FetchCustomer.aspx) and we have a value to pass to the querystring
for the page. So the next thing to do is to add an event handler to the DropDownList
that will fire the GetCustomer()
event and pass a CustomerID
value:
if (!Page.IsPostBack)
{
DropDownList1.DataBind();
DropDownList1.Items.Insert(0, "");
}
DropDownList1.Attributes["onChange"] = "GetCustomer(this.value);";
HttpResponse myHttpResponse = Response;
HtmlTextWriter myHtmlTextWriter = new HtmlTextWriter(myHttpResponse.Output);
DropDownList1.Attributes.AddAttributes(myHtmlTextWriter);
And finally, the code for the FetchCustomer.aspx page. Everything has been removed from the .aspx file except the first line. We don't want DocTypes or default <form>
tags disturbing the piece of HTML to be emitted in the response:
<%@ Page Language="C#" AutoEventWireup="true"
CodeFile="FetchCustomer.aspx.cs" Inherits="FetchCustomer" %>
And the code-behind makes doubly sure by calling Response.Clear()
before querying the database and emitting the resulting data as HTML:
using System;
using System.Data;
using System.Data.OleDb;
using System.Text;
public partial class FetchCustomer : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
Response.Clear();
StringBuilder sb = new StringBuilder();
sb.Append("
");
string provider = "Provider=Microsoft.Jet.OLEDB.4.0;";
string db = "Data Source=|DataDirectory|Northwind.mdb";
string connstr = provider + db;
string query = "SELECT * FROM Customers WHERE CustomerID = ?";
OleDbConnection conn = new OleDbConnection(connstr);
OleDbCommand cmd = new OleDbCommand(query, conn);
cmd.Parameters.AddWithValue("", Request.QueryString["CustomerID"]);
conn.Open();
OleDbDataReader dr = cmd.ExecuteReader();
while(dr.Read())
{
sb.Append(dr[1].ToString() + "
");
sb.Append(dr[4].ToString() + "
");
sb.Append(dr[5].ToString() + "
");
sb.Append(dr[6].ToString() + "
");
sb.Append(dr[7].ToString() + "
");
sb.Append(dr[8].ToString() + "
");
sb.Append("Tel: " + dr[9].ToString() + "
");
}
dr.Close();
dr.Dispose();
conn.Close();
conn.Dispose();
Response.Write(sb.ToString())
Response.End();
}
}
History
- 30th October, 2009: Initial post
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