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Developing AutoComplete Textbox using ASP.NET AJAX 1.0, Web Service and JavaScript
by Jesudas Chinnathampi (Das)
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Invoking a web service method from JavaScript code (client side) is the essence of this article. Due to the introduction of AJAX 1.0 extension for ASP.NET 2.0 invoking a web service method from client side can be implemented with less lines of code. We can perform a variety of tasks combining AJAX and Web Service. One of the techniques is to create an AutoComplete Textbox using AJAX. To begin with, let us see how we can expose a web service method that can be invoked from a client side JavaScript code.


As a user types into the Textbox, a client side call is made to the web service; this in turn returns the possible word that matches the typed-in text. The Textbox is udpated with the word received from the Web Service. This is achieved without refreshing the web page. The call between the client and web service happens in such a fast manner that the user will not be aware that we are indeed making a call to the server when they type in.


The complete steps required to build an AutoComplete Textbox control are elaborated below.

Exposing a Web Service method to be invoked from Client Side

The following is a partial code snippet for a web service class definition.

Listing 1

[WebService(Namespace = "")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class myServices : System.Web.Services.WebService {

In order to expose the class, "myServices" to be invoked from a client side, all we have to do is insert the following attribute above the class definition.

Listing 2


By adding the above line, the AJAX run time extensions will know that we are going to invoke the methods in the myServices class which is going to be invoked from the client side.

Now the methods defined in the myServices class web service are ready to be invoked from the client side. Let us pay some attention to how we can invoke the methods from client side using AJAX.

Invoking the web service method using AJAX

Any AJAX enabled web page should at least have one ScriptManager tag. One of the collections for this ScriptManager tag is the Services collection. Through this Services collection, we can refer to the asmx file that we are going to invoke. Now allow me to show the syntax for the Services collection.

Listing 3

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

Using the above code, we can invoke the methods defined in the myServices.asmx from client side.

JavaScript code to invoke a Web Service method

Listing 4

ret = className.methodName(Parameter1, OnComplete, OnError, OnTimeOut);

The above is the syntax to invoke a Web Service method from client side JavaScript code. Parameter1 is the parameter for the method. OnComplete, OnError and OnTimeOut are client side JavaScript function names. For example, the client side function "OnComplete" will be invoked once the asynchronous call to the web service method completes successfully. The function "OnError" will be invoked if the asynchronous call to the method results in any error. And finally, the client side function "OnTimeOut" will be invoked, if the asynchronous call to the method does not complete after a certain timeout. These three functions are user defined functions. So, you can name them to your wish. These functions will look as follows:

Listing 5

function OnComplete(arg) 
  alert (arg);
function OnTimeOut(arg) 
  alert("TimeOut encountered");
function OnError(arg) 
  alert("Error : " + arg);

How does an AutoComplete Textbox works?

Now, using the above technique, let us see how we can implement an AutoComplete Textbox. To begin with, the web service method should receive a parameter. This parameter will be the textbox content that the user will be typing in. The web method can compare against a dictionary of words that matches with the passed in parameter. When I say dictionary, it can be an ArrayList or a database call to get the list of matching words. This can be implemented according to your business logic. For our discussion we will use an ArrayList which holds the possible combination of words. For better performance, a binary search will be performed against the Array. .NET Framework has an inbuilt method called BinarySearch. The only pre-condition to use this method is that the Array should be sorted. Again, we can use the inbuilt Sort method before invoking the BinarySearch method. The web service method will return the closest match for the passed in parameter.

The JavaScript "OnComplete" function will receive the text returned by the web service method. Once we have the text, all we have to do is to assign the text to the Textbox. To simplify, the above discussion can be summarized as follows:

1.    User types in text to the textbox.

2.    The "onkeyup" event for the textbox will fire a client side JavaScript function.

3.    The JavaScript function will invoke the web service method. The typed in text will be passed as a parameter to the web service method.

4.    The web service method will return the closest match for the passed in text.

5.    The JavaScript "OnComplete" function will receive the value returned by the web service method and assign it back to the Textbox.

Selecting the text inside the AutoComplete Textbox

One of the cosmetic features of an AutoComplete Textbox is to select the text in the Textbox. For example, if the value returned by the web service method is "hello," then we have to select the text "hello" inside the Textbox. This can be performed by the following code.

Listing 6

function SelectText(intStart, intLength)
  var myElement = document.getElementById('txtSearch');
  // get reference to the Textbox
  if (myElement.createTextRange)
  // If IE
    var myRange = myElement.createTextRange();
    myRange.moveStart("character", intStart);
    myRange.moveEnd("character", intLength - myElement.value.length);;
  else if (myElement.setSelectionRange)
  // if FireFox
    myElement.setSelectionRange(intStart, intLength);

Invoking the Web Service method only when specific keys are pressed

In order to make the AutoComplete Textbox more user friendly, we should not invoke the web service method for any keys pressed. For example, if the user presses the Backspace, we should invoke the web service method. If we allow, then the user will never be able to delete any text that he/she types in to the Textbox. Key trapping is a very simple process in JavaScript. Using the following line we can capture the ASCII value of the key pressed.

Listing 7


Once we have the ASCII value, we can control which keys should be allowed. Here is a code snippet which performs what we discussed in this section.

Listing 8

intKeyCode = event.keyCode;
// See if a valid key key is pressed such as 0-9, A-Z, a-z, Hyphen, Underscore
if (((intKeyCode >= 48) && (intKeyCode <= 57)) ||  // Numbers 0-9
((intKeyCode >= 65) && (intKeyCode <= 90)) ||  // Upper case A-Z
((intKeyCode >= 97) && (intKeyCode <= 122)) ||  // Lower case a-z
(intKeyCode == 189))
// Hyphen
  txtSearch = document.getElementById('txtSearch').value;
  strOldText = txtSearch;
  ret = myServices.WhatIsNext(txtSearch, OnComplete, OnError, OnTimeOut);
  return (true);
// return false.
return (false);

Web Service method which finds the closest match

Here is the code snippet for the web service method which searches for the closest match for the typed in text.

Listing 9

[WebMethod]public string WhatIsNext(string strText)
  int intIndex;
  string strWord;
  strWord = strText;
  intIndex = Array.BinarySearch(arrItems, strText);
  // if an exact match is found
  if (intIndex >= 0)
    // if an Exact match is found, check the next item in the 
    // dictionary to see if it matches
    if (arrItems[intIndex + 1].Length >= strText.Length)
      if (arrItems[intIndex + 1].Substring(0, strText.Length).ToLower() ==
        strWord = arrItems[intIndex + 1];
        strWord = arrItems[intIndex];
    if ((~intIndex) < (arrItems.Length))
      // when we return the next word, make sure that it is really a closest match
      if (arrItems[~intIndex].Length >= strText.Length)
        if (arrItems[~intIndex].Substring(0, strText.Length).ToLower() ==
          strWord = arrItems[~intIndex];
    return strWord;
Live Demo

The ASP.NET AJAX Control Toolkit already provides us with an AutoComplete Textbox control. You may ask why we are re-inventing the wheel. Well, this is a learning exercise to understand what is happening behind the scenes. I am not saying that this is how the control Toolkit AutoComplete Textbox is performed. This is one of the ways to build an AutoComplete Textbox. The essence of this article is to show how we can invoke a web service method from a client side JavaScript code. I hope this article comes in handy to you!

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