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Multiline TextBox with MaxLength Validation

, 12 Mar 2006 CPOL
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A simple extension of the existing TextBox control to attach JavaScript for maxlength validation.


In almost all web projects, we require a multiline TextBox control. It was annoying to find that the maxLength property doesn't work on multiline textboxes. There are various solutions suggesting to use validator controls to validate length. I suggest that we should put a validator as well as extend our control so that the user will not be able to enter characters more than the length specified.

Extending the TextBox

To achieve this, I tried extending the ASP.NET TextBox WebControl.

using System;
using System.Collections.Generic;
using System.Text;
using System.Web.UI.WebControls;

namespace CustomWebControls
    /// <summary>
    /// Extended TextBox control that applies
    /// javascript validation to Multiline TextBox
    /// </summary>
    public class TextArea: TextBox
        /// <summary>
        /// Override PreRender to include custom javascript
        /// </summary>
        /// <param name="e"></param>
        protected override void OnPreRender(EventArgs e)
            if (MaxLength > 0 && TextMode == TextBoxMode.MultiLine)
                // Add javascript handlers for paste and keypress
                // Add attribute for access of maxlength property on client-side
                Attributes.Add("maxLength", this.MaxLength.ToString());
                // Register client side include - only once per page

Now our custom control is ready to use. In the above code block, it should be self-explanatory that if I find the TextBox's TextMode property to be MultiLine, I add custom JavaScript and a property to pass the maxLength on the client side. I take advantage of ClientScript to attach a JavaScript, only one per page. I chose to include JavaScript as an external file because this allows us to modify the JavaScript easily. In addition, the page will not be bloated and the script will be cached.

I created this control in a Project Library so that I can use this control in any of our projects.

JavaScript File

// Keep user from entering more than maxLength characters
function doKeypress(control){
    maxLength = control.attributes["maxLength"].value;
    value = control.value;
     if(maxLength && value.length > maxLength-1){
          event.returnValue = false;
          maxLength = parseInt(maxLength);
// Cancel default behavior
function doBeforePaste(control){
    maxLength = control.attributes["maxLength"].value;
          event.returnValue = false;
// Cancel default behavior and create a new paste routine
function doPaste(control){
    maxLength = control.attributes["maxLength"].value;
    value = control.value;
          event.returnValue = false;
          maxLength = parseInt(maxLength);
          var oTR = control.document.selection.createRange();
          var iInsertLength = maxLength - value.length + oTR.text.length;
          var sData = window.clipboardData.getData("Text").substr(0,iInsertLength);
          oTR.text = sData;

Using the Custom Control

To test the custom control, I created a test website and added a reference to the project. I wanted to avoid the hassle of adding the Register tag on each page, so I went ahead and added the following statements in the system.web section of web.config.

    <add tagPrefix="CustomControls" 

Now I created a test web page and added the control on the page as:

<CustomControls:TextArea runat="server" ID="testTextArea" 
   MaxLength="10" TextMode="MultiLine"></CustomControls:TextArea>

Then I added the JavaScript file in the JS folder.

Tested the solution and voila! My first attempt to create an extended custom Web Control is successful. I will appreciate your comments.

Summary of things I learned

  • Extending a Web Control is a breeze.
  • The new ClientScript property of Page. And the related IsClientScriptIncludeRegistered and RegisterClientScriptInclude properties.
  • Added the tag prefix in Web.Config instead of specifying it on each page.

Client Browsers Supported

  • IE 6.0

Other Notes

  • To get this control to work on other browsers also, we might need some modifications in the JS file.
  • This solution was developed in Visual Studio 2005. To convert it to Visual Studio 2003, we just need to change the calls for IsClientScriptIncludeRegistered and RegisterClientScriptInclude.
  • It's necessary to add validation controls also for length check. This will allow you to validate the input even if client-side script fails.

Thanks CodeProject for all the help!


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Yen Dutt
Web Developer Airtel Broadband
India India
Despite my lack of interest, started trying coding at the age of 15 in Visual FoxPro. Got introduced to ASP in 2003 and started working for a US based client for a Youth Sports Management Application. In 2005, got Introduced to ASP.NET/ VB.NET and since then, it has become my passion. Recently joined Rapidigm Inc. as a software developer and looking forward to expand my knowledge base by sharing information with fellow coders in the Web World.

My interests include listening to music, trekking, traveling apart from coding!

Comments and Discussions

QuestionThanks Pinmembertidkerohit12-Oct-11 4:18 
GeneralWell done thanks PinmemberMelih Gümüşçay18-Jul-09 4:32 
RantRe: Well done thanks Pinmemberhfrmobile4-Mar-10 0:06 
GeneralClient code work on ff & ie Pinmemberramip4-Apr-09 2:25 
Generalgud 1 : Multiline TextBox with MaxLength Validation PinmemberCoder Sumi2-Feb-09 0:51 
GeneralI'm sorry, but forget about TextArea behaving like a TextBox. Firefox / IE / Chrome [modified] Pinmemberviolentkun12-Dec-08 3:25 
QuestionWhy not use a Regular Expression Validator? PinmemberJoe Gakenheimer )23-Jun-08 3:26 
GeneralProblems w/ AJAX update panels PinmemberJoAnn Morse31-May-08 9:13 
GeneralRe: Problems w/ AJAX update panels PinmemberAstynax77727-Dec-08 8:50 
GeneralThanks Pinmembercarladinmc14-Apr-08 3:32 
Very nice
GeneralFixed for Firefox PinmemberMember 436197826-Feb-08 3:16 
GeneralRe: Fixed for Firefox Pinmemberjmoore653-Mar-08 4:57 
GeneralRe: Fixed for Firefox Pinmemberhfrmobile4-Mar-10 0:05 
GeneralNew line PinmemberAstynax77720-Feb-08 8:52 
GeneralRe: New line Pinmembersar11234-Sep-09 11:27 
GeneralRe: New line PinmemberAstynax7774-Sep-09 11:47 
GeneralFirefox Pinmemberhalexic30-Mar-07 2:32 
GeneralRe: Firefox Pinmemberhfrmobile4-Mar-10 0:19 
QuestionUpdatePanel Pinmemberjlweand13-Mar-07 23:24 
GeneralGreat work, paste issue PinmemberJuan Martin23-Jan-07 10:27 
GeneralThank you very much Pinmembermsg2rk17-Jan-07 21:43 
GeneralError Pinmembermishenkovks9-Nov-06 0:25 
Generalthanx for the article... another nice solution... PinmemberSina Falahati31-Oct-06 8:59 
GeneralRe: thanx for the article... another nice solution... PinmemberYen Dutt31-Oct-06 18:48 
GeneralRe: thanx for the article... another nice solution... PinmemberSina Falahati1-Nov-06 4:13 

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