Multiline TextBox with MaxLength Validation

• Download source code - 13.7 KB

Introduction

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
                Attributes.Add("onkeypress","doKeypress(this);");
                Attributes.Add("onbeforepaste","doBeforePaste(this);");
                Attributes.Add("onpaste","doPaste(this);");
                // Add attribute for access of maxlength property on client-side
                Attributes.Add("maxLength", this.MaxLength.ToString());
                // Register client side include - only once per page
                if(!Page.ClientScript.IsClientScriptIncludeRegistered("TextArea"))
                {
                    Page.ClientScript.RegisterClientScriptInclude("TextArea", 
                        ResolveClientUrl("~/js/textArea.js"));
                }
            }
            base.OnPreRender(e);
        }
    }
}

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;
     if(maxLength)
     {
          event.returnValue = false;
     }
}
// Cancel default behavior and create a new paste routine
function doPaste(control){
    maxLength = control.attributes["maxLength"].value;
    value = control.value;
     if(maxLength){
          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.

<pages>
   <controls>
    <add tagPrefix="CustomControls" 
      namespace="CustomWebControls" 
      assembly="CustomWebControls"></add>
   </controls>
</pages>

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!