Developers use multiline TextBox controls in almost all web projects. Since MaxLength property of a TextBox control does not work when the TextMode property is set to Multiline, we usually use Validator controls to validate the length. In this hands-on article, we are going to extend the TextBox control using JavaScript in order to limit the number of characters entered by the user to the length specified.  

TextArea Custom Server Control - C# Code

The code above creates a new TextArea custom server control by extending ASP.NET’s TextBox control. By overriding the OnPreRender function, we include attributes to the HTML of the control. We add custom javascripts and a property to pass maxLength on client side.

To show the working TextArea control, I prepared the following html:

TextArea Custom Server Control - HTML Code

In the above HTML, I register the custom control with the web page by using the following line:

<%@ Register TagPrefix="csc" Namespace="CustomServerControls" %>

If you don’t want to add the above registration line on each page that you use the TextArea control, you may add the following statement in system.web section of the web.config file.

<pages>
   <controls>
    <add tagPrefix="csc" namespace="CustomServerControls"></add>
   </controls>
</pages>

I added the control on page as:

<csc:TextArea id="TextArea" runat="server" MaxLength="105" Rows="10" Width="300px"></csc:TextArea>

Let’s compare the rendered output of a multiline textbox control and our text area control:

Rendered output of a standard multiple line Asp.Net TextBox is:

<textarea name="TextArea" id="TextArea" rows="10" cols="20" style="width:300px;" ></textarea>

Rendered output of our TextArea custom server control is:

<textarea name="TextArea" rows="10" cols="20" id="TextArea" onkeypress="LimitInput(this)" onbeforepaste="doBeforePaste(this)" onpaste="doPaste(this)" onmousemove="LimitInput(this)" maxLength="105" style="width:300px;"></textarea>

The javascript event handlers doBeforePaste and doPaste are only implemented in Internet Explorer. These event handlers are used to check the length of characters that are pasted by using a mouse in Internet Explorer. Unfortunately, doBeforePaste and doPaste event handlers  are not defined in other browsers and we cannot catch a mouse paste in browsers other than IE. Therefore,   I added an onmousemove event handler in order to check the length of characters that are pasted by using a mouse after a mouse move. The onkeypress event handler handles the standard character input.

TextArea Custom Server Control - JavaScript Code

Download the demo project

Try TextArea Custom Server Control with MaxLength property

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