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Published: Wednesday, May 14, 2008

Warning the User when Caps Lock is On

By Scott Mitchell


Most security systems' passwords are case-sensitive. Case sensitivity nearly doubles the number of possible characters that can appear in the password, which makes it harder for nefarious users trying to break into the system. As a result, if a user logging into the system has Caps Lock turned on, they'll enter letters in the opposite case and not be able to login. Because the textboxes that collect user input typically are masked to prevent an onlooker from seeing a user's password, the user typing in her password may not realize that Caps Lock is on. To help prevent this type of user error, many login screens for desktop-based applications display some sort of warning if the user's Caps Lock is on when they start typing in their password.

Unfortunately, such functionality is rarely seen in web applications. A 4Guys reader recently asked me if there was a way to provide such feedback to a user logging into an ASP.NET website. The short answer is, Yes. It is possible to detect whether a user has Caps Lock enabled through JavaScript. Likewise, it's possible to have this client-side logic execute whenever a user starts typing in a particular textbox. With a little bit of scripting, such an interface could be devised.

After thinking about the implementation a bit, I decided to create a compiled server-side control that would emit the necessary JavaScript. This article introduces this control, WarnWhenCapsLockIsOn, and shows how it works and how to use it in an ASP.NET web page. Read on to learn more!

- continued -

An Overview of the WarnWhenCapsLockIsOn Control's Functionality

The WarnWhenCapsLockIsOn control is designed to display a page developer-specified message if a user types in a specified TextBox control with Caps Lock on. The control extends the Label class so it has the same set of properties as the Label control: Text, Font, Width, CssClass, and so on. Use these properties to configure the message and the message's appearance.

In addition to the Label control's properties, the WarnWhenCapsLockIsOn control also has a TextBoxControlId property, which you must set to the ID of a TextBox control. This property is required. (To provide a Caps Lock warning for multiple TextBox controls on the page, add a WarnWhenCapsLockIsOn control for each TextBox.) If the user starts typing in the specified textbox with the Caps Lock on, the WarnWhenCapsLockIsOn control is displayed. The WarnWhenCapsLockIsOn control is hidden again when one of the following conditions apply:

  • The user types in the same textbox with Caps Lock off.
  • The WarnWhenCapsLockIsOn control has been displayed for WarningDisplayTime milliseconds. The WarningDisplayTime property defaults to a value of 2500, meaning that the control will be displayed for 2.5 seconds after the last character with Caps Lock on has been typed. You can lengthen or shorten this property value as needed. A value of 0 displays the warning until the user types in a character without Caps Lock on or after there's a postback.
To provide a warning to the user if Caps Lock is on, then, you'll add this control to the page, set its Text property (and, perhaps, other formatting properties), and indicate the TextBox to "watch" via the control's TextBoxControlId property. That's all there is to it! We'll discuss how to add the WarnWhenCapsLockIsOn control to your ASP.NET website project and look at an end-to-end example in the "Using the WarnWhenCapsLockIsOn Control in an ASP.NET Web Page" section later on in this article.

Initially Hiding the WarnWhenCapsLockIsOn Control

Whenever the ASP.NET page is loaded in the user's browser, the WarnWhenCapsLockIsOn control needs to be hidden so that it does not appear. The WarnWhenCapsLockIsOn control should only appear when the user starts typing in the specified textbox with Caps Lock on. To initially hide the WarnWhenCapsLockIsOn control, I overrided the control's AddAttributesToRender method and added code that set the visibility and display style attributes to "hidden" and "none", respectively.

public class WarnWhenCapsLockIsOn : System.Web.UI.WebControls.Label
   protected override void AddAttributesToRender(HtmlTextWriter writer)

      // Add a style attribute that hides this element (by default)
      writer.AddStyleAttribute(HtmlTextWriterStyle.Visibility, "hidden");
      writer.AddStyleAttribute(HtmlTextWriterStyle.Display, "none");


The effects of these style attribute settings are evinced by the markup sent to the browser. Because the WarnWhenCapsLockIsOn control extends the Label class, it renders as a <span> element with the value of its Text property rendered as the <span> element's inner content, just like the Label control. In the style attribute you can see that the visibility and display settings have been configured to hide the control:

<span id="ID" style="visibility:hidden;display:none;">Value of Text property</span>

We now need to write JavaScript that sets the visibility and display settings to "visible" and "inline" whenever the user types into the corresponding textbox with Caps Lock on.

Executing JavaScript in Response to Typing in a Textbox

It is possible to execute JavaScript in response to a variety of client-side events, such as when the document has loaded, when the user changes the selection of a drop-down list, when an input field receives (or loses) focus, or when the user presses a key. To execute client-side script in response to a user pressing a key, use the onkeypress event like so:

<input type="text" ... onkeypress="action" />

The action can be inline script or a call to a function. In addition to noting that a key has been pressed, JavaScript offers the event object, which includes information such as what key was pressed and whether or not the Shift key was depressed, among other useful tidbits. In short, we need to add an onkeypress client-side event handler to the TextBox control referenced by the WarnWhenCapsLockIsOn control's TextBoxControlId property. The event handler called must determine whether Caps Lock is on or off and show or hide the WarnWhenCapsLockIsOn control in response.

To add client-side event handlers to a server-side Web control use the Attributes collection like so:

WebControlID.Attributes["attribute"] = "value";

The above code adds the markup attribute="value" to the Web control's rendered output. Usually this sort of logic is applied during the PreRender stage of the page lifecycle. Therefore, to implement this functionality I overrode the OnPreRender method in the WarnWhenCapsLockIsOn class (which is fired during the PreRender stage) and added the following code:

public class WarnWhenCapsLockIsOn : System.Web.UI.WebControls.Label
   protected override void OnPreRender(EventArgs e)

      if (this.Enabled)
         /* We need to add two bits of JavaScript to the page:
          * (1) The include file that has the JavaScript function to check if Caps Lock is on
          * (2) JavaScript that will call the appropriate function in (1) when a key is pressed in the TextBoxControlId TextBox

         // (1) Register the client-side function using WebResource.axd (if needed)
         if (this.Page != null && !this.Page.ClientScript.IsClientScriptIncludeRegistered(this.GetType(), "skmControls2"))
            this.Page.ClientScript.RegisterClientScriptInclude(this.GetType(), "skmValidators", this.Page.ClientScript.GetWebResourceUrl(this.GetType(), "skmControls2.skmControls2.js"));

         // (2) Call skm_CountTextBox onkeyup
         TextBox tb = GetTextBoxControl();

tb.Attributes["onkeypress"] += string.Format("skm_CheckCapsLock( event, '{0}', {1});", this.ClientID, this.WarningDisplayTime);


The GetTextBoxControl method (not shown in this article) returns a reference to the TextBox control specified by the TextBoxControlId property. This referenced TextBox has its Attributes collection updated to include an onkeypress event handler that calls the JavaScript function skm_CheckCapsLock. The skm_CheckCapsLock function (which we'll examine shortly), is passed three input parameters:

  • event - information about the keypress event, including what key was pressed and whether the Shift key was depressed.
  • The value of the WarnWhenCapsLockIsOn's ClientID property - we need to supply the client-side id of the WarnWhenCapsLockIsOn control so that it can be displayed or hidden, depending on whether the user has Caps Lock on.
  • WarningDisplayTime - the number of milliseconds to display the Caps Lock warning; defaults to 2500 milliseconds (2.5 seconds).
The WarnWhenCapsLockIsOn control is one of a few controls in my open-source control library, skmControls2. Another control in the same library is the TextBoxCounter control, which interactively displays how many characters the user has currently typed into a given textbox. (For more information on the TextBoxCounter control, read: Creating a TextBox Word / Character Counter Control.) The TextBoxCounter control requires a bit of JavaScript, and that JavaScript is already defined in a file named skmControls2.js. This file is compiled as an embedded resource in the assembly and is loaded into the ASP.NET page on which the TextBoxCounter or WarnWhenCapsLockIsOn controls are used via a call to the GetWebResourceUrl method. For more information on this technique see: Accessing Embedded Resources through a URL using WebResource.axd.

Because the skmControls2 library already has a JavaScript file, I decided to place the skm_CheckCapsLock function (and ancillary helper functions) there. It is injected into the ASP.NET page in the OnPreRender method (see step 1 in the comments in OnPreRender).

Determining Whether Caps Lock is On

When the keypress event is raised in client-side script, the event object contains information as to what key was pressed and whether the Shift key was depressed (along with whether Alt and Ctrl were depressed). It does not, however, indicate whether Caps Lock was on. The good news is that with a bit of code we can determine whether Caps Lock is on. If the user has entered a capital letter and the Shift key is not depressed, then Caps Lock must be on; likewise, if the user enters a lowercase letter and the Shift key is depressed, then Caps Lock is on.

The skm_CheckCapsLock function determines whether Caps Lock is on and, if so, calls the skm_ShowCapsWarning function, which displays the WarnWhenCapsLockIsOn control for a specified interval. If Caps Lock is not on then the skm_HideCapsWarning function is called, which hides the WarnWhenCapsLockIsOn control. The skm_CheckCapsLock function uses a modified version of a script created by John G. Wang, available online at http://javascript.internet.com/forms/check-cap-locks.html.

function skm_CheckCapsLock( e, warnId, dispTime ) {
   var myKeyCode = 0;
   var myShiftKey = e.shiftKey;
   if ( document.all ) {
      // Internet Explorer 4+
      myKeyCode = e.keyCode;
   } else if ( document.getElementById ) {
      // Mozilla / Opera / etc.
      myKeyCode = e.which;
   if ((myKeyCode >= 65 && myKeyCode <= 90 ) || (myKeyCode >= 97 && myKeyCode <= 122)) {
      if (
         // Upper case letters are seen without depressing the Shift key, therefore Caps Lock is on
         ( (myKeyCode >= 65 && myKeyCode <= 90 ) && !myShiftKey )


         // Lower case letters are seen while depressing the Shift key, therefore Caps Lock is on
         ( (myKeyCode >= 97 && myKeyCode <= 122 ) && myShiftKey )
         skm_ShowCapsWarning(warnId, dispTime);
      else {

The keen reader will note that this check doesn't really identify when Caps Lock is on or off. Rather, it identifies if Caps Lock is on or off when typing in an alphabetic character. If the user types in the number "9" or presses the left arrow, the first conditional statement will evaluate to false. In other words, the skm_ShowCapsWarning or skm_HideCapsWarning functions are only called when the user enters an alphabetic character.

The skm_ShowCapsWarning and skm_HideCapsWarning functions are not shown in this article; you'll have to download the source code to inspect them. They're both fairly straightforward: both reference the WarnWhenCapsLockIsOn control and then set the visibility and display properties to show or hide the warning message. The only trickiness deals with the code that displays the warning for at most a specified number of milliseconds after the user types in with Caps Lock on. Specifically, these functions use the JavaScript setTimeout and clearTimeout methods. For more information on these functions, see the JavaScript Timing Events tutorials at W3 Schools.

Using the WarnWhenCapsLockIsOn Control in an ASP.NET Web Page

The download available at the end of this article includes the complete source code for the WarnWhenCapsLockIsOn controls, as well as a demo ASP.NET website. To use the skmControls2 controls in an ASP.NET website, copy the DLL to the website's /Bin directory and then add the following @Register directive to the tops of the .aspx pages where you want to use the controls:

<%@ Register Assembly="skmControls2" Namespace="skmControls2" TagPrefix="skm" %>

(Alternatively, you can add this @Register directive in the Web.config file so that you do not need to add it to every ASP.NET page that uses the controls. See Tip/Trick: How to Register User Controls and Custom Controls in Web.config.)

The demo included in the download has an ASP.NET page that shows using the WarnWhenCapsLockIsOn control in two scenarios: on a stand-alone TextBox and on the Password TextBox in the Login control. Let's look at the stand-alone TextBox example first:

<b>Please enter your name:</b>
<asp:TextBox ID="YourName" runat="server"></asp:TextBox>

<skm:WarnWhenCapsLockIsOn runat="server" ID="WarnOnYourName"
           CssClass="CapsLockWarning" TextBoxControlId="YourName"
           WarningDisplayTime="5000"><b>Warning:</b> Caps Lock is on.</skm:WarnWhenCapsLockIsOn>

<br />
<asp:Button ID="Button1" runat="server" Text="Submit" />

The declarative markup above shows three controls: a TextBox (YourName); a WarnWhenCapsLockIsOn control; and a Button Web control. The WarnWhenCapsLockIsOn control is configured to display the message "Warning: Caps Lock is on" when the user types into the YourName textbox with Caps Lock on. It uses the CSS class CapsLockWarning for its styling, which I've defined on the page; the class specifies a light yellow background, padding, centered text, and a red border. The warning message is displayed for (at most) five seconds.

The following screen shot shows the demo page in action. Note that when Caps Lock is on and an alphabetic character is typed, the warning is displayed. This warning immediately disappears when Caps Lock is turned off and another alphabetic character is typed, or after five seconds, whatever comes first.

A warning is displayed when typing in the textbox with Caps Lock on.

This demo also shows how to apply the WarnWhenCapsLockIsOn control to the Password TextBox of a Login control. Start by adding a Login control to the page and then, from the Designer, select the "Convert to Template" from its smart tag. This will convert the Login control into a template, at which point you can add the WarnWhenCapsLockIsOn control in the Login user interface as needed, referencing the Password TextBox (whose ID property is "Password"). Be sure to put the WarnWhenCapsLockIsOn control within the Login control's template so that it can "see" the Password TextBox.

The WarnWhenCapsLockIsOn control can also be used in a Login control.

Happy Programming!

  • By Scott Mitchell


  • Download the Complete Source Code (in ZIP format)
  • Further Readings:

  • ASP.NET Security: Forms Authentication, User Accounts, Membership, and Roles
  • Accessing Embedded Resources through a URL using WebResource.axd
  • Creating a TextBox Word / Character Counter Control
  • JavaScript for Checking Caps Lock
  • JavaScript for Clearing All Timers

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