Home > Articles > Programming > General Programming/Other Languages

This chapter is from the book

This chapter is from the book

Nonstandard Messages

Until now, the discussion has centered on regular Windows messages (those that begin with WM_XXX). However, two other major categories of messages merit some discussion: notification messages and user-defined messages.

Notification Messages

Notification messages are messages sent to a parent window when something happens in one of its child controls that might require the parent's attention. Notification messages occur only with the standard Windows controls (button, list box, combo box, and edit control) and with the Windows Common Controls (tree view, list view, and so on). For example, clicking or double-clicking a control, selecting some text in a control, and moving the scrollbar in a control all generate notification messages.

You can handle notification messages by writing message-handling procedures in the form that contains a particular control. Table 3.2 lists the Win32 notification messages for standard Windows controls.

Table 3.2 Standard Control Notification Messages



Button Notification



The user clicked a button.


A button is disabled.


The user double-clicked a button.


The user highlighted a button.


The button should be painted.


The highlight should be removed.

Combo Box Notification



The list box of a combo box has closed.


The user double-clicked a string.


The list box of a combo box is dropping down.


The user has changed text in the edit control.


Altered text is about to be displayed.


The combo box is out of memory.


The combo box is losing the input focus.


A new combo box list item is selected.




The user's selection should be canceled.


The user's selection is valid.


The combo box is receiving the input focus.

Edit Notification



The display is updated after text changes.


The edit control is out of memory.


The user clicked the horizontal scrollbar.


The edit control is losing the input focus.


The insertion is truncated.


The edit control is receiving the input focus.


The edit control is about to display altered text.


The user clicked the vertical scrollbar.

List Box Notification



The user double-clicked a string.


The list box is out of memory.


The list box is losing the input focus.


The selection is canceled.


The selection is about to change.


The list box is receiving the input focus.

Internal VCL Messages

VCL has an extensive collection of its own internal and notification messages. Although you don't commonly use these messages in your Delphi applications, Delphi component writers will find them useful. These messages begin with CM_ (for component message) or CN_ (for component notification), and they are used to manage VCL internals such as focus, color, visibility, window re-creation, dragging, and so on. You can find a complete list of these messages in the "Creating Custom Components" portion of the Delphi online help.

A common inquiry is how to detect that the mouse is entered or left a controls space. This can be handled by processing the custom messages CM_MOUSEENTER and CM_MOUSELEAVE. Consider the following component:

 TSpecialPanel = class(TPanel)
 procedure CMMouseEnter(var Msg: TMessage); message CM_MOUSEENTER;
 procedure CMMouseLeave(var Msg: TMessage); message CM_MOUSELEAVE;
procedure TSpecialPanel.CMMouseEnter(var Msg: TMessage);
 Color := clWhite;

procedure TSpecialPanel.CMMouseLeave(var Msg: TMessage);
 Color := clBtnFace; 

This component handles the custom messages by turning the panel white when the mouse has entered the component's surface area and then turns the color back to clBtnFace when the mouse leaves. You'll find an example of this code on the CD under the directory CustMessage.

User-Defined Messages

At some point, you'll come across a situation in which one of your own applications must send a message to itself, or you have to send messages between two of your own applications. At this point, one question that might come to mind is, "Why would I send myself a message instead of simply calling a procedure?" It's a good question, and there are actually several answers. First, messages give you polymorphism without requiring knowledge of the recipient's type. Messages are therefore as powerful as virtual methods but more flexible. Also, messages allow for optional handling: If the recipient doesn't do anything with the message, no harm is done. Finally, messages allow for broadcast notifications to multiple recipients and "parasitic" eavesdropping, which isn't easily done with procedures alone.

Messages Within Your Application

Having an application send a message to itself is easy. Just use the Perform(), SendMessage(), or PostMessage() function and use a message value in the range of WM_USER + 100 through $7FFF (the value Windows reserves for user-defined messages):


 SomeForm.Perform(SX_MYMESSAGE, 0, 0);
 { or }
 SendMessage(SomeForm.Handle, SX_MYMESSAGE, 0, 0);
 { or }
 PostMessage(SomeForm.Handle, SX_MYMESSAGE, 0, 0);

Then create a normal message-handling procedure for this message in the form in which you want to handle the message:

TForm1 = class(TForm)
 procedure SXMyMessage(var Msg: TMessage); message SX_MYMESSAGE;

procedure TForm1.SXMyMessage(var Msg: TMessage);
 MessageDlg('She turned me into a newt!', mtInformation, [mbOk], 0);

As you can see, there's little difference between using a user-defined message in your application and handling any standard Windows message. The real key here is to start at WM_USER + 100 for interapplication messages and to give each message a name that has something to do with its purpose.


Never send messages with values of WM_USER through $7FFF unless you're sure that the intended recipient is equipped to handle the message. Because each window can define these values independently, the potential for bad things to happen is great unless you keep careful tabs on which recipients you send WM_USER through $7FFF messages to.

Messaging Between Applications

When you want to send messages between two or more applications, it's usually best to use the RegisterWindowMessage() API function in each application. This method ensures that every application uses the same message number for a given message.

RegisterWindowMessage() accepts a null-terminated string as a parameter and returns a new message constant in the range of $C000 through $FFFF. This means that all you have to do is call RegisterWindowMessage() with the same string in each application between which you want to send messages; Windows returns the same message value for each application. The true benefit of RegisterWindowMessage() is that because a message value for any given string is guaranteed to be unique throughout the system, you can safely broadcast such messages to all windows with fewer harmful side effects. It can be a bit more work to handle this kind of message, though; because the message identifier isn't known until runtime, you can't use a standard message handler procedure, and you must override a control's WndProc() or DefaultHandler() method or subclass an existing window procedure. A technique for handling registered messages is demonstrated in Chapter 13, "Hard-Core Techniques," of Delphi 5 Developer's Guide, found on this book's CD-ROM. This useful demo shows how to prevent multiple copies of your application from being launched.


The number returned by RegisterWindowMessage() varies between Windows sessions and can't be determined until runtime.

Broadcasting Messages

TWinControl descendants can broadcast a message record to each of their owned controls—thanks to the Broadcast() method. This technique is useful when you need to send the same message to a group of components. For example, to send a user-defined message called um_Foo to all of Panel1's owned controls, use the following code:

 M: TMessage;
 with M do
 Message := UM_FOO;
 wParam := 0;
 lParam := 0;
 Result := 0;

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020