When to Use a Dialog Box - Win32 Programmer's Reference

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When to Use a Dialog Box

Most applications use dialog boxes to prompt for additional information for commands that require user input. Using a dialog box is the only recommended way for an application to retrieve the input. For example, the File Open command requires the name of a file to open, so an application should use a dialog box to prompt the user for the name. In such cases, the application creates the dialog box when the user chooses the command and destroys the dialog box immediately after the user supplies the information.

Many applications also use dialog boxes to display information or options while the user works in another window. For example, word processing applications often use a dialog box with a text-search command. While the application searches for the text, the dialog box remains on the screen. The user can then return to the dialog box and search for the same word again; or the user can change the entry in the dialog box and search for a new word. Applications that use dialog boxes in this way typically create one when the user chooses a command and continue to display it for as long as the application runs or until the user explicitly closes the dialog box.

To support the different ways applications use dialog boxes, Windows provides two types of dialog box: modal and modeless. A modal dialog box requires the user to supply information or cancel the dialog box before allowing the application to continue. Applications use modal dialog boxes in conjunction with commands that require additional information before they can proceed. A modeless dialog box allows the user to supply information and return to the previous task without closing the dialog box. Modal dialog boxes are simpler to manage than modeless dialog boxes because they are created, perform their task, and are destroyed by calling a single function.

To create either a modal or modeless dialog box, an application must supply a dialog box template to describe the dialog box style and content; the application must also supply a dialog box procedure to carry out tasks. The dialog box template is a binary description of the dialog box and the controls it contains. The developer can create this template as a resource to be loaded from the application's executable file, or created in memory while the application runs. The dialog box procedure is an application-defined callback function that Windows calls when it has input for the dialog box or tasks for the dialog box to carry out. Although a dialog box procedure is similar to a window procedure, it does not have the same responsibilities.

An application typically creates a dialog box by using either the DialogBox or CreateDialog function. DialogBox creates a modal dialog box; CreateDialog creates a modeless dialog box. These two functions load a dialog box template from the application's executable file and create a pop-up window that matches the template's specifications. There are other functions that create a dialog box by using templates in memory; they pass additional information to the dialog box procedure as the dialog box is created.

Dialog boxes usually belong to a predefined, exclusive window class. Windows uses this window class and its corresponding window procedure for both modal and modeless dialog boxes. When the function is called, it creates the window for the dialog box, as well as the windows for the controls in the dialog box, then sends selected messages to the dialog box procedure. While the dialog box is visible, the predefined window procedure manages all messages, processing some messages and passing others to the dialog box procedure so that the procedure can carry out tasks. Applications do not have direct access to the predefined window class or window procedure, but they can use the dialog box template and dialog box procedure to modify the style and behavior of a dialog box.

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