Multiple Document Interface Enhancements in Windows 3.1

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

SUMMARY

The Windows multiple document interface (MDI) provides a way to standardize the user interface of applications that can process more than one "document" simultaneously. Applications that use MDI automatically benefit from improvements in the MDI user interface code within Windows.

The MDI under Windows version 3.1 introduces a number of significant enhancements to the features provided by Windows version 3.0, and it also removes a number of problems. When an application that is designed to use MDI under Windows 3.0 is run on a system equipped with Windows 3.1, the application automatically receives the benefit of many of these enhancements. In addition, an application targeted specifically for Windows 3.1 can use additional new MDI features.

MORE INFORMATION

The Windows 3.1 MDI corrects a number of problems found in the Windows 3.0 MDI. For a list of problems in Windows 3.0 that have been corrected in Windows 3.1, query on the following words in the Microsoft Knowledge Base:

   buglist3.00 and fixlist3.10 and mdi

The "Double-Click" Controversy Finally Comes to a Close

Under Windows 3.0, when the user double-clicks the mouse on the system menu bitmap of a maximized MDI child window, the child window does not close. This behavior caused many complaints when MDI was incorporated into Windows 3.0. A controversy arose regarding this behavior: some believed it to be a design choice; others saw it as a problem with the software.

The design has been changed under Windows 3.1. Using the mouse to double-click the system menu bitmap of a maximized MDI child window closes the child window. The new behavior removes an inconsistency among applications that use the MDI functions built into Windows and two applications that perform their own MDI processing: Microsoft Word for Windows and Microsoft Excel for Windows.

A Difference with Scroll Bars on MDI Children

Under Windows 3.0, if an application specifies WS_HSCROLL and WS_VSCROLL in the style field of an MDICREATESTRUCT data structure and creates an MDI child window, the scroll bars do not appear. Windows automatically sets the scroll range to zero when it creates the scroll bars. The application must call the SetScrollRange function to specify a positive, non-zero, upper bound for the scroll range to make the scroll bars appear.

Under Windows 3.1, the application behaves somewhat differently. Because Windows 3.1 does not reset the scroll range to zero, the scroll bars appear if the application specifies the WS_HSCROLL and WS_VSCROLL styles. This change might cause problems for an application designed under Windows 3.0 that expected the scroll bars to be hidden until the application explicitly set the scroll range.

The CS_NOCLOSE Class Style

Under Windows 3.0, specifying the CS_NOCLOSE window class style does not change the behavior of MDI child windows. This behavior has been corrected under Windows 3.1. For example, an application specifies CS_NOCLOSE in the style field of a WNDCLASS structure and registers a window class using that structure. If the application creates an MDI child window using the new class, Windows will disable the Close menu option on the child window's system menu.

However, Windows makes other changes to the MDI child's system menu that are not correct. For more information on this problem, query on the following words in the Microsoft Knowledge Base:

   CS_NOCLOSE and MDI

MDI Child Windows No Longer Required to Use Standard Styles

One of the most requested enhancements to Windows MDI provided by Windows 3.1 is the ability to create MDI child windows using any window style. If an application specifies the MDIS_ALLCHILDSTYLES style for its MDI client window, Windows creates the MDI child windows using only the styles specified in the MDICREATESTRUCT data structure. Otherwise, Windows enforces the default MDI child window styles. For example, an application that uses the MDIS_ALLCHILDSTYLES style can create MDI child windows without maximize or minimize buttons.

Multiple Document Interface Message Changes

Windows 3.1 includes several enhancements to MDI system messages. The WM_MDICASCADE and WM_MDITILE messages have been enhanced to accept a cascade flag. If an application sends either of these messages with wParam set to MDITILE_SKIPDISABLED, Windows does not change the position of disabled MDI child windows.

The WM_MDITILE message accepts two additional flags in wParam: MDITILE_HORIZONTAL and MDITILE_VERTICAL. These flags specify that the MDI child windows should be as wide as possible or as tall as possible, respectively.

Under Windows 3.0, the WM_MDINEXT message was not accompanied by any parameters in lParam or wParam. Under Windows 3.1, both parameters are used. The value of wParam specifies the handle of the MDI child window receiving the message. The lParam parameter is a Boolean value specifying whether the next or previous MDI child window is being activated.

The WM_MDISETMENU message also has new functionality. If wParam contains the Boolean value TRUE, the current menus are refreshed. If wParam contains the Boolean value FALSE, the frame menu and window menu are replaced with the menus specified in lParam.