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A Single Document Interface (SDI) is an application design in which each document appears in its own separately managed top-level window. The operating system’s window manager handles each of those windows on its own, and each window carries its own interface controls. SDI describes how windows are arranged on screen. It does not by itself say how many documents an application can have open, or whether those windows run in separate processes.
What SDI means
Microsoft describes SDI as a way of organizing graphical user interface applications into individual windows that the operating system window manager handles separately. In Microsoft’s Excel example from its programming guidance, each Excel 2013 window contains one workbook and its own ribbon user interface. The guidance is written for Excel 2010 and Excel 2013, so the specific behavior it describes applies to those versions. Newer releases may differ. (Source: Microsoft Learn, “Programming for the Single Document Interface in Excel”.)
Key properties of an SDI application
- Each document has its own top-level window.
- Each window has its own menus, ribbon, or other interface controls.
- The window manager treats each window as a separate item, so it can be moved, minimized, or arranged independently.
- Several documents can be open at once. SDI does not mean that only one document can be open in the entire application.
SDI versus MDI
The main alternative is the Multiple Document Interface (MDI). In an MDI application, one parent application window contains multiple document child windows. In the Excel 2010 example that Microsoft uses for comparison, workbooks shared an application-level ribbon rather than each carrying its own. (Sources: Microsoft Learn; Microsoft Press training material, “Lesson 4: Document/View Architecture”.)
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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →| Question | SDI | MDI |
|---|---|---|
| Where do documents live? | Each document is a top-level window handled separately by the window manager. | Documents are child windows inside one parent application window. |
| Do documents have their own interface controls? | Yes. The Excel 2013 example gives each workbook window its own ribbon. | Controls are generally shared at the application level. The Excel 2010 example shows a shared ribbon. |
| Can documents be viewed side by side? | Depends on the application and the window manager; the sources do not establish a general rule. | Depends on the application’s child-window layout; the sources do not establish a general rule. |
| Do separate windows mean separate processes? | Not implied by the label. In Excel 2013, opening another workbook window can leave both workbooks in the same Excel instance. | Not implied by the label; verify the behavior in the specific application. |
Those four questions are the most useful way to test a particular application. The SDI or MDI label alone is not enough to answer them.
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Separate windows do not mean separate processes
A common misunderstanding is that every SDI window is a separate program instance. That is not established by the design itself. Microsoft notes that Excel 2013 can keep multiple workbook windows inside one Excel instance. The window arrangement is what SDI describes; process boundaries are a separate architectural decision.
National Instruments’ archived Visual C++ guidance uses Notepad as an example of an application used to work with one document at a time. Behavior of this kind is implementation-specific, and an application can change its window behavior between versions, so treat that example as a description of one program at one point in time. (Source: National Instruments, archived “Document/View Architecture in Visual C++ Test and Measurement Applications”.)
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Why SDI matters for developers
For developers, SDI changes assumptions about user-interface state. If each document window has its own ribbon, custom task pane, or set of controls, code that assumed one shared set of controls can produce confusing results when the user switches between workbooks. Microsoft’s Excel guidance discusses these considerations for custom task panes, custom ribbons, VBA, and window-related behavior. Those details apply to the Excel versions and programming context Microsoft describes; check the target application’s current behavior before applying them elsewhere.
Tabbed and hybrid designs
Many modern applications do not fit cleanly into either category. Some display documents as tabs inside one window, and others mix top-level document windows with shared panels. The SDI or MDI label may not capture that behavior. When precision matters, describe what the window does: whether each document is a top-level window, whether controls are per document or shared, and whether separate documents share one process. (Source: Microsoft Press training material.)
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Learning SDI in a software development course
SDI is one of the basic user-interface design concepts covered in introductory software development material. The Pearson/Certiport study guide for the Microsoft Technology Associate (MTA) Software Development Fundamentals exam is one example of an entry-level reference that covers it. (Source: Pearson/Certiport, “Software Development Fundamentals Study Guide”.)
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common questions readers ask
People searching this topic usually ask three things: what SDI means, how it differs from MDI, and whether SDI limits an application to one open document. The short answers are above. SDI means one top-level window per document, MDI means child windows inside one parent window, and SDI does not restrict how many documents can be open.
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