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A Single Document Interface (SDI) is a way of designing a graphical application so that each document appears in its own top-level window, and the operating system’s window manager handles each of those windows separately. Each window can carry its own menus, ribbon, or other interface controls. The term is most often contrasted with the Multiple Document Interface (MDI), where documents live inside one shared parent window.
What SDI means
Microsoft’s Learn article “Programming for the Single Document Interface in Excel” defines the concept directly: “SDI is a method of organizing graphical user interface (UI) applications into individual windows that the operating system window manager handles separately.”
The practical result is that a user who opens two documents in an SDI application sees two separate windows, and each window presents its own set of interface controls. In the Excel 2013 example Microsoft uses, each workbook window contains one workbook and its own ribbon.
SDI compared with MDI
SDI and MDI are two answers to the same question: how should an application organize the documents a user has open? The table below sets out the comparable behaviors. The Excel 2010 and Excel 2013 references come from Microsoft’s article, so treat them as descriptions of those versions rather than of every release.
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| Behavior | SDI (Single Document Interface) | MDI (Multiple Document Interface) |
|---|---|---|
| Where each document lives | In its own top-level window | As a child window inside one parent application window |
| Interface controls | Each document window can have its own menus, ribbon, or other controls; the Excel 2013 example shows one ribbon per workbook window | Controls are typically shared at the application level; the Excel 2010 example shows workbooks sharing one application-level ribbon |
| Who manages the windows | The operating system window manager handles each window separately | The parent window contains and arranges its document child windows |
| Separate processes | Not implied by the label; both Excel 2013 workbooks can run in one Excel instance | Not implied by the label |
The MDI description, including the parent-and-child arrangement, is drawn from Microsoft Press training material, “Lesson 4: Document/View Architecture”, available at https://flylib.com/books/en/2.906.1.20/1/.
What SDI does not mean
It does not mean only one document can be open
SDI describes how windows are arranged, not how many documents an application can hold. An SDI application can have many documents open at once; each one simply sits in its own window. A common misreading is that SDI forces a one-document-at-a-time limit. Archived National Instruments guidance, “Document/View Architecture in Visual C++ Test and Measurement Applications”, cites Notepad as an example of an application used to work with one document at a time. That example reflects one implementation, and application window behavior can change across versions, so it should not be read as a general rule. The archived guidance is available at this National Instruments knowledge article.
Separate windows do not guarantee separate processes
Whether each window is its own top-level window and whether each window runs in its own process are two different questions. Microsoft notes that opening another workbook window in Excel 2013 can leave both workbooks in the same Excel instance. The window arrangement is visible to the user; the process layout is a separate implementation detail. Do not infer process boundaries from the label alone.
Why the distinction matters for Excel developers
For people writing add-ins or VBA, SDI changes assumptions about interface state. Each workbook window has its own ribbon, so code that assumed one shared set of controls may need to track which window is active as the user switches among workbooks. Microsoft’s article discusses considerations for custom task panes, custom ribbons, VBA, and window-related behavior in that context.
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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →These details describe the Excel versions and programming model in Microsoft’s article. Confirm the target application’s current behavior before applying them to other software or to newer releases.
Tabbed and hybrid designs
Many modern applications do not fit cleanly into either category. A tabbed interface, for example, may show several documents within one window while still behaving differently from a classic MDI child layout. Microsoft Press’s training material notes that the SDI or MDI label may not fully capture these designs. When precision matters, describe the window behavior itself rather than relying on the label.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to classify an application’s window model
Use these observable checks instead of assuming a label from the product’s reputation:
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- Open two documents in the application. If each one appears as a separate top-level window with its own menus or ribbon, the layout is consistent with SDI.
- Check whether the controls change with the document. Per-document controls point toward SDI; a single shared set of controls inside one frame points toward MDI.
- Check whether the documents can be arranged side by side as separate windows. This is a behavior to observe in the specific application, not a guaranteed feature of either model.
- If you need to know about processes, check the process list of your operating system’s task manager after opening documents. Two windows listed under one executable generally indicate a shared process, while separate entries indicate separate processes. Confirm this against the application’s own documentation, because the process layout is not defined by the window model.
Key points to remember
- SDI places each document in its own top-level window, managed separately by the operating system.
- MDI nests document windows inside one parent window.
- SDI does not mean only one document can be open, and it does not establish separate processes.
- Labels such as SDI and MDI may not capture tabbed or hybrid designs, so describe the observed behavior when precision matters.
Further reading
- Microsoft Learn: Programming for the Single Document Interface in Excel
- Microsoft Press training material: Lesson 4: Document/View Architecture
- Pearson/Certiport, Software Development Fundamentals Study Guide: MTA Software Development Fundamentals study guide (PDF)
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