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Extend custom functions with XLL add-ins

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Note

An XLL add-in is an Excel add-in file with the file extension .xll. An XLL file is a type of dynamic link library (DLL) file that can only be opened by Excel. XLL add-in files must be written in C or C++. See Developing Excel XLLs to learn more.

If you have existing Excel XLL add-ins, you can build equivalent custom function add-ins using the Excel JavaScript API to extend your solution features to other platforms, such as Excel on the web or on a Mac. However, Excel JavaScript API add-ins don't have all of the functionality available in XLL add-ins. Depending on the functionality your solution uses, the XLL add-in may provide a better experience in Excel on Windows than the Excel JavaScript API add-in.

Important

The equivalent add-in feature is supported by the following platform and applications. COM add-ins can't be installed on any other platform, so on those platforms the manifest element that is discussed later in this article, EquivalentAddins, is ignored.

  • Excel, Word, and PowerPoint on Windows (Version 1904 or later)
  • Classic Outlook on Windows (Version 2102 or later) against a supported Exchange server version
    • Exchange Online
    • Exchange 2019 Cumulative Update 10 or later (KB5003612)
    • Exchange 2016 Cumulative Update 21 or later (KB5003611)

Specify equivalent XLL in the manifest

To enable compatibility with an existing XLL add-in, identify the equivalent XLL add-in in the manifest of your Excel JavaScript API add-in. Excel then uses the XLL add-in functions when running on Windows, instead of your Excel JavaScript API add-in custom functions.

To set the equivalent XLL add-in for your custom functions, specify the FileName of the XLL file. When the user opens a workbook with functions from the XLL file, Excel converts the functions to compatible functions. The workbook then uses the XLL file when opened in Excel on Windows, but it continues to use custom functions from your Excel JavaScript API add-in when opened on the web or on Mac.

The following example shows how to specify both a COM add-in and an XLL add-in as equivalents in an Excel JavaScript API add-in manifest file. Often you specify both. For completeness, this example shows both equivalents in context. They're identified by their ProgId and FileName respectively. The EquivalentAddins element must be positioned immediately before the closing VersionOverrides tag. For more information on COM add-in compatibility, see Make your Office Add-in compatible with an existing COM add-in.

<VersionOverrides>
  ...
  <EquivalentAddins>
    <EquivalentAddin>
      <ProgId>ContosoCOMAddin</ProgId>
      <Type>COM</Type>
    </EquivalentAddin>

    <EquivalentAddin>
      <FileName>contosofunctions.xll</FileName>
      <Type>XLL</Type>
    </EquivalentAddin>
  </EquivalentAddins>
</VersionOverrides>

Note

If an Excel JavaScript API add-in declares its custom functions to be compatible with an XLL add-in, changing the manifest at a later time could break a user's workbook because it will change the file format.

Custom function behavior for XLL compatible functions

An add-in's XLL functions are converted to XLL compatible custom functions when a spreadsheet is opened and there is an equivalent add-in available. On the next save, the XLL functions are written to the file in a compatible mode so that they work with both the XLL add-in and Excel JavaScript API add-in custom functions (when on platforms unsupported by XLL).

The following table compares features across XLL user-defined functions, XLL compatible custom functions, and Excel JavaScript API add-in custom functions.

XLL user-defined function XLL compatible custom functions Excel JavaScript API add-in custom function
Supported platforms Windows Windows, macOS, web browser Windows, macOS, web browser
Supported file formats XLSX, XLSB, XLSM, XLS XLSX, XLSB, XLSM XLSX, XLSB, XLSM
Formula autocomplete No Yes Yes
Streaming Possible via xlfRTD and XLL callback. Yes Yes
Localization of functions No No. The Name and ID must match the existing XLL's functions. Yes
Volatile functions Yes Yes Yes
Multi-threaded recalculation support Yes Yes Yes
Calculation behavior No UI. Excel can be unresponsive during calculation. Users see #BUSY! until a result is returned. Users see #BUSY! until a result is returned.
Requirement sets N/A CustomFunctions 1.1 and later CustomFunctions 1.1 and later

See also