xll

XLL is the extension file format used by Microsoft Excel for native add-ins. An XLL is a dynamic library with the .xll extension that is loaded by Excel to expose additional worksheet functions, command items, and automation capabilities. XLLs are built using the Excel/XLL API, a C/C++ based interface that allows add-ins to register user-defined functions (UDFs), menus, and event handlers. UDFs defined in an XLL appear in cells just like built-in functions, and may return scalar values or arrays; XLLs can also provide custom function wizard entries and help text.

Development and distribution: Creating an XLL typically requires C or C++ tooling on Windows; you compile a DLL and export the required entry points, such as xlAutoOpen and xlAutoClose, to integrate with Excel. The official Microsoft Excel Software Development Kit (SDK) provides headers, samples and docs. In recent years, technologies such as Excel-DNA provide a way to create XLLs using .NET languages, simplifying the process for some developers, while still producing a .xll that Excel can load.

Usage considerations: XLLs run with the process privileges of Excel and can access workbook data; security settings may require trust or signing before loading. They are distributed as standalone files or part of an installation package. They are primarily Windows-based; cross-platform support is limited, as Excel for Mac uses different add-in mechanisms.

Comparison: XLLs differ from VBA macros and older XLA add-ins; newer development often leverages Office.js for web-based add-ins on modern Excel versions.