XLLs

XLLs are a type of Excel add-in implemented as native dynamic-link libraries with the .xll extension. They extend Excel by providing custom worksheet functions, commands, and automation capabilities beyond what standard VBA or COM add-ins offer. XLLs are primarily used to deliver high-performance computations, access to external libraries, or specialized data sources inside Excel.

Most XLLs are written in C or C++ and built against the Excel XLL SDK, then loaded by Excel as a plug-in. Because they execute as native code, XLLs can operate more quickly than interpreted add-ins and can call into external C libraries or perform operations not easily achieved with VBA.

An XLL exposes functions to Excel as user-defined functions and can also provide commands that appear in menus or custom toolbars. During initialization, the XLL’s entry point (commonly xlAutoOpen) runs to register the available functions, argument types, and help text; cleanup may be performed by an xlAutoClose entry point. Registered UDFs can be used in worksheet formulas, including certain array formulas, depending on the API capabilities.

XLLs are primarily a Windows technology; distribution and support for macOS have been limited and version-dependent. They are distributed as native binaries and require trusted sources due to the risk of executing arbitrary code. They are incompatible with standard Excel Online and may not be supported in all environments.

XLLs are distinct from VBA-based add-ins (XLA) and COM or .NET add-ins, which use different hosting and security models. They are sometimes referred to as Excel XLLs or Excel add-ins and remain in use in specialized industries where performance or access to low-level APIs matters.

See also: Excel add-ins, XLA, VBA, COM add-ins, Excel C API.