The Excel C SDK dates from the early 1990s. Memory management is manual, the type system is painful, and there’s almost no tooling. Despite all of this, it remains the only way to build add-ins that run truly in-process with Excel, supporting multi-threaded recalculation and the full breadth of what the host application can do. If you want the best possible performance, you need an XLL. But it’s a foot gun....
Excel is the original rapid application development tool. People do unholy things to get the job done with no code, or perhaps with just a smattering of VBA. These solutions can be brilliant in the moment but, long term, tend to end up brittle and hard to extend. Over the years there have been many approaches for extending Excel beyond formula soup and VBA. There is the excellent Excel-DNA, PyXLL, Microsoft’s own Office....
xllify is a platform I’ve released for creating custom Excel functions by describing what you need or pasting some existing formulas or VBA. The resulting functions get packaged into a ready-to-deploy Office Add-in for all versions of Excel, as well as an optional-extra native XLL build for Windows. Right now you can write your prompt for your functions in a browser or Excel add-in to iterate, debug and even manually code....