Archidamos wrote:

What is XLL?



You can write UDF functions (and more) in C++, compile them to DLL
(called XLL) and use them as built-in functions.
it is 30 - 300 times faster Comparing to vba udfs


Go to Add-in click browse and you can see that allowed extensions are
*.xla, *.xll ....


whatever is not formula (i.e. events) must be done in .net but you do
not have to deal with calculation and pasting outputs to excel.