ExcelFunction IsMacroType questions

[emphasis87]

What are the (performance) implications using [ExcelFunction(IsMacroType=true)] - is there a reason not to always use it?
Is there a documentation for it?
Can it be combined with IsThreadSafe?

[emphasis87]

I have found this: https://learn.microsoft.com/en-us/office/client-developer/excel/multithreaded-recalculation-in-excel

Excel imposes an additional restriction: thread-safe functions cannot be registered as macro-sheet equivalents, and therefore cannot call XLM information functions or get the values of un-recalculated cells.

Excel uses a single main thread to run or execute the following:

• Microsoft Visual Basic for Applications (VBA) user-defined commands (often referred to as macros)
• XLM macro sheet user-defined commands and functions

Not sure if the previous applies, especially since I don't know what exactly XLM is.

[keithalewis]

Excel macros. The C API calls into Excel using the Excel4/Excel12 functions. The first argument is the "function number" to call. Functions numbers for functions start with xlf and function numbers for macros start with xlc. See https://xlladdins.github.io/Excel4Macros/ for documentation of the arguments used for macros.

[govert]

Functions that are both marked as IsMacroType=true and have at least one parameter marked as AllowReference=true will be considered volatile by Excel (even if marked IsVolatile=false). You can internally clear the flag, but that might not always be a good idea.

I suspect functions registered as IsMacroType=true might be considered differently in the dependency processing, though I have no evidence or documentation to offer.

[keithalewis]

I'm not sure what AllowReference=true means in Excel-DNA. In the Excel C API pointers to XLOPER arguments can be specified as either type "Q" or type "U". If type "U" then Excel can hand you a reference and the function becomes volatile. If type "Q" then Excel will handily coerce that to the value that the reference points to and will not be marked volatile.

I believe IsMacroType=true allows functions to fetch cell values that are stale and call information functions that do not change the state of the workbook. It does not make a function volatile.