I have a scenario where I have an activeX EXE GUI component to
interact with Excel. The GUI component should allow certain controls
within it to be dragged and dropped to a cell in Excel. As
usual, the control that is dropped is called the "Drop source". The
drop source, written by me, returns a string like
this "=EXTERNAL_LIVE_DATA(param, param, param,...)".

The above part is too easy, but the difficulties are not there. Once
the drop source's string has been inserted into the cell, the
function call contained in the string should be invoked, regularly in
real time.

It is pretty tough just to handle the repeated invocation above;
however not impossible, by virtue of Excel's RTD mechanism.

I am stuck at a bigger challenge: the function contained in the
string will return not just 1 returned value, rather, it returns an
array of values which must be populated into a series of cells in a
row starting from the cell containing the function call string. This
one is hard. As we are well aware that user defined functions (UDF)
are restricted and not allowed to make any changes in the environment
except just returning a single value (or a single array, or a single
whatever which simply has to be a single blah).

I have not figured out how my function can populate values from the
returned array into not just the cell containing the function call
string (the drop source), but also a series of neighboring cells in
that row.