Excel12 and Excel12v

There are two API functions that enable you to interact with Excel.
These are both callback functions that allows the DLL to call Excel.
The only difference between them is the number of arguments.
Excel12 - takes a variable number of arguments. The maximum is 255.
Excel12v - takes 4 arguments, with the last one being a variable sized array.
Excel12 and Excel12v ensure backwards compatibility with Excel 11 (2003) and earlier.

Excel12

uses UNICODE based strings

int Excel12(int iFunction, 
            xloper12 *pxReturnValue, 
            int iCount, 
            xloper12 *pxArgument1, 
            .., 
            xloper12 *pxArgument255)

iFunction - The number corresponding to a pre-defined function or command.
pxReturnValue - A pointer that will hold the result.
iCount - The number of arguments that are being passed in.
pxArgument1 - The first argument. Passed as a pointer.
pxArgument255 - The last argument. Passed as a pointer.

Excel12v

int Excel12v(int iFunction, 
             xloper12 *pxReturnValue, 
             int iCount, 
             xloper12 *pxArguments[])

iFunction - The number corresponding to a pre-defined function or command.
pxReturnValue - A pointer that will hold the result.
iCount - The number of arguments that are being passed in.
pxArguments - The array of arguments. They are all passed as pointers.

XLOPER12 data type

This is a multi-type data structure that can represent any type of worksheet data type.

XLOPER12 vs LPXLOPER12

In most cases the prototype is written as:

int Excel12v(int iFunction, xloper12 *pRetVal, int count, xloper12 *args[]);

but sometimes it is written as:

int Excel12v(int iFunction, LPXLOPER12 operRes, int count, xloper12 *args[]);

This is just to represent that the argument is a pointer without having to include the asterisk (*)

Return Values

What is returned will indicate whether the call has been successful or not.
If a non-zero is returned then the function was not successful.
If zero is returned (or xlretSuccess) then the function might have been successful.
You always need to check the "pxReturnValue"

0	xlretSuccess	Called successfully but you need to check the return value
1	xlretAbort	The macro that has been stopped by the user or the system
2	xlretInvX1fn	The function is not recognised, not supported or has the wrong context
4	xlretInvCount	The number of arguments is wrong
8	xlretInvXloper	One or more of the arguments passed in are not valid
16	xlretStackOvfl	Excel's pre-call stack check indicates a possibility that the stack might overflow
32	xlretFailed	The command failed or unable to allocate enough memory
64	xlretUncalced	A worksheet function tries to access cells that have not yet been recalculated
128	xlretNotThreadSafe	Trying to call a not thread safe method from a function indicated as thread safe

Excel Data Types

Excel will pass arguments and accepr return values for the following data types.

Data Type	Type
unsigned short *	String	UNICODE wide char string, length prepended
unsigned short *	String	UNICODE wide char string, null prepended
unsigned char *	String	ASCII byte string, length prepended
signed char *	String	ASCII byte string, null terminated
double	Number
double *	Number
signed long int	Number	32-bit
signed long int *	Number	32-bit
signed short int	Number	16-bit. can be used for bool
signed short int *	Number	16-bit
unsigned short int	Number	DWORD = wchar_t

Data types such as bool, char and float are not supported.
If an Excel worksheet function cannot convert an input value to the necessary type, it will return #VALUE!

Excel Events

The C API command functions are xlcOnData, xlcOnDoubleclick, xlcOnEntry, xlcOnKey, xlcOnRecalc, xlcOnWindow, xlcOnTime

Excel4

Excel 2003 and earlier.
uese the XLOPER data structure
uses ASCII based strings
An XLL is only loaded once per session
These can be browsed to from the Excel add-ins dialog box ?