Introduced in build 9.1
The RegisterCustomFunction method registers an IFunction
object with the workbook to be used when calculating a specific custom Excel formula.
Disabling Excel Calculation
Registering a formula allows ExcelWriter to calculate a function a certain way. However, once the workbook opens inside of Microsoft Excel, Excel will calculate the formula again with its own calculation engine. This means that if you registered a function name that does not exist in Excel, your formula will evaluate to #NAME? once Excel recalculates it. If you overwrote a formula with you own implementation, the formula will be recalculated using the implementation that is built into Excel. To keep Excel from re-calculating the formulas upon opening the workbook or upon changing values in the workbook, go to File -> Options -> Formulas. Under the "Calculation Options" header, there is an option called "Workbook Calculation". Please select the "Manual" option and uncheck the "Recalculate workbook before saving" checkbox. Now Excel will not re-calculate formulas. These settings will allow you to preserve the values that ExcelWriter calculated.
The name of the Excel Function you want to bind your custom IFunction logic to.
The custom logic you want to use for a specific Excel formula. You implement this logic by using the IFunction
If the custom function has already been registered with the workbook.