How to return a custom object or double[,] to a cell in Excel from ExcelDna?

Question

Ideally my UDF would return some double results, either in a form of double[,], double[] or as a custom object. I would like it to be all stored in a single cell in Excel and then use another UDF to extract them. It is similar like cache the result of a calculation and display it later on demand.

Is it possible?

Answer 1

One approach is to have an internal 'object store' in your Excel-DNA add-in, and then have a function that returns some kind of 'handle' to Excel, together with accessor functions that take the 'handle' and access the object as required. Cleaning up the objects you can create is a possible issue, but can be dealt with using some of the RTD features.

This discussion on the Excel-DNA group has an example of implementing this idea in F# - https://groups.google.com/group/exceldna/browse_frm/thread/138bc83923701e6d.

Answer 2

It certainly is possible, but it is difficult to give you a detailed answer without knowing how you are using ExcelDNA. What I mea by that is whether you are wrapping your C# methods in vba code, or are getting a handle on the Excel application in C# and writing to the workbook directly from there. In both cases you can do what you want, but the way to do it would be slightly different. The wrapper method would also be slightly less flexible, since you have to first pass out your values to the vba UDF and then write them to the cell from there, so you will be restricted in the data type you can return (as far as I know anyway).

The way to do this would be to write the results to a specific cell, maybe on a hidden sheet, so it can't be tampered with, and then retrieve those using another UDF. You would have to hardcode the cell address, or maybe parse the sheet to find the values you want.

In case of a method returning a double[,], you would probably have to write the values to two different cells, or in one cell as text with a separator and then convert from text to double when you retrieve the cell value, something like Double.Parse(cell.value.ToString().Split(',')[0]) to get the first values (assuming that you are storing the values as a comma-separated string) or similar code in vba if you use a pure vba UDF to get the values...

If you want to do this, I think you should definitely use a hidden sheet with a well-defined structure to store your values. If you only need to store the values for the duration of the session, then I think you should store them in global variables in a vba module.

UPDATE

Since you are just writing functions, I don't think you will be able to pass out a custom object (unless you implement your own converter, convert it to text and then read it back in that way).

You pass back the double or double[,] to a variable in your UDF and write it to a cell from there. Then read it back again from that cell with any other UDF.

The rest is the same as I wrote above. If you store two values in the same cell, you will have to do that as text, so you will have to split and parse the values first in your UDF before passing them to your C# method (or you can do the parsing in the method).

In practice there should be no problem at all with what you are trying to do.