Execute a user-defined function into another cell VBA Excel

Question

I need to automatize a process which execute various (a lot) user-defined function with different input parameters.

I am using the solution of timer API found in I don't want my Excel Add-In to return an array (instead I need a UDF to change other cells) .

My question is the following: "Does anybody can explain to me HOW IT IS WORKING?" If I debug this code in order to understand and change what I need, I simply go crazy.

1) Let say that I am passing to the public function AddTwoNumbers 14 and 45. While inside AddTwoNumber, the Application.Caller and the Application.Caller.Address are chached into a collection (ok, easier than vectors in order not to bother with type). Application.Caller is kind of a structured object where I can find the function called as a string (in this case "my_function"), for example in Application.Caller.Formula. !!! Nowhere in the collection mCalculatedCells I can find the result 59 stored.

2)Ok, fair enough. Now I pass through the two UDF routines, set the timers, kill the timers. As soon as I am inside the AfterUDFRoutine2 sub, the mCalculatedCell(1) (the first -- and sole -- item of my collection) has MAGICALLY (??!?!?!!!??) obtained in its Text field exactly the result "59" and apparently the command Set Cell = mCalculatedCells(1) (where on the left I have a Range and on the right I have ... I don't know) is able to put this result "59" into the variable Cell that afterward I can write with the .Offset(0,1) Range property on the cell to the right.

I would like to understand this point because I would like to give MORE task to to inside a single collection or able to wait for the current task to be finished before asking for a new one (otherwise I am over-writing the 59 with the other result). Indeed I read somewhere that all the tasks scheduled with the API setTimer will wait for all the callback to be execute before execute itself (or something like this).

As you can see I am at a loss. Any help would be really really welcomed.

In the following I try to be more specific on what (as a whole) 
I am planning to achieved.

To be more specific, I have the function

public function my_function(input1 as string, Field2 as string) as double

/*some code */

end function

I have (let's say) 10 different strings to be given as Field2.

My strategy is as follow:

1)I defined (with a xlw wrapper from a C++ code) the grid of all my input values

2)define as string all the functions "my_function" with the various inputs

3)use the nested API timer as in the link to write my functions IN THE RIGHT CELLS as FORMULAS (not string anymore)

3)use a macro to build the entire worksheet and then retrieve the results.

4)use my xlw wrapper xll to process further my data.

You may wonder WHY should I pass through Excel instead of doing everything in C++. (Sometime I ask myself the same thing...) The prototype my_function that I gave above has inside some Excel Add-In that I need to use and they work only inside Excel.

It is working pretty well IN THE CASE I HAVE ONLY 1 "instance" of my_function to write for the give grid of input. I can even put inside the same worksheet more grids, then use the API trick to write various different my_functions for the different grids and then make a full calculation rebuild of the entire worksheet to obtain the result. It works. However, as soon as I want to give more tasks inside the same API trick (because for the same grid of input I need more calls to my_function) I am not able to proceed any further.

After Axel Richter's comment I would like to ad some other information

@Axel Richter Thank you very much for your reply.

Sorry for that, almost surely I wasn't clear with my purposes.

Here I try to sketch an example, I use integer for simplicity and let's say that my_function works pretty much as the SUM function of Excel (even if being an Excel native function I could call SUM directly into VBA but it is for the sake of an example).

If I have these inputs: input1 = "14.5" a vector of different values for Field2, for instance (11;0.52;45139) and then I want to write somewhere my_function (which makes the sum of the two values given as input).

I have to write down in a cell =my_function(14.5;11), in the other =my_function(14.5;0.52) and in a third one =my_function(14.5;45139).

These input changes any time I need to refresh my data, then I cannot use directly a sub (I think) and, in any case, as far as I understand, in writing directly without the trick I linked, I will always obtain strings : something like '=my_function(14.5;0.52). Once evaluated (for example by a full rebuild or going over the written cell and make F2 + return) will give me only the string "=my_function(14.5;0.52)" and not its result. I tried at the beginning to use an Evaluate method which works well as soon as I write something like 14.5+0.52, but it doesn't work as soon as a function (nor a user-defined function) is used instead.

This is "as far as I can understand". In the case you can enlighten me (and maybe show an easier track to follow), it would be simply GREAT.

Answer 1

So far the comments are correct in that they repeat the simple point that a User-Defined Function called a worksheet can only return a value, and all other actions that might inject values elsewhere into the worksheet calculation tree are forbidden.

That's not the end of the story. You'll notice that there are add-ins, like the Reuters Eikon market data service and Bloomberg for Excel, that provide functions which exist in a single cell but which write blocks of data onto the sheet outside the calling cell.

These functions use the RTD (Real Time Data) API, which is documented on MSDN:

How to create a RTD server for Excel

How to set up and use the RTD function in Excel

You may find this link useful, too:

Excel RTD Servers: Minimal C# Implementation

However, RTD is all about COM servers outside Excel.exe, you have to write them in another language (usually C# or C++), and that isn't the question you asked: you want to do this in VBA.

But I have, at least, made a token effort to give the 'right' answer.

Now for the 'wrong' answer, and actually doing something Microsoft would rather you didn't do. You can't just call a function, call a subroutine or method from the function, and write to the secondary target using the subroutine: Excel will follow the chain and detect that you're injecting values into the sheet calculation, and the write will fail.

You have to insert a break into that chain; and this means using events, or a timer call, or (as in RTD) an external process.

I can suggest two methods that will actually work:

1: Monitor the cell in the Worksheet_Change event:

Private Sub Worksheet_Change(ByVal Target As Range)
Dim strFunc As String

strFunc = "NukeThePrimaryTargets"

If Left(Target.Formula, Len(strFunc) + 1) = strFunc Then
    Call NukeTheSecondaryTargets
End If

End Sub

Alternatively...

2: Use the Timer callback API:

However, I'm not posting code for that: it's complex, clunky, and it takes a lot of testing (so I'd end up posting untested code on StackOverflow). But it does actually work.

I can give you an example of a tested Timer Callback in VBA:

Using the VBA InputBox for passwords and hiding the user's keyboard input with asterisks.

But this is for an unrelated task. Feel free to adapt it if you wish.

Answer 2

Edited with following requirements: It is necessary to run a user defined worksheet function, because there are addins called in this function and those work only within a Excel sheet. The function has to run multiple times with different parameters and its results have to be gotten from the sheet.

So this is my solution now:

Public Function my_function(input1 As Double, input2 As Double) As Double
 my_function = input1 + input2
End Function

Private Function getMy_Function_Results(input1 As Double, input2() As Double) As Variant
 Dim results() As Double

 'set the Formulas
 With Worksheets(1)
  For i = LBound(input2) To UBound(input2)
   strFormula = "=my_function(" & Str(input1) & ", " & Str(input2(i)) & ")"
   .Cells(1, i + 1).Formula = strFormula
  Next

 'get the Results
  .Calculate
  For i = LBound(input2) To UBound(input2)
   ReDim Preserve results(i)
   results(i) = .Cells(1, i + 1).Value
  Next
 End With
 getMy_Function_Results = results
End Function

Sub test()
 Dim dFieldInput2() As Double
 Dim dInput1 As Double

 dInput1 = Val(InputBox("Value for input1"))

 dInput = 0
 iIter = 0
 Do
  dInput = InputBox("Values for fieldInput2; 0=END")
  If Val(dInput) <> 0 Then
   ReDim Preserve dFieldInput2(iIter)
   dFieldInput2(iIter) = Val(dInput)
   iIter = iIter + 1
  End If
 Loop While dInput <> 0

 On Error GoTo noFieldInput2
 i = UBound(dFieldInput2)
 On Error GoTo 0

 vResults = getMy_Function_Results(dInput1, dFieldInput2)

 For i = LBound(vResults) To UBound(vResults)
  MsgBox vResults(i)
 Next

noFieldInput2:
End Sub

The user can input first a value input1 and then input multiple fieldInput2 until he inputs the value 0. Then the results will be calculated and presented.

Greetings

Axel