FunctionExecutionHandlers Called Twice

[Tom Baxter]

Govert, and others,

I'm using .NET 6 and Excel 365 with FunctionExecutionHandlers on an async UDF (i.e., decorated with ExcelAsyncFunction), although the behavior is the same if the UDF is not async.

Below I have a little schematic that shows the fact that the OnEntry(), OnSuccess() and OnExit() all get called twice. Why are they getting called twice? I need to reliably differentiate the OnEntry(), OnSuccess() and OnExit() marked with asterisks from the non-asterisk-marked ones.

Even if I remove the async service call and replace ExcelAsyncFunction with ExcelFunction , I see the same behavior.

Here is the sequence of calls I am seeing when calling an async service:

*OnEntry() completes

UDF begins execution

UDF awaits an async service call

While awaiting the service call...

   OnSuccess() completes

   OnExit()  completes 

After async service call completes, UDF runs to completion

OnEntry() completes  (again)

*OnSuccess() completes   (again)

*OnExit() completes  (Again)

If I remove the async service call, we still end up with this schematic:

OnEntry() completes

UDF runs to completion

OnSuccess() completes  

OnExit() runs (before UDF has completed)

OnEntry() completes 

OnSuccess() completes 

OnExit() completes 

[Govert van Drimmelen]

Hi Tom,

 

Can you check with a simple function like this:

 

public static object GetValues()

{

    return new object[,] { { 1, 2}, { "abc", "def" }  };
}

 

I don’t expect  it to be called twice.

 

If your function is being wrapped as an async function (so if your function is returning Task<T> and being processed by the Registration helper) then it will be called twice, even if it does no async work internally. That is a consequence of the RTD-based implementation that we use for async functions.

The first time it runs, it initiates the async call, then the async call completes, notifies Excel that the call is complete (which invalidates the calling cell(s)) and then Excel recalculates those cells, which call the function again, this time returning the value.

 

You can somewhat distinguish the calls by looking at the return value – it would be #N/A (ExcelError.ExcelErrorNA) for the first call, and an actual value for the second.

 

-Govert

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