Render LIST, ARRAY, STRUCT, and MAP columns as JSON strings instead of #VALUE!

[pep-sanwer]

DuckDB.NET returns .NET collection objects (List<T>, Dictionary<K,V>) for complex column types
Excel-DNA does not marshal these, producing #VALUE! errors

This change JSON-serializes collection-typed columns so they display as readable strings in Excel

Given

FROM (
    VALUES (
        ['apple', 'banana', 'cherry']
        , { 'id': 101, 'status': 'active', 'score': 9.5 }
        , MAP(['color', 'shape', 'material'], ['red', 'circle', 'wood'])
        , '{"meta": {"tags": ["sql", "duckdb"], "version": 1.2}}'::JSON
    )
) AS test (example_list, example_struct, example_map, example_json)
;

Before

example_list	example_struct	example_map	example_json
#VALUE!	#VALUE!	#VALUE!	{"meta": {"tags": ["sql", "duckdb"], "version": 1.2}}

After

example_list	example_struct	example_map	example_json
["apple","banana","cherry"]	{"id":101,"status":"active","score":9.5}	{"color":"red","shape":"circle","material":"wood"}	{"meta": {"tags": ["sql", "duckdb"], "version": 1.2}}

Changes

• DuckDbHelper.cs — Added jsonSerializeField flag for IList/IDictionary column types, serialized via JsonSerializer.Serialize(); null collections guard via IsDBNull → #N/A
• UnitTestDuckDbQueries.cs — Added tests for LIST, STRUCT, MAP, and null collections (LIST)

Assisted-by: Claude:claude-opus-4-6 [dotnet-test]

[pep-sanwer]

Hello!

My apologies for this drive by and thank you for the incredible tool and work here!
Please feel free to completely ignore this PR if it isn't suitable

[RusselWebber]

Hi, thanks very much for the PR! I like the idea, but was wondering if maybe it would be better to return array constants? https://support.microsoft.com/en-gb/office/use-array-constants-in-array-formulas-477443ea-5e71-4242-877d-fcae47454eb8
What do you think?

[pep-sanwer]

Hello - appreciate the feedback!
That would be interesting functionality, specifically for LIST / ARRAY types, but I'm not sure what native Excel Spill functionality would mean for STRUCT or MAP.

My thoughts were:
I was also hoping to preserve duckdb semantics here (i.e. number of rows returned matches duckdb output).
For example:

[Fig 1]

is equivalent to:

---

this also allows roundtrip back into duckdb for user controlled "spill" like:
*last two rows in Fig 1. above is xlRange below

or

or

[RusselWebber]

Nice work. Thank you for the PR!

[pep-sanwer]

Cheers! Thank you for all your work again!