Copilot commented on code in PR #474:
URL: https://github.com/apache/arrow-dotnet/pull/474#discussion_r4309311180
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src/Apache.Arrow/C/CArrowSchemaExporter.cs:
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@@ -59,17 +67,25 @@ public static unsafe void ExportType(IArrowType datatype,
CArrowSchema* schema)
throw new ArgumentNullException(nameof(schema));
}
+ // A dictionary-encoded field's extension metadata describes its
value type. IPC
+ // keeps it on the field, but the C Data Interface puts it on the
dictionary child.
+ IReadOnlyDictionary<string, string> dictionaryMetadata = null;
+ if (datatype is DictionaryType)
+ {
+ metadata = SplitExtensionMetadata(metadata, out
dictionaryMetadata);
Review Comment:
This also moves metadata for an unresolved extension whose storage is a
dictionary. For example, importing an extension over `dictionary<int32,
string>` without its registry definition leaves a plain `DictionaryType` with
the extension keys on the field. Re-exporting now puts those keys on the
string-valued dictionary child, changing the extension's storage type and
potentially causing the consumer to reject it. Preserve whether unresolved
metadata describes the dictionary itself or its values, while retaining the IPC
metadata relocation, and add a no-registry import/re-export regression test.
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