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Spike: how does EF Core map value objects without a per-type registration?

Question. Can CodoMetis.TypeKit.EntityFrameworkCore make EF Core 10 map every value object, discovered at run time from the model, with no scan of the consumer's types and no Properties<T>() per type? And do keys, foreign keys, nullable properties and primitive collections of value objects then behave as with an explicit per-type HasConversion?

Answer: yes, through an additive IRelationalTypeMappingSourcePlugin. Measured 2026-09-27 with EF Core 10.0.12 on SQLite and on PostgreSQL 17 (Npgsql 10.0.3), against value objects woven by the real generators. A replaced IValueConverterSelector maps the same, but conflicts with any other library that replaces the selector (findings 11 and 12); the plugin does not (finding 14).

dotnet run --project spikes/EfMapping    # needs Docker for the PostgreSQL half

Mechanisms

Both work because EF asks the type mapping source per CLR type wherever it maps one: property discovery, keys, foreign keys, collection elements and query parameters. Neither needs a list of types.

  • Selector (Mapping.cs): IValueConverterSelector supplies the conversions EF may apply to a type it cannot map directly, which is how an enum reaches an integer column. One that also answers "any IValueObject<TSelf, T> converts to T" replaces EF's.
  • Plugin (Plugin.cs): the type mapping source asks its plugins first. One that answers a value object with the provider's mapping for T, with the converter composed onto it, is added beside EF's services. It fetches that mapping from the type mapping source at lookup time, because injecting the source would be a cycle; resolving it lazily is not.

The converter's from-provider expression calls a plain generic helper that calls IValueObjectMaterializer<TSelf, T>.Materialize, since an expression tree cannot call a static abstract member directly (CS8927).

Findings

# Claim SQLite PostgreSQL
1 Without any mechanism, the model does not build ✅ fails (List<Tag> mapped as an entity type) ✅ same
2 A value-object key maps as a scalar with the converter ✅ TEXT, key ✅ uuid, key
3 A value-object foreign key ✅ ✅ uuid, foreign key
4 A nullable value-object property ✅ INTEGER, nullable ✅ integer, nullable
5 A primitive collection of value objects, elements converted ✅ JSON column, element converter ✅ text[], element converter
6 Round trip with a navigation loaded through the value-object foreign key ✅ ✅
7 A stored value that Create refuses is read back as stored (the materializer, not Create) ✅ ✅
8 Equality, Contains over a list, nullable comparison, collection Contains, OrderBy translate ✅ ✅ = ANY (@ids), @tag = ANY (o."Tags")
9 The selector works whether it is registered before or after the provider ✅ ✅
10 .Value in a predicate translates ❌ needs the member translator ❌ same
11 Selector, then another library's selector (ReplaceService, as strongly-typed-id guides do): ours is gone and the model fails, but only because this model has a List<Tag>, and the message blames that collection ✅ fails, misleading ✅ same
12 Another library's selector, then ours: ours wins and the other library loses its mappings ✅ ✅
13 The plugin: findings 2–9 hold unchanged ✅ ✅
14 The plugin beside another library's replaced selector: findings 2–9 still hold ✅ ✅

Rows 11 to 14 were added the same day. The first version of this README chose the selector and rejected the plugin for a dependency cycle that was assumed, not measured; resolving the type mapping source at lookup time has none.

Consequences for the design

  • Plugin, not a replaced selector. Both map the same (findings 2–9, 13), but two replaced selectors cannot both win (findings 11, 12), and the loser fails with a message that does not name the cause. Plugins are additive (finding 14).
  • Not a pre-convention. Properties<T>() needs the list of types up front, which means a scan.
  • Facets. The plugin passes the lookup's store type, size, precision, Unicode and key flags on to the wrapped type's lookup, so a configured column keeps them.
  • .Value needs the member translator (finding 10), as measured in spikes/ValueTranslation for the other libraries.