NpgsqlModelValidator rejects properties mapped via ValueConverter in EF Core 10

Start by reading EF Core 10's ModelValidator.ValidatePropertyMapping and the related relational validation code described in the report; compare their behavior with EF Core 9. Reproduce the failure with the supplied ValueConverter example and check the provider's model validation tests for a suitable regression test. Done means converter-backed properties with a valid provider type mapping pass validation, while genuinely unmapped properties still fail.

NpgsqlModelValidator rejects properties mapped via ValueConverter (no native type mapping) in EF Core 10

Environment

• Microsoft.EntityFrameworkCore: 10.0.2
• Npgsql.EntityFrameworkCore.PostgreSQL: 10.0.0
• .NET target framework: net10.0
• PostgreSQL: any 10+

Symptoms

A model that maps a readonly struct (or any non-primitive CLR type) to a primitive database type via ValueConverter<TModel, TProvider> fails to build with:

System.InvalidOperationException: The property 'ItemDefinition.Name' could not be mapped
because it is of type 'LocalizedString', which is not a supported primitive type or a valid entity type.
Either explicitly map this property, or ignore it using the '[NotMapped]' attribute or by
using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelValidator.ValidatePropertyMapping(IConventionTypeBase, IConventionModel, IDiagnosticsLogger`1)
   at MUnique.OpenMU.Persistence.EntityFramework.EntityDataContext.OnModelCreating(...)
   at ...

The same model builds and works correctly under EF Core 9 (Microsoft.EntityFrameworkCore 9.0.x + Npgsql.EntityFrameworkCore.PostgreSQL 9.0.x).

Root cause

EF Core 10's ModelValidator.ValidatePropertyMapping(IConventionTypeBase, IConventionModel, IDiagnosticsLogger<Validation>) runs:

IConventionProperty conventionProperty = structuralType.GetDeclaredProperties()
    .FirstOrDefault(p =>
        (!ConfigurationSource.Convention.Overrides(p.GetConfigurationSource())
            || !p.IsImplicitlyCreated())
        && p.FindTypeMapping() == null);
if (conventionProperty != null)
    ThrowPropertyNotMappedException(...);

FindTypeMapping() returns null for a property whose CLR type has no native relational mapping and whose mapping is established solely through a ValueConverter registered via OnModelCreating. EF Core 9's relational validator consulted the registered converter before this check; EF Core 10 does not.

Confirmed by decompiling both Microsoft.EntityFrameworkCore.Relational 10.0.0 and Npgsql.EntityFrameworkCore.PostgreSQL 10.0.0 with ILSpy.

Minimal reproduction

// Program.cs
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Diagnostics;
using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
using Npgsql;

public readonly struct LocalizedString
{
    public string Value { get; }
    public LocalizedString(string? value) { Value = value; }
    public static implicit operator string(LocalizedString ls) => ls.Value;
}

public class LocalizedStringConverter : ValueConverter<LocalizedString, string>
{
    public LocalizedStringConverter() : base(ls => ls.Value ?? "", s => new LocalizedString(s)) { }
}

public class Item
{
    public int Id { get; set; }
    public LocalizedString Name { get; set; }
}

public class TestContext : DbContext
{
    public DbSet<Item> Items => Set<Item>();
    protected override void OnConfiguring(DbContextOptionsBuilder o)
        => o.UseNpgsql("Host=localhost;Database=test;Username=postgres;Password=postgres");
    protected override void OnModelCreating(ModelBuilder m)
    {
        m.Entity<Item>(e =>
        {
            e.Property(p => p.Name).HasConversion(new LocalizedStringConverter());
        });
    }
}

public class Program
{
    public static void Main()
    {
        using var ctx = new TestContext();
        ctx.Database.OpenConnection(); // Triggers model validation
    }
}

Expected: ctx.Database.OpenConnection() returns without throwing.
Actual: throws InvalidOperationException: The property 'Item.Name' could not be mapped because it is of type 'LocalizedString'.

The same code runs cleanly under EF Core 9 + Npgsql 9.0.4 (the same ValueConverter<LocalizedString, string> is sufficient to make EF Core 9 happy).

Impact

Many production codebases map readonly struct value objects (translated-text wrappers, currency, units of measure, JSON-shaped DTOs) through ValueConverter. EF Core 10's regression in this area forces users either to:

• mark the type as a complex type via [ComplexType], which has different semantics and does not always work (verified against EF Core 10.0.2 with a converter-backed property on a complex type — the same exception is raised);
• or to fork the provider with a custom IModelValidator that catches the diagnostic (what I am doing locally to unblock OpenMU — see ffuhr91/OpenMU@fix/npgsql-validator-localizedstring).

Neither is a satisfying fix for the long term.

Suggested fix

ModelValidator.ValidatePropertyMapping(IConventionTypeBase, IConventionModel, IDiagnosticsLogger<Validation>) should consult property.GetValueConverter() (and, by extension, the resulting provider-side type mapping) before declaring the property unmapped. The EF Core 9 relational validator effectively did this; the EF Core 10 rewrite dropped the check.

A targeted fix could be:

IConventionProperty conventionProperty = structuralType.GetDeclaredProperties()
    .FirstOrDefault(p =>
        (!ConfigurationSource.Convention.Overrides(p.GetConfigurationSource())
            || !p.IsImplicitlyCreated())
        && p.FindTypeMapping() == null
        && p.GetValueConverter() == null);  // <-- new clause

This restores the EF Core 9 behaviour: properties with a registered ValueConverter are not flagged as "no type mapping", because the converter establishes the mapping to a supported provider type.

Suggested labels

• bug
• area: model validation
• severity: high
• affects: 10.0.0

Related

• OpenMU ffuhr91/OpenMU@fix/npgsql-validator-localizedstring — a working IModelValidator subclass that catches this diagnostic so the game server can boot while the upstream issue is open.
• I also plan to file a separate report on the EF Core 10 SQL-generation regression that references a non-existent ItemDefinitionTempId3 column when serializing the OpenMU GameConfiguration aggregate.