Detect primitive collections automatically once the element type is known

[CheloXL]

I'm not sure if this is already implemented or not. Found some closed issues, but no documentation on it.

I have a column definition that is of type IEnumerable<LanguageId>. LanguageId is basically a value object that has a custom valueconverter/valuecomparer implemented and already registered in EF (and it's working fine when I have a column of type LanguageId).

Now, on the above example, I'm getting the following error: The 'IEnumerable' property 'Installation.Languages' could not be mapped because the database provider does not support this type. Consider converting the property value to a type supported by the database using a value converter.

I understand the error message, but I already implemented a valueconverter that converts the object LanguageId from/to a primitive value (string). What I don't know is how to configure that property, so EF understands that it has to run the converter on every entry, much like when its working on a simple property.

Is there a way to do that? I would like to do so, so I can retain the ability to later query that column. Having to write a custom converter for the IEnumerable<> would not allow me to query the column via "Contains", for example.

[ajcvickers]

This issue is lacking enough information for us to be able to fully understand what is happening. Please attach a small, runnable project or post a small, runnable code listing that reproduces what you are seeing so that we can investigate.

[CheloXL]

Sorry, here you have. After instantiating a new AppDbContext and trying to run dbContext.Database.EnsureCreated() is when I got the above error.

public readonly struct LanguageId {
	private readonly bool _isInitialized;
	private readonly string _value;
	public LanguageId()
	{
		_isInitialized = false;
		_value = null!;
	}

	public LanguageId(string value)
	{
		_value = value;
		_isInitialized = true;
	}

	public class EfCoreValueConverter : ValueConverter<LanguageId, string>
	{
		public EfCoreValueConverter() : this(null) { }
		public EfCoreValueConverter(ConverterMappingHints? mappingHints) : base(vo => vo._value, value => new(value), mappingHints) {}
	}
	
	public class EfCoreValueComparer : ValueComparer<LanguageId>
	{
		public EfCoreValueComparer() : base(
			(left, right) => DoCompare(left, right), 
			instance => instance._isInitialized ? instance.GetHashCode() : 0) 
		{ 
		}

		private static bool DoCompare(LanguageId left, LanguageId right)
		{
			if(!left._isInitialized && !right._isInitialized) return true;
			return left._isInitialized && right._isInitialized && left._value.Equals(right._value);
		}
	}
}

public sealed class Installation {
	public long Id Languages { get; private set; }
	public required IEnumerable<LanguageId> Languages { get; init; }
}

public class AppDbContext : DbContext
{
	public AppDbContext(DbContextOptions<AppDbContext> options):base(options) { }

	/// <inheritdoc />
	protected override void ConfigureConventions(ModelConfigurationBuilder configurationBuilder)
	{
		configurationBuilder.Properties<LanguageId>().HaveConversion<LanguageId.EfCoreValueConverter, LanguageId.EfCoreValueComparer>();
		base.ConfigureConventions(configurationBuilder);
	}

	/// <inheritdoc />
	protected override void OnModelCreating(ModelBuilder modelBuilder)
	{
		modelBuilder.ApplyConfigurationsFromAssembly(typeof(AppDbContext).Assembly);
		base.OnModelCreating(modelBuilder);
	}
}

public sealed class InstallationsConfiguration : IEntityTypeConfiguration<Installation>
{
	public void Configure(EntityTypeBuilder<Installation> builder)
	{
		builder.ToTable("installations");
		builder.HasKey(x => x.Id);
	}
}

[ajcvickers]

@CheloXL I don't think this is currently possible; we will consider it. For now, primitive collections like this will need to be explicitly configured:

public void Configure(EntityTypeBuilder<Installation> builder)
{
	builder
		.PrimitiveCollection(e => e.Languages)
		.ElementType(e => e.HasConversion<LanguageId.EfCoreValueConverter, LanguageId.EfCoreValueComparer>());
	
	builder.ToTable("installations");
	builder.HasKey(x => x.Id);
}

[CheloXL]

Ah, perfect. I have no problem configuring the property. I didn't find on the documentation what you just posted (it would be great if that could be added to the primitive collection docs).
Thank you very much!

[ajcvickers]

Related to #31413

[CodingOctocat]

@ajcvickers An important reason for using PrimitiveCollection is the need to support Where + Any operations, but ValueConverter will cause the query expression translate to fail.