CSharpHelper: support more types of collection literals

[AdrianoAE-Surface]

I've been hacking around trying to extend the modelbuilder and I came across this CSharpHelper and need some clarifications.
Class:

public class ConfigTest
{
    public Type Type { get; set; }
    public string ForeignKeyName { get; set; }
}

And i'm adding a annotation to the EntityTypeBuilder

IEnumerable<ConfigTest> config = new List<ConfigTest>() { ... }
builder.Metadata.AddAnnotation("ConfigTestKey", config);

But then when creating the migration the following error is thrown

The current CSharpHelper cannot scaffold literals of type 'System.Collections.Generic.List`1[ConfigTest]'. Configure your services to use one that can.

So after digging around I got to the CSharpHelper and the UnknownLiteral. After messing around trying to understand what I had to do I ended up doing this for no reason and it worked

public override string UnknownLiteral(object value)
{
    if (value is IEnumerable<ConfigTest> configuration)
    {
        var test = configuration.Select(x => "").ToArray();
        return Literal(test);
    }

    return base.UnknownLiteral(value);
}

After that and injecting my custom CSharpHelper everything works and I can use my List as value of the annotation. But how does it work? Why passing a empty array of strings with the size of my List works and the actual value can be casted back to the List when building the model. What is going on under the hood?

I couldn't find mutch information about the CSharpHelper or even how to implement the UnkownLiteral, I just got it working by accident and I don't understand why it works.

[ajcvickers]

@AdrianoAE-MA It would probably help here if you could describe your goal in some more details. What is the reason for the annotation and how do you intend to use it?

We will consider updating the C# helper to support generating literals from IEnumerables.

[AdrianoAE]

I'm trying to learn some of the inner working of the entity framework so I can build some extensions. In this particular case I ended up just refactoring the code to use a static configuration class. During the process I just ended up in that class and while trying to understand it I got to that solution that (for the knowledge I have) makes no sense how the value goes to the actual annotation while I'm just passing to the CSharpHelper a array of empty strings.

In terms of what I'm trying to do, I'm trying to automate localization for my domain but I don't want my domain to have any knowledge of it since it's not it's responsability. Giving a concrete example:
Domain Class:

public class Product
{
    public int Id { get; private set; }
    public string Name { get; private set; }
    public string Description { get; private set; }
    public decimal Price { get; private set; }
    //...
}

In my persistence I've created some EntityTypeBuilder extension where I can configure like that

internal class ProductConfigurations : IEntityTypeConfiguration<Product>
{
    public void Configure(EntityTypeBuilder<Product> productConfiguration)
    {
        productConfiguration.PropertyWithTranslation(p => p.Name)
            .HasMaxLength(100)
            .IsRequired();

        productConfiguration.PropertyWithTranslation(p => p.Description)
            .HasMaxLength(400)
            .IsRequired();
    }
}

And since I don't have access to the LanguageEntity (because it's a completely different project) I had to allow some way to configure a "ShadowTable" where I had to say what are the primary Keys (that's where that problem emerged. I was trying to use a Annotation in the model builder to get those values into my extension.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.ApplyConfigurationsFromAssembly(typeof(T).Assembly);
    modelBuilder.ApplyTranslationsConfigurations(new List<PrimaryKeyConfiguration>
        {
            new PrimaryKeyConfiguration(typeof(int), "LanguageId"),
            new PrimaryKeyConfiguration(typeof(Guid), "UserId")
        });
}

Repository of the actual project: https://github.com/AdrianoAE/AdrianoAE.EntityFrameworkCore.Translations

Now I'm stuck trying to understand how I can allow my entity to be used normally and on save changes / on querying use the selected language key and get my domain object. No luck so far 😕

[ajcvickers]

@AdrianoAE-MA That's a fairly ambiguous goal. I'm not sure this will be possible without significant hacking.

With regards to the original question, we are putting this on the backlog to support more collection types. For now you should be able to just use an array instead of a List.

[AdrianoAE-Surface]

@ajcvickers Regarding the CSharpHelper what I don't understand is why a empty array of string works.
That was my Class:

public class PrimaryKeyConfiguration
{
    public readonly Type Type;
    public readonly string ForeignKeyName;

    public PrimaryKeyConfiguration(Type type, string foreignKeyName)
    {
        Type = type;
        ForeignKeyName = !string.IsNullOrWhiteSpace(foreignKeyName) ? foreignKeyName : throw new ArgumentNullException(nameof(ForeignKeyName));
    }
}

I was passing to the Annotation

var keys = new List<PrimaryKeyConfiguration> { ... }
builder.Metadata.AddAnnotation("keyConfig", keys).

And then during the OnModelCreating I was casting that annotation back to IEnumerable.

That's when I got the error in the CSharpHelper but this works.
return Literal(configuration.Select(x => "").ToArray());
That's what I don't understand, how does it work when I'm just creating a array of empty string from my collection and the values are successfully casted back to the actual List.

---

In terms of the actual goal, I know that it will require quite a lot of work and probably won't be able to do the way I was originally thinking. But at least It's being a good project to help learn some of the internals of EFCore even tho in a very basic level.

[ajcvickers]

Regarding the CSharpHelper what I don't understand is why a empty array of string works.

Because the code only handles arrays, not Lists.

And then during the OnModelCreating I was casting that annotation back to IEnumerable.

It's not casting to IEnumerable. List implements IEnumerable. So List can be used any place an IEnumerable is expected.

[yinzara]

I'm going to implement List literals in CSharpHelper and support for ListInit Expressions which should make this work as well as add support needed by:
npgsql/efcore.pg#2402

[roji]

Leaving open to support yet other collection literal types; Dictionary specifically could be useful for JSON scenarios.

[yinzara]

If I was to suddenly implement Dictionary literals, would you prefer the pre .NET 6 syntax using collection initializers i.e.:

new Dictionary<string, string>
{
   { "key1", "value1" },
   ...
}

or the .NET 6 indexed based variety:

new Dictionary<string, string>
{
   ["key1"] = "value1",
   ...
}

I know this would go into EF for .NET 7 but I didn't know if we prefered to be backward compatible.

Also should we put a space after the , in new Dictionary<string, string>() or not new Dictionary<string,string>()