ReadOnlyMemoryConverter.cs

await using var context = new BlogContext();
await context.Database.EnsureDeletedAsync();
await context.Database.EnsureCreatedAsync();

public class BlogContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }

    protected override void ConfigureConventions(ModelConfigurationBuilder configurationBuilder)
        => configurationBuilder.Properties<ReadOnlyMemory<byte>>().HaveConversion<ReadOnlyMemoryConverter>();

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        => optionsBuilder
            .UseSqlServer("Server=localhost;Database=test;User=SA;Password=Abcd5678;Connect Timeout=60;ConnectRetryCount=0;Encrypt=false")
            .LogTo(Console.WriteLine, LogLevel.Information)
            .EnableSensitiveDataLogging();

    private class ReadOnlyMemoryConverter() : ValueConverter<ReadOnlyMemory<byte>, byte[]>(v => v.ToArray(), v => v);
}

public class Blog
{
    public int Id { get; set; }
    public ReadOnlyMemory<byte> Bytes { get; set; }
}

Is there something like a IModelCustomizer for configuring conventions?

You can take a look at user conventions, but I'm not sure exactly what you're looking for (and specifically what's needed beyond the ConfigureConventions snippet above.

I'm trying to configure stuff without changing the dbcontext code.

I can customize the model "from the outside":

services
    .AddPooledDbContextFactory<MyDbContext<TData>>(options =>
    {
        options.UseSqlite(connectionString);
        options.ReplaceService<IModelCustomizer, SqliteModelCostumizer<TMessage>>();
    })
    ;

But I haven't found a way to do the same for conventions.

Unless I inject them in the service provider and retrieve them from there.

Then you're probably looking for type mapping plugin - that allows you to write a plugin that adds/overrides type mappings in addition to the provider's ones. You can then even publish that plugin as a nuget package, and people can reference it and use it.

For a good example, take a look at the NetTopologySuite plugins in the EF Core repo.

Looks great.

Now I can write one for DateTimeOffset for Sqlite too. 😄

There already is a value converter for DateTimeOffset for Sqlite (converting it to a string) - what exactly are you looking for?

Is it on by default? Or do I have to do something?
Does it allow queries comparing dates?

    System.InvalidOperationException : The LINQ expression 'DbSet<QueueEntity<TestMessage>>()
    .Where(q => q.DequeueAfter <= __dequeueAfter_0 && q.DequeueAfter < __dequeueAfter_0)' could not be translated.

Is it on by default? Or do I have to do something?

It's enabled by default - have you seen some other behavior?

Does it allow queries comparing dates?

No, because comparing the string representations of DateTimeOffset doesn't work - imagine having different time zones there, and then comparing string representations. If you're only storing timestamps in a single timezone (e.g. UTC), you can just use DateTime, which has a default value where comparison is supported.