Return data from a query with Linq in an EF Core repository

With Entity Framework Core there is a possibility to load the and / or relationships later, example strong>:

using (Db db = new Db())
{
    var pessoa = db.Pessoa.Find(1);
    if (pessoa.PessoaNatureza == 0)
    {
        db.Entry(pessoa)
            .Reference(x => x.PessoaFisica)
            .Load(); // Carregando só se for Pessoa Fisica
    }
    else
    {
        db.Entry(pessoa)
            .Reference(x => x.PessoaJuridica)
            .Load(); // Carregando só se for Pessoa Juridica
    }
}

This type of loading is a new feature, which also has filtering elements that could be loaded, but this is not the case.

In your layer you could drive that same code with some changes, example :

public class PessoaRepository : Repository<Pessoa>, IPessoaRepository
{
    public PessoaRepository(SistemaComercialContext context)
        :base(context)
    {

    }

    public List<Pessoa> GetAllJoin()
    {
        var pessoas = context.Pessoa.ToList();
        pessoas.ForEach(x =>
        {                    
            if (x.PessoaNatureza == 0)
            {
                context.Entry(x)
                    .Reference(f => f.PessoaFisica)
                    .Load();
            }
            else
            {
                context.Entry(x)
                    .Reference(j => j.PessoaJuridica)
                    .Load();
            }
        });
        return pessoas;     
    }        
}

On performance, if the registry quantity is too large may have poor performance, then, have to ponder all this, maybe a results pagination would be ideal, but, wanted to show the resource loading relations after loading some data.

There is no "pretty" way for you to do this, since Include does not support conditions.

One way is to separate it into two queries and then merge it:

return DbSet.Where(p => p.PessoaNatureza == 0)
            .Include("PessoaFisica")
            .Concat(
                   DbSet.Where(p => p.PessoaNatureza == 1)
                        .Include("PessoaJuridica")
            ).AsNoTracking();

This way you keep your work in the bank and have the result you expect.

There are other ways, using anonymous objects to return the data and then filter in memory. An example this link which also contains a code for a IQueryable extension for this "problem" of Include .