C＃EF6 LINQ从include（）中的子项中选择多个第二子项

Question

So I'm trying to include multiple nested properties from a child with LINQ

var templates = context.Templates
                        .Include(t => t.template_fields)
                        .Include(t => t.templateinstances.Select(ti => ti.templateinstance_fields))
                        .Include(t => t.templateinstances.Select(ti => ti.templateinstance_categories.Select(tic => tic.category)))
                        .ToList();

But when I include t.templateinstances more than once, I get a NullPointerException when the ToList() call is made.

There is no problem if t.templateinstances is only included once.

Answer 1

You should check possible null properties. Perhabs it should be;

    var templates = context.Templates
                            .Include(t => t.template_fields)
                            .Include(t => t.templateinstances.Where(ti => ti != null).Select(ti => ti.templateinstance_fields))
                            .Include(t => t.templateinstances.Where(ti => ti != null && ti.templateinstance_categories != null).Select(ti => ti.templateinstance_categories.Select(tic => tic.category)))
                            .ToList();

Answer 2

For EF 6

using System.Data.Entity;

query.Include(x => x.Collection.Select(y => y.Property))

See Remarks for more examples.

Make sure to add using System.Data.Entity; to get the version of Include that takes in a lambda.

---

If you are using EF Core you can use the new method ThenInclude

query.Include(x => x.Collection)
     .ThenInclude(x => x.Property);