使用LinqKit和EFCore进行快速加载

Question

B"H

C#sharps inclusion of linq right into the language is one of its most powerful features. It is also what makes Entity Framework such an enticing option for working with a database.

Unfortunately due to typing restrictions its often difficult to create centralized expressions to use throughout your project/solution.

One answer to that question is LinqKit. This was working great for me in EF 6.x. However when I moved to EF Core, I am facing a showstopper.

Instead of properly compiled expressions which should then be converted into SQL statement. What I am getting are function like expressions which don't include their sub expressions in the SQL that they generate. Instead they create AsyncLinqOperatorProvider.EnumerableAdapter s which .Net tries to execute (out of an async context) when you access the IEnumerables a few lines later.

So the question is: How do I get EF Core to execute the entire expression as one SQL statement and return a complete materialized object?

Given two classes

    class OrderItemDTO
    {
        public string OrderItemName { get; set; }
    }
    class OrderDTO
    {
        public string OrderName { get; set; }
        public ICollection<OrderItemDTO> OrderItems { get; set; }
    }

I would like to create a global expressions somewhere

    public static Expression<Func<Order, OrderDTO>> ToDTO = x => new OrderDTO
    {
        OrderName = x.Name,
        OrderItems = x.Items.Select(y => new OrderItemDTO { OrderItemName = y.Name })
    };

Which I would then use somewhere as var orders = await db.Orders.AsExpandable().Select(ToDTO).ToListAsync(); Expecting a fully materialized OrderDTO.

Instead what I get is a DTO with OrderItems as a AsyncLinqOperatorProvider.EnumerableAdapter which causes a race condition when executed

Answer 1

Call AsQueryable() and then ToList() on your inner collections - EF will then be able to correctly treat this as full projection:

public static Expression<Func<Order, OrderDTO>> ToDTO = x => new OrderDTO
{
    OrderName = x.Name,
    OrderItems = x.Items.AsQueryable().Select(y => new OrderItemDTO { OrderItemName = y.Name }).ToList()
};