使用 LINQ 的动态表达式。如何找到厨房？

Question

我尝试实现一个用户动态过滤器，其中使用选择一些属性，选择一些运算符并选择值。

由于我还没有找到这个问题的答案，因此我尝试使用 LINQ 表达式。
主要是我需要识别所有主要房间是厨房的房屋（任何感觉，我知道）。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
//using System.Linq.Dynamic;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            Room aRoom = new Room() { Name = "a Room" };
            Room bRoom = new Room() { Name = "b Room" };
            Room cRoom = new Room() { Name = "c Room" };

            House myHouse = new House
            {
                Rooms = new List<Room>(new Room[] { aRoom }),
                MainRoom = aRoom
            };
            House yourHouse = new House()
            {
                Rooms = new List<Room>(new Room[] { bRoom, cRoom }),
                MainRoom = bRoom
            };
            House donaldsHouse = new House()
            {
                Rooms = new List<Room>(new Room[] { aRoom, bRoom, cRoom }),
                MainRoom = aRoom
            };

            var houses = new List<House>(new House[] { myHouse, yourHouse, donaldsHouse });

            //var kitchens = houses.AsQueryable<House>().Where("MainRoom.Type = RoomType.Kitchen");
            //Console.WriteLine("kitchens count = {0}", kitchens.Count());

            var houseParam = Expression.Parameter(typeof(House), "house");
            var houseMainRoomParam = Expression.Property(houseParam, "MainRoom");
            var houseMainRoomTypeParam = Expression.Property(houseMainRoomParam, "Type");

            var roomTypeParam = Expression.Parameter(typeof(RoomType), "roomType");

            var comparison = Expression.Lambda(
                Expression.Equal(houseMainRoomTypeParam,
                Expression.Constant("Kitchen", typeof(RoomType)))
                );

            // ???????????????????????? DOES NOT WORK
            var kitchens = houses.AsQueryable().Where(comparison);

            Console.WriteLine("kitchens count = {0}", kitchens.Count());
            Console.ReadKey();

        }
    }

    public class House
    {
        public string Address { get; set; }
        public double Area { get; set; }
        public Room MainRoom { get; set; }
        public List<Room> Rooms { get; set; }
    }

    public class Room
    {
        public double Area { get; set; }
        public string Name { get; set; }
        public RoomType Type { get; set; }
    }

    public enum RoomType
    {
        Kitchen,
        Bedroom,
        Library,
        Office
    }
}

Answer 1

var kitchens = from h in houses
               where h.MainRoom.Type == RoomType.Kitchen
               select h;

但您必须先设置房间的 RoomType 属性。

好的，编辑：

所以你必须重新定义：

var comparison = Expression.Lambda<Func<House, bool>>(...

然后，当你使用它时：

var kitchens = houses.AsQueryable().Where(comparison.Compile());

编辑#2：

好的，你开始：

var roomTypeParam = Expression.Parameter(typeof(RoomType), "roomType");



// ???????????????????????? DOES NOT WORK
var comparison = Expression.Lambda<Func<House, bool>>(
    Expression.Equal(houseMainRoomTypeParam,
    Expression.Constant(Enum.Parse(typeof(RoomType), "Kitchen"), typeof(RoomType))), houseParam);



// ???????????????????????? DOES NOT WORK
var kitchens = houses.AsQueryable().Where(comparison);

编辑#3：当然，为了你的需要，我现在没有想法。我给你最后一个：

在 String 类型上声明一个扩展方法：

internal static object Prepare(this string value, Type type)
{
    if (type.IsEnum)
        return Enum.Parse(type, value);

    return value;
}

然后在该表达式中使用它，例如：

Expression.Constant("Kitchen".Prepare(typeof(RoomType)), typeof(RoomType))

那是因为显然枚举的处理方式不同。该扩展将使其他类型的字符串保持不变。缺点：您必须在那里添加另一个 typeof() 。

Answer 2

// ???????????????????????? DOES NOT WORK
var kitchens = houses.AsQueryable().Where(comparison);

Where 方法采用 Func 或 Expression> 作为参数，但变量 comparison 的类型为 LambdaExpression，不匹配。您需要使用该方法的另一个重载：

var comparison = Expression.Lambda<Func<House, bool>>(
                Expression.Equal(houseMainRoomTypeParam,
                Expression.Constant("Kitchen", typeof(RoomType))));
//now the type of comparison is Expression<Func<House, bool>>

//the overload in Expression.cs
public static Expression<TDelegate> Lambda<TDelegate>(Expression body, params ParameterExpression[] parameters);

Answer 3

我不会以这种方式构建 where 子句 - 我认为它比您的需求所需的更复杂。相反，您可以组合这样的 where 子句：

var houses = new List<House>(new House[] { myHouse, yourHouse, donaldsHouse });

// A basic predicate which always returns true:
Func<House, bool> housePredicate = h => 1 == 1;

// A room name which you got from user input:
string userEnteredName = "a Room";

// Add the room name predicate if appropriate:
if (!string.IsNullOrWhiteSpace(userEnteredName))
{
    housePredicate += h => h.MainRoom.Name == userEnteredName;
}

// A room type which you got from user input:
RoomType? userSelectedRoomType = RoomType.Kitchen;

// Add the room type predicate if appropriate:
if (userSelectedRoomType.HasValue)
{
    housePredicate += h => h.MainRoom.Type == userSelectedRoomType.Value;
}

// MainRoom.Name = \"a Room\" and Rooms.Count = 3 or 
// ?????????????????????????
var aRoomsHouses = houses.AsQueryable<House>().Where(housePredicate);

我测试了这个，诚实:)

Answer 4

这个怎么样

var kitchens = houses
                .SelectMany(h => h.Rooms, (h, r) => new {House = h, Room = r})
                .Where(hr => hr.Room.Type == RoomType.Kitchen)
                .Select(hr => hr.House);

Answer 5

添加以下代码：

typeof(Enum),
typeof(T)

T : Enum type

要向动态 Linq 添加新的 Enum 类型，您必须在预定义的动态类型中 这对我有用。