Linq至少必须有一个对象实现IComparable

本文关键字：一个对象 实现 IComparable Linq | 更新日期: 2023-09-27 18:00:09

我正在尝试订购一个包含另一个实体列表的实体列表。我已经为所有实体实现了IComparable，但仍然得到了异常。我看到的所有例子都解决了这样一个问题，即你有一个列表，你按该列表中的给定字段排序，但没有列表列表。下面的Linq to Objects和Linq to Entity都会出现此问题。我错过了什么？

[TestClass]
public class OrderBy
{
    [TestMethod]
    public void OrderByTest()
    {
        var hobbies = new Collection<Hobby> { new Hobby { HobbyId = 1, Name = "Eating" }, new Hobby() { HobbyId = 2, Name = "Breathing" } };
        var p1 = new Person
        {
            PersonId = 1,
            Name = "A",
            PersonHobbies = new Collection<PersonHobby> { new PersonHobby() { PersonHobbyId = 1}}
        };
        var p2 = new Person
        {
            PersonId = 2,
            Name = "Z",
            PersonHobbies = new Collection<PersonHobby> { new PersonHobby() { PersonHobbyId = 2 }}
        };
        var people = new List<Person> { p1, p2 };
        var pplEnumerable = people.AsEnumerable();
        pplEnumerable = pplEnumerable.OrderByDescending(r => r.PersonHobbies.OrderByDescending(p => p.Hobby.Name));
        foreach (var person in pplEnumerable)
        {
            Console.WriteLine(person.Name);
        }
    }
    public class Person : IComparable
    {
        public int PersonId { get; set; }
        public string Name { get; set; }
        public virtual ICollection<PersonHobby> PersonHobbies { get; set; }
        public int CompareTo(object obj)
        {
            if (obj == null) return 1;
            var otherPerson = obj as Person;
            return PersonId.CompareTo(otherPerson.PersonId);
        }
    }
    public class PersonHobby : IComparable
    {
        public int PersonHobbyId { get; set; }
        public int HobbyId { get; set; }
        public virtual Person Person{ get; set; }
        public int PersonId { get; set; }
        public virtual Hobby Hobby { get; set; }
        public int CompareTo(object obj)
        {
            if (obj == null) return 1;
            var otherPersonHobby = obj as PersonHobby;
            return PersonHobbyId.CompareTo(otherPersonHobby.PersonHobbyId);
        }
    }
    public class Hobby : IComparable
    {
        public int HobbyId { get; set; }
        public string Name { get; set; }
        public int CompareTo(object obj)
        {
            if (obj == null) return 1;
            var otherHobby = obj as Hobby;
            return HobbyId.CompareTo(otherHobby.HobbyId);
        }
    }
}

Linq至少必须有一个对象实现IComparable

默认情况下，不能对列表应用排序。您需要编写一个自定义类（类似EquatableList等）或使用LINQ Except&交集运算符以比较列表。

但根据你的评论，如果你正在寻找LINQ的等价物：

select * from Person p join PersonHobby ph 
on ph.PersonId = p.PersonId join Hobby h 
on h.HobbyId = ph.HobbyId order by h.Name

则可以实现为：

var query = people.SelectMany(p => p.PersonHobbies)
                  .Join(hobbies, ph => ph.HobbyId, h => h.HobbyId, 
                        (ph, h) => new 
                        { 
                         Person = ph.Person, PersonHobby = ph, Hobby = h 
                        })
                  .OrderBy(r => r.Hobby.Name);

基本上，我们在键上加入个人、个人爱好和爱好，并投影所有列，并按照您的SQL中提到的hobby.name字段对其进行排序。

相关文章：