Linq At least one object must implement IComparable

Question

I am trying to order a List of Entities that contains another list of Entities. I have implemented IComparable for all entities and still get the exception. All of the examples I have seen address the issue where you have one list and you order by a given field in that list but not where you have a list of lists. This issue is happening for Linq to Objects per below and also for Linq to Entities. What am I missing? ``` [TestClass] public class OrderBy { [TestMethod] public void OrderByTest() { var hobbies = new Collection { new Hobby { HobbyId = 1, Name = "Eating" }, new Hobby() { HobbyId = 2, Name = "Breathing" } }; var p1 = new Person { PersonId = 1, Name = "A", PersonHobbies = new Collection { new PersonHobby() { PersonHobbyId = 1}} }; var p2 = new Person { PersonId = 2, Name = "Z", PersonHobbies = new Collection { new PersonHobby() { PersonHobbyId = 2 }} }; var people = new List { p1, p2 }; var pplEnumerable = people.AsEnumerable(); pplEnum…

Accepted Answer

You cannot apply ordering to lists by default. You need to write up a custom class (sort of EquatableList etc.) or use LINQ Except & Intersect operators to compare lists.

But based on your comment, if you're looking for the LINQ equivalent of:

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

then that can be achieved as:

```
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);
```

basically we join person, person hobbies and hobby on the keys, and project all columns and sort it by the hobby.name field, as mentioned in your SQL.

Linq At least one object must implement IComparable

Solution

Problem