Sorting Generic Lists with CompareTo

Question

I am trying to figure out the correct way to sort a generic list of objects. A quick example of my data structure:

// The base class
public abstract class Item : IComparable<Item> {
    public enum Category { Hats, Shirts, ... }
    public Category category;

    public int CompareTo (Item that) {
       ...
    }
}

// One of several classes extending Item
public class Hat : Item {
    public int CompareTo (Hat that) {
       ...
    }
}

I have a manager class that maintains lists for each class that extends Item:

Dictionary<Item.Category, List<Item>> _items;

...

foreach (Item.Category category in Enum.GetValues(typeof(Item.Category))) {
    List<Item> list = _items[category];
    list.Sort();
}

The issue I'm having is that when I call List<Item>.Sort(), it's obviously not making use of the class-specific CompareTo() functions. What's the proper way to approach this?

score 1 · Accepted Answer · answered May 23 '13 at 03:11

The problem is that List<Item>.Sort() is looking for an implementation of IComparable<Item> which your derived class does not implement. You've added a CompareTo(Hat hat) method which has nothing to do with your IComparable<Item> implementation other than they happen to have the same name. You could make CompareTo virtual and overload it, but then you'd have to change the signature of the override:

// The base class
public abstract class Item : IComparable<Item> {
    public enum Category { Hats, Shirts, ... }
    public Category category;

    public virtual int CompareTo (Item that) {
       // default implementation
    }
}

// One of several classes extending Item
public class Hat : Item {
    public override int CompareTo (Item that) {
       // override for Hats - can Hats be compared to other Items?
    }
}

also be very careful that your CompareTo is symmetric and transitive or your sort will fail:

Symmetric : if a < b then b > a

Transitive: If a < b and b < c then a < c

I've seen implementations of CompareTo that aren't transitive and it is tough to diagnose and fix.

You should also take the time to override Equals and the comparison operators

Thank you so much, that was exactly the solution I was looking for, but couldn't figure out :). Works perfectly! — Mike, May 23 '13 at 03:49

score 0 · Answer 2 · answered May 23 '13 at 02:19

0

is there a reason you can't just use the built in compare that c# comes with ?

list.OrderByDescending(..) or list.OrderBy(..)

if you want to choose to use Icomparable then you have to tell it what to compare ex.

list.Sort((x,y) => x.Category.CompareTo(y.Category));

answered May 23 '13 at 02:19

Scott Selby

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Each class has its own sorting logic implemented in CompareTo, so I don't think I can make that work with OrderByDescending/OrderBy. – Mike May 23 '13 at 02:28
no you cant , I was just curious , since you didn't post the sorting logic , if it was just generic sort. but it should work when you pass what parameter to sort with lambda like my example – Scott Selby May 23 '13 at 02:29

Sorting Generic Lists with CompareTo

2 Answers2