Rewriting an IComparer to use a lambda expression

Question

I'm trying to rewrite my IComparer<Animal> to use a lambda expression, but I can't seem to get this done correctly

public class AnimalCarriageSorter : IComparer<Animal>
{
    public int Compare(Animal animal, Animal targetAnimal)
    {
        //Compare diet first.
        if (animal.FoodDiet.CompareTo(targetAnimal.FoodDiet) != 0) 
        {
            return animal.FoodDiet.CompareTo(targetAnimal.FoodDiet);
        }

        //Compare size.
        return targetAnimal.Size.CompareTo(animal.Size);
    }
}

Please elaborate on how you want your comparer to work exactly. — Amir Molaei, Apr 09 '19 at 19:33
Where is the lambda expression? What can't you get done correctly? — Lance U. Matthews, Apr 09 '19 at 19:40

Johnny · Accepted Answer · 2019-05-02T09:33:09.107

3

You could easily rewrite it using Func<Animal, Animal, int> instead of the IComparer<Animal>, e.g.

Func<Animal, Animal, int> comparer = (x, y) => 
    x.FoodDiet.CompareTo(y.FoodDiet) != 0 ? 
        x.FoodDiet.CompareTo(y.FoodDiet) : 
        x.Size.CompareTo(y.Size)

edited May 02 '19 at 09:33

answered Apr 09 '19 at 19:42

Johnny

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score 1 · Answer 2 · answered Apr 09 '19 at 19:36

The problem is that you've switched the comparison from animal.CompareTo(targetAnimal) to targetAnimal.CompareTo(animal), so you will not get consistent results. You should have animal as the first object in both cases.

For example:

public int Compare(Animal animal, Animal targetAnimal)
{
    // Compare diet first.
    if (animal.FoodDiet.CompareTo(targetAnimal.FoodDiet) != 0)
    {
        return animal.FoodDiet.CompareTo(targetAnimal.FoodDiet);
    }

    // Compare size.
    return animal.Size.CompareTo(targetAnimal.Size);
}

Or you can simplify it a little:

public int Compare(Animal animal, Animal targetAnimal)
{
    return animal.FoodDiet.CompareTo(targetAnimal.FoodDiet) != 0 
        ? animal.FoodDiet.CompareTo(targetAnimal.FoodDiet) 
        : animal.Size.CompareTo(targetAnimal.Size);
}

Rewriting an IComparer to use a lambda expression

2 Answers2