Comparers

The last comparison library you'll ever need! Wide platform support; fluent syntax.

Creating Comparers

Install the Nito.Comparers NuGet package. By default, this includes the extension package for LINQ support.

The comparer types are in the namespace Nito.Comparers.

Let's say you've got a collection of your POCOs:

class Person
{
    public string FirstName { get; }
    public string LastName { get; }
}
List<Person> list = ...;

Here's an easy way to sort them all by last name and then first name:

IComparer<Person> nameComparer =
    ComparerBuilder.For<Person>()
                   .OrderBy(p => p.LastName)
                   .ThenBy(p => p.FirstName);
list.Sort(nameComparer);

Implementing Comparable Types

How about having Person implement it? Let's face it: implementing comparison in .NET is a real pain. IComparable<T>, IComparable, IEquatable<T>, Object.Equals, and Object.GetHashCode?!?! But it's easy with a base type:

class Person : ComparableBase<Person>
{
    static Person()
    {
        DefaultComparer =
            ComparerBuilder.For<Person>()
                           .OrderBy(p => p.LastName)
                           .ThenBy(p => p.FirstName);
    }

    public string FirstName { get; }
    public string LastName { get; }
}

ComparableBase<T> auto-magically implements all the comparable interfaces, including correct overrides of Object.Equals and Object.GetHashCode.

Using Comparers in Hash Containers

What about hash-based containers? Every single comparer produced by the Comparers library also implements equality comparison!

IEqualityComparer<Person> nameComparer =
    ComparerBuilder.For<Person>()
                   .OrderBy(p => p.LastName)
                   .ThenBy(p => p.FirstName);
Dictionary<Person, Address> dict = new Dictionary<Person, Address>(nameComparer);

Equality Comparers

Sometimes, you can only define equality. Well, good news: there are equality comparer types that parallel the full comparer types.

class Entity : EquatableBase<Entity>
{
    static Entity()
    {
        DefaultComparer =
            EqualityComparerBuilder.For<Entity>()
                                   .EquateBy(e => e.Id);
    }

    public int Id { get; }
}

Working with Sequences

Sequences are sorted lexicographically. The Sequence operator takes an existing comparer for one type, and defines a lexicographical comparer for sequences of that type:

var nameComparer =
    ComparerBuilder.For<Person>()
                   .OrderBy(p => p.LastName)
                   .ThenBy(p => p.FirstName);
List<IEnumerable<Person>> groups = ...;
groups.Sort(nameComparer.Sequence());

There's also natural extensions for LINQ that allow you to define comparers on-the-fly (particularly useful for anonymous types):

IEnumerable<Person> people = ...;
var anonymousProjection = people.Select(x => new { GivenName = x.FirstName, Surname = x.LastName });
var reduced = anonymousProjection.Distinct(c => c.EquateBy(x => x.Surname));

Dynamic Sorting

Need to sort dynamically at runtime? No problem!

var sortByProperties = new[] { "LastName", "FirstName" };
IComparer<Person> comparer = ComparerBuilder.For<Person>().Null();
foreach (var propertyName in sortByProperties)
{
    var localPropertyName = propertyName;
    Func<Person, string> selector = p => p.GetType().GetProperty(localPropertyName).GetValue(p, null) as string;
    comparer = comparer.ThenBy(selector);
}

Complex Sorting

Want a cute trick? Here's one: true is "greater than" false, so if you want to order by some weird condition, it's not too hard:

// Use the default sort order (last name, then first name), EXCEPT all "Smith"s move to the head of the line.
var comparer =
    ComparerBuilder.For<Person>()
                   .OrderBy(p => p.LastName == "Smith", descending: true)
                   .ThenBy(ComparerBuilder.For<Person>().Default());
list.Sort(comparer);

By default, null values are "less than" anything else, but you can use the same sort of trick to sort them as "greater than" non-null values (i.e., nulls will be last in a sorted collection):

List<int?> myInts = ...;
var comparer =
    ComparerBuilder.For<int?>()
                   .OrderBy(i => i == null, specialNullHandling: true)
                   .ThenBy(ComparerBuilder.For<int?>().Default());
myInts.Sort(comparer);
// Note: we need to pass "specialNullHandling"; otherwise, the default null-ordering rules will apply.

More?!

For full details, see the detailed docs.

What's with the flying saucer?

Other languages provide a comparison operator <=>, which is called the "spaceship operator". This library provides similar capabilities for C#, hence the "spaceship logo".

StephenCleary / Comparers