Key Comparers

Simple Key Comparers

You can create a simple key comparer by invoking Compare<T>.OrderBy (or OrderByDescending).

Consider a simple Person class:

public sealed class Person
{
  public string Name { get; set; }
}

You can write code like this to create a comparer for people that uses Name as a key:

IComparer<Person> comparer = Compare<Person>.OrderBy(p => p.Name);

Of course, you don't have to save the comparer in a variable. If you're only going to use it once, you can just create the comparer as needed:

List<Person> people = ...;
people.Sort(Compare<Person>.OrderBy(p => p.Name));

For equality comparers, use EqualityCompare<T>.EquateBy:

IEqualityComparer<Person> comparer = EqualityCompare<Person>.EquateBy(p => p.Name);

Compound Key Comparers

If we change Person to have both first and last names:

public sealed class Person
{
  public string FirstName { get; set; }
  public string LastName { get; set; }
}

Then we can create an ordering by last name and then by first name:

var comparer = Compare<Person>.OrderBy(p => p.LastName).ThenBy(p => p.FirstName);

ThenBy is an extension method that can actually be applied to any comparable.

Similarly, equality comparers may use ThenEquateBy.

Null Values

By default, OrderBy and ThenBy handle null values behind the scenes, using the standard approach (every null value is less than every non-null value, and every null value is equal to every other null value).

If you want to override that handling, you can set the allowNulls parameter to true. For example, here is a comparer that works exactly like the default int? comparer, except that nulls are last instead of first:

var comparer = Compare<int?>.OrderBy(x => x == null, allowNulls: true)
    .ThenBy(Compare<int?>.Default());

The equality comparer methods also provide allowNulls overloads.