What's the Comparer class for?

Question

What purpose does the Comparer class serve if the type that you specify already implements IComparable?

If I specify Comparer.Defaul

Answer 1

The type doesn't need to implement IComparable, it can be any type - there are no constrains on T:

public abstract class Comparer : IComparer, IComparer

The new Comparer you create implements IComparer and the non-generic IComparer, and can be used for comparisons and sorting of collections.

You are correct: if your type, Customer implements IComparable, and you don't need another comparison, Comparer isn't useful to you. Most classes in the .net framework can accept both IComparable or Comparer, so you can use either one.

However, you are wrong to assume that is always the case. It is very possible to create a Comparer for a non-Comparable type. Note that the following is not required:

public abstract class Comparer : IComparer, IComparer
                                    where T : IComparable, IComparable

Suppose you have a simple class, Person and you want to sort a list of Persons, your best bet it to write a Comparer:

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