As the name suggests, IComparable reads out I'm comparable. IComparable when defined for T lets you compare the current instance with another instance of same type. IComparer reads out I'm a comparer, I compare. IComparer is used to compare any two instances of T, typically outside the scope of the instances of T.
As to what they are for can be confusing at first. From the definition it should be clear that hence IComparable (defined in the class T itself) should be the de facto standard to provide the logic for sorting. The default Sort on List etc relies on this. Implementing IComparer on T doesn't help regular sorting. Subsequently, there is little value for implementing IComparable on any other class other than T. This:
class MyClass : IComparable
rarely makes sense.
On the other hand
class T : IComparable
{
public int CompareTo(T other)
{
//....
}
}
is how it should be done.
IComparer can be useful when you require sorting based on a custom order, but not as a general rule. For instance, in a class of Person at some point you might require to Sort people based on their age. In that case you can do:
class Person
{
public int Age;
}
class AgeComparer : IComparer
{
public int Compare(Person x, Person y)
{
return x.Age - y.Age;
}
}
Now the AgeComparer helps in sorting a list based on Age.
var people = new Person[] { new Person { age = 23 }, new Person(){ age = 22 } };
people.Sort(p, new AgeComparer()); //person with age 22 comes first now.
Similarly IComparer on T doesn't make sense.
class Person : IComparer
True this works, but doesn't look good to eyes and defeats logic.
Usually what you need is IComparable. Also ideally you can have only one IComparable while multiple IComparer is possible based on different criteria.
The IComparer and IComparable are exactly analogous to IEqualityComparer and IEquatable which are used for testing equality rather than comparing/sorting; a good thread here where I wrote the exact same answer :)
