What is difference between IComparable and IComparer in C#?

In C#, both IComparable and IComparer are interfaces used for sorting and comparing objects, but they serve different purposes:

1. IComparable:

   • IComparable is used when you want to define a natural ordering for your objects.
   • When a class implements IComparable, it means that the class instances know how to compare themselves with other instances of the same type.
   • The IComparable interface defines a single method CompareTo(object obj) which returns an integer indicating whether the current object is less than, equal to, or greater than the object being compared to.

   Example using IComparable:

   Suppose we have a Person class that we want to compare based on their age.

   using System;
   
   public class Person : IComparable<Person>
   {
   	public string Name { get; set; }
   	public int Age { get; set; }
   
   	public int CompareTo(Person other)
   	{
   		// Compare persons based on their age
   		return this.Age.CompareTo(other.Age);
   	}
   }
   

   In this example, the Person class implements the IComparable<Person> interface. The CompareTo method is implemented to compare Person objects based on their ages.

   Now, you can use the CompareTo method for sorting Person objects:

   Person[] people = new Person[]
   {
   	new Person { Name = "Alice", Age = 30 },
   	new Person { Name = "Bob", Age = 25 },
   	new Person { Name = "Charlie", Age = 35 }
   };
   
   Array.Sort(people);
   
   foreach (Person person in people)
   {
   	Console.WriteLine($"{person.Name} - {person.Age}");
   }
   

   This will output:

   Bob - 25
   Alice - 30
   Charlie - 35
   

2. IComparer:

   • IComparer is used when you want to define an external way of comparing objects.
   • It allows you to define a separate class to perform the comparison.
   • You might use IComparer when you want to sort objects in a different way from their natural ordering or when you don't have control over the class definition to implement IComparable.
   • The IComparer interface defines a single method Compare(object x, object y) which compares two objects and returns an integer indicating their relative order.

   Example using IComparer:

   Suppose we have the same Person class, but now we want to sort people based on their names.

   using System;
   using System.Collections;
   
   public class PersonNameComparer : IComparer
   {
   	public int Compare(object x, object y)
   	{
   		Person person1 = (Person)x;
   		Person person2 = (Person)y;
   
   		// Compare persons based on their names
   		return string.Compare(person1.Name, person2.Name);
   	}
   }
   

   In this example, we've created a separate class PersonNameComparer that implements the IComparer interface. The Compare method is implemented to compare Person objects based on their names.

   Now, you can use this comparer to sort Person objects:

   Person[] people = new Person[]
   {
   	new Person { Name = "Alice", Age = 30 },
   	new Person { Name = "Bob", Age = 25 },
   	new Person { Name = "Charlie", Age = 35 }
   };
   
   Array.Sort(people, new PersonNameComparer());
   
   foreach (Person person in people)
   {
   	Console.WriteLine($"{person.Name} - {person.Age}");
   }
   

   This will output:

   Alice - 30
   Bob - 25
   Charlie - 35
   

In summary, IComparable is implemented by the class whose instances need to be sorted, defining their natural order, while IComparer is a separate class used to provide a custom comparison method for objects of a particular type.