IComparerT

IComparer<T> is a generic interface in the .NET platform, defined in System.Collections.Generic, that provides a way to supply external comparison logic for objects of type T. It enables custom ordering without requiring the objects themselves to implement any particular comparison behavior. This makes it useful for sorting, searching, and other ordered operations on collections of T.

The interface defines a single method: int Compare(T x, T y). The method returns a negative value when x is less than y, zero when they are equal, and a positive value when x is greater than y. Implementations should establish a total, consistent ordering. For reference types, null handling is typically considered: null is usually treated as less than non-null values, though custom logic may vary. Comparisons should be deterministic and transitive to ensure predictable sorting results.

Usage often involves implementing IComparer<T> in a dedicated class and passing an instance to sorting methods, such as List<T>.Sort(IComparer<T>). It can also be used with LINQ methods that accept a comparer to influence ordering, such as OrderBy with an explicit comparer. When a type T already defines a natural ordering via IComparable<T> or IComparable, Comparer<T>.Default can supply a standard comparer that defers to that natural order.

Relation to other comparers: IComparer<T> is the generic counterpart to the non-generic IComparer in System.Collections. If T implements IComparable<T> or IComparable, a default comparer can provide that natural ordering. IComparer<T> is particularly useful when multiple, distinct sort orders are needed for the same type, or when you want to externalize sorting logic from the type itself.