When To Use IEquatable<T> And Why

From the MSDN:

The IEquatable(T) interface is used by generic collection objects such as Dictionary(TKey, TValue), List(T), and LinkedList(T) when testing for equality in such methods as Contains, IndexOf, LastIndexOf, and Remove.

The IEquatable<T> implementation will require one less cast for these classes and as a result will be slightly faster than the standard object.Equals method that would be used otherwise. As an example see the different implementation of the two methods:

public bool Equals(T other) 
{
  if (other == null) 
     return false;

  return (this.Id == other.Id);
}

public override bool Equals(Object obj)
{
  if (obj == null) 
     return false;

  T tObj = obj as T;  // The extra cast
  if (tObj == null)
     return false;
  else   
     return this.Id == tObj.Id;
}

I'm amazed that the most important reason is not mentioned here.

IEquatable<> was introduced mainly for structs for two reasons:

1. For value types (read structs) the non-generic Equals(object) requires boxing. IEquatable<> lets a structure implement a strongly typed Equals method so that no boxing is required.

2. For structs, the default implementation of Object.Equals(Object) (which is the overridden version in System.ValueType) performs a value equality check by using reflection to compare the values of every field in the type. When an implementer overrides the virtual Equals method in a struct, the purpose is to provide a more efficient means of performing the value equality check and optionally to base the comparison on some subset of the struct's field or properties.

Both of which improves performance.

Reference types (read classes) don't benefit as much. The IEquatable<> implementation does let you avoid a cast from System.Object but that's a very trivial gain. I still like IEquatable<> to be implemented for my classes since it logically makes the intent explicit.

I use IEquatable<T> quite a lot, although from a pure technical perspective, it doesn't really give me any particular benefits. Overriding System.Object.Equals can provide you with the same functionality.

However, I like the explicitness of implementing IEquatable<T>. I use the concepts of Entities and Value Objects from Domain-Driven Design quite a lot, and use IEquatable<T> particularly for Value Objects, simply because it signals that a type has well-defined equality.