This interface is implemented by types whose values can be equated (for example, the numeric and string classes). A value type or class implements the Equals method to create a type-specific method suitable for determining equality of instances.
The IComparable<T> interface defines the CompareTo method, which determines the sort order of instances of the implementing type. The IEquatable<T> interface defines the Equals method, which determines the equality of instances of the implementing type.
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. It should be implemented for any object that might be stored in a generic collection.
Notes to Implementers
Replace the type parameter of the IEquatable<T> interface with the type that is implementing this interface.
If you implement IEquatable<T>, you should also override the base class implementations of Object.Equals(Object) and GetHashCode so that their behavior is consistent with that of the IEquatable<T>.Equals method. If you do override Object.Equals(Object), your overridden implementation is also called in calls to the static Equals(System.Object, System.Object) method on your class. In addition, you should overload the op_Equality and op_Inequality operators. This ensures that all tests for equality return consistent results.
For a value type, you should always implement IEquatable<T> and override Object.Equals(Object) for better performance. Object.Equals boxes value types and relies on reflection to compare two values for equality. Both your implementation of Equals and your override of Object.Equals should return consistent results.
If you implement IEquatable<T>, you should also implement IComparable<T> if instances of your type can be ordered or sorted. If your type implements IComparable<T>, you should also always implement IEquatable<T>.