IEquatable<T>.Equals 是Object.Equals方法的泛型版本

The implementation of the Equals method is intended to perform a test for equality with another object of type
T, the same type as the current object. The
Equals method is called in the following circumstances:

When the Equals method is called and the
other parameter represents a strongly-typed object of type
T. (If other is of type
Object, the base
Object.Equals(Object) method is called. Of the two methods,
IEquatable<T>.Equals offers slightly better performance.)

When the search methods of a number of generic collection objects are called. Some of these types and their methods include the following:

Some of the generic overloads of the BinarySearch method.

The search methods of the List<T> class, including
List<T>.Contains(T),
List<T>.IndexOf,
List<T>.LastIndexOf, and
List<T>.Remove.

The search methods of the Dictionary<TKey, TValue> class, including
ContainsKey and
Remove.

The search methods of the generic LinkedList<T> class, including
LinkedList<T>.Contains and
Remove.

In other words, to handle the possibility that objects of a class will be stored in an array or a generic collection object, it is a good idea to implement
IEquatable<T> so that the object can be easily identified and manipulated.

两点注意：

1）如果对象需要存储在集合类中，编写这些类时要考虑实现IEquatable<T>.Equals方法以实现判等，实现IComparable<T>接口以实现排序；

2) 集合类中的某些方法例如LinkedList.Find 方法等要利用对象的Equals方法来判断是否相等。

Sample: 观察对象类的Equals, GetHashCode以及operator override

using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;

public class Person : IEquatable<Person>
{
private string uniqueSsn;
private string lName;

public Person(string lastName, string ssn)
{
this.SSN = ssn;
this.LastName = lastName;
}

public string SSN
{
get { return this.uniqueSsn; }
set {
if (Regex.IsMatch(value, @"\d{9}"))
uniqueSsn = String.Format("{0}-(1}-{2}", value.Substring(0, 3),
value.Substring(3, 2),
value.Substring(5, 4));
else if (Regex.IsMatch(value, @"\d{3}-\d{2}-\d{4}"))
uniqueSsn = value;
else
throw new FormatException("The social security number has an invalid format.");
}
}

public string LastName
{
get { return this.lName; }
set {
if (String.IsNullOrEmpty(value))
throw new ArgumentException("The last name cannot be null or empty.");
else
this.lName = value;
}
}

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

if (this.uniqueSsn == other.SSN)
return true;
else
return false;
}

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

Person personObj = obj as Person;
if (personObj == null)
return false;
else
return Equals(personObj);
}

public override int GetHashCode()
{
return this.SSN.GetHashCode();
}

public static bool operator == (Person person1, Person person2)
{
if ((object)person1 == null || ((object)person2) == null)
return Object.Equals(person1, person2);

return person1.Equals(person2);
}

public static bool operator != (Person person1, Person person2)
{
if (person1 == null || person2 == null)
return ! Object.Equals(person1, person2);

return ! (person1.Equals(person2));
}
}

Test :

public class TestIEquatable
{
public static void Main()
{
// Create a Person object for each job applicant.
Person applicant1 = new Person("Jones", "099-29-4999");
Person applicant2 = new Person("Jones", "199-29-3999");
Person applicant3 = new Person("Jones", "299-49-6999");

// Add applicants to a List object.
List<Person> applicants = new List<Person>();
applicants.Add(applicant1);
applicants.Add(applicant2);
applicants.Add(applicant3);

// Create a Person object for the final candidate.
Person candidate = new Person("Jones", "199-29-3999");
if (applicants.Contains(candidate))
Console.WriteLine("Found {0} (SSN {1}).",
candidate.LastName, candidate.SSN);
else
Console.WriteLine("Applicant {0} not found.", candidate.SSN);

// Call the shared inherited Equals(Object, Object) method.
// It will in turn call the IEquatable(Of T).Equals implementation.
Console.WriteLine("{0}({1}) already on file: {2}.",
applicant2.LastName,
applicant2.SSN,
Person.Equals(applicant2, candidate));
}
}
// The example displays the following output:
//       Found Jones (SSN 199-29-3999).
//       Jones(199-29-3999) already on file: True.