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 typeT, the same type as the current object. The Equals method is called in the following circumstances:

• When the Equals method is called and theother parameter represents a strongly-typed object of typeT. (If other is of typeObject, the baseObject.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, includingList<T>.Contains(T),List<T>.IndexOf,List<T>.LastIndexOf, andList<T>.Remove.

  • The search methods of the Dictionary<TKey, TValue> class, includingContainsKey andRemove.

  • The search methods of the generic LinkedList<T> class, includingLinkedList<T>.Contains andRemove.

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 implementIEquatable<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.