IEquatable<T>.Equals Method (T)


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Indicates whether the current object is equal to another object of the same type.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)

bool Equals(
	T other


Type: T

An object to compare with this object.

Return Value

Type: System.Boolean

true if the current object is equal to the other parameter; otherwise, false.

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:

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.

When implementing the Equals method, define equality appropriately for the type specified by the generic type argument. For example, if the type argument is Int32, define equality appropriately for the comparison of two 32-bit signed integers.

Notes to Implementers:

If you implement Equals, 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, which the example illustrates.

The following example shows the partial implementation of a Person class that implements IEquatable<T> and has two properties, LastName and SSN. The Equals method returns True if the SSN property of two Person objects is identical; otherwise, it returns False.

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

      uniqueSsn = ssn;
      this.LastName = lastName;

   public string SSN
      get { return this.uniqueSsn; }

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

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

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

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

      Person personObj = obj as Person;
      if (personObj == null)
         return false;
         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 (((object)person1) == null || ((object)person2) == null)
         return ! Object.Equals(person1, person2);

      return ! (person1.Equals(person2));

Person objects can then be stored in a List<T> object and can be identified by the Contains method, as the following example shows.

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>();

       // 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);
         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}.",  
                        Person.Equals(applicant2, candidate)); 
// The example displays the following output:
//       Found Jones (SSN 199-29-3999).
//       Jones(199-29-3999) already on file: True.

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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