IEquatable<(Of <(T>)>).Equals Method

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

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

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

Syntax

Function Equals ( _
    other As T _
) As Boolean
bool Equals(
    T other
)

Parameters

  • 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.

Remarks

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<(Of <(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.

Examples

The following example shows the partial implementation of a Person class that implements IEquatable<(Of <(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.

Imports System.Collections.Generic
Imports System.Text.RegularExpressions

Public Class Person : Implements IEquatable(Of Person)
   Private uniqueSsn As String
   Private lName As String

   Public Sub New(lastName As String, ssn As String)
      Me.SSN = ssn
      Me.LastName = lastName
   End Sub

   Public Property SSN As String
      Set
         If Regex.IsMatch(value, "\d{9}") Then
            uniqueSsn = String.Format("{0}-(1}-{2}", value.Substring(0, 3), _
                                                     value.Substring(3, 2), _
                                                     value.Substring(5, 4))
         ElseIf Regex.IsMatch(value, "\d{3}-\d{2}-\d{4}") Then
            uniqueSsn = value
         Else 
            Throw New FormatException("The social security number has an invalid format.")
         End If
      End Set
      Get
         Return Me.uniqueSsn
      End Get      
   End Property

   Public Property LastName As String
      Get
         Return Me.lName
      End Get
      Set
         If String.IsNullOrEmpty(value) Then
            Throw New NullReferenceException("The last name cannot be null or empty.")
         Else
            lname = value
         End If   
      End Set
   End Property

   Public Overloads Function Equals(other As Person) As Boolean _
                   Implements IEquatable(Of Person).Equals
      If other Is Nothing Then Return False

      If Me.uniqueSsn = other.SSN Then
         Return True
      Else
         Return False
      End If
   End Function

   Public Overrides Function Equals(obj As Object) As Boolean
      If obj Is Nothing Then Return MyBase.Equals(obj)

      Dim personObj As Person = TryCast(obj, Person)
      If personObj Is Nothing Then
         Return False
      Else   
         Return Equals(personObj)   
      End If
   End Function   

   Public Overrides Function GetHashCode() As Integer
      Return Me.SSN.GetHashCode()
   End Function

   Public Shared Operator = (person1 As Person, person2 As Person) As Boolean
      If person1 Is Nothing OrElse person2 Is Nothing Then
         Return Object.Equals(person1, person2)
      End If

      Return person1.Equals(person2)
   End Operator

   Public Shared Operator <> (person1 As Person, person2 As Person) As Boolean
      If person1 Is Nothing OrElse person2 Is Nothing Then
         Return Not Object.Equals(person1, person2) 
      End If

      Return Not person1.Equals(person2)
   End Operator
End Class
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 NullReferenceException("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 base.Equals(obj);

      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));
   }
}

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

Module Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      ' Create a Person object for each job applicant.
      Dim applicant1 As New Person("Jones", "099-29-4999")
      Dim applicant2 As New Person("Jones", "199-29-3999")
      Dim applicant3 As New Person("Jones", "299-49-6999")

      ' Add applicants to a List object.
      Dim applicants As New List(Of Person)
      applicants.Add(applicant1)
      applicants.Add(applicant2)
      applicants.Add(applicant3)

      ' Create a Person object for the final candidate.
      Dim candidate As New Person("Jones", "199-29-3999")

      If applicants.Contains(candidate) Then
         outputBlock.Text += String.Format("Found {0} (SSN {1}) & vbCrLf.", _
                            candidate.LastName, candidate.SSN)
      Else
         outputBlock.Text += String.Format("Applicant {0} not found.", candidate.SSN) & vbCrLf
      End If

      ' Call the shared inherited Equals(Object, Object) method.
      ' It will in turn call the IEquatable(Of T).Equals implementation.
      outputBlock.Text += String.Format("{0}({1}) & vbCrLf already on file: {2}.", _
                        applicant2.LastName, _
                        applicant2.SSN, _
                        Person.Equals(applicant2, candidate))
   End Sub
End Module
' The example displays the following output:
'       Found Jones (SSN 199-29-3999).
'       Jones(199-29-3999) already on file: True.
public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      // 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))
         outputBlock.Text += String.Format("Found {0} (SSN {1}).",
                            candidate.LastName, candidate.SSN) + "\n";
      else
         outputBlock.Text += String.Format("Applicant {0} not found.", candidate.SSN) + "\n";

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

Version Information

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Platforms

Windows Phone

See Also

Reference

IEquatable<(Of <(T>)>) Interface

System Namespace