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IEquatable(Of T).Equals Method

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

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

'Declaration
Function Equals ( _
	other As T _
) As Boolean

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.

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.

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(Of 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. This ensures that all invocations of the Equals method return consistent results, which the example illustrates.

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 ArgumentException("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 False

      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


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 TestIEquatable
   Public Sub Main()
      ' 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
         Console.WriteLine("Found {0} (SSN {1}).", _
                            candidate.LastName, candidate.SSN)
      Else
         Console.WriteLine("Applicant {0} not found.", candidate.SSN)
      End If         

      ' 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)) 
   End Sub
End Module
' The example displays the following output:
'       Found Jones (SSN 199-29-3999).
'       Jones(199-29-3999) already on file: True.


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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