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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. 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(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.uniqueSsn 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: 4.6, 4.5, 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

.NET for Windows Store apps

Supported in: Windows 8

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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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