IEquatable<T>.Equals Method
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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)
Syntax
'Declaration
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:
When the Equals method is called and other represents a strongly-typed object of type T. (Otherwise, if other is of type Object, the base Object.Equals(Object) method is called. Of the two methods, IEquatable<T>.Equals offers slightly better performance.
When the search methods of some generic collection objects are called. 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, including List<T>.Contains(T), List<T>.IndexOf, List<T>.LastIndexOf, and List<T>.Remove.
The search methods of the Dictionary<TKey, TValue> class, including ContainsKey and Remove.
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.
Examples
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.
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, 3))
ElseIf Regex.IsMatch(value, "\d{3}-\d{2}-\d{3}") 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, 3));
else if (Regex.IsMatch(value, @"\d{3}-\d{2}-\d{3}"))
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<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
Silverlight
Supported in: 5, 4, 3
Silverlight for Windows Phone
Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0
XNA Framework
Supported in: Xbox 360, Windows Phone OS 7.0
Platforms
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.