IEquatable<T>.Equals(T) Method
Definition
Important
Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.
Indicates whether the current object is equal to another object of the same type.
public:
bool Equals(T other);public bool Equals (T other);public bool Equals (T? other);abstract member Equals : 'T -> boolPublic Function Equals (other As T) As BooleanParameters
- other
- T
An object to compare with this object.
Returns
true if the current object is equal to the other parameter; otherwise, false.
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.
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 = $"{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;
else
throw new FormatException("The social security number has an invalid format.");
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.");
else
this.lName = value;
}
}
public bool Equals(Person other)
{
if (other == null)
return false;
if (this.uniqueSsn == other.uniqueSsn)
return true;
else
return false;
}
public override bool Equals(Object obj)
{
if (obj == null)
return false;
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 (((object)person1) == null || ((object)person2) == null)
return ! Object.Equals(person1, person2);
return ! (person1.Equals(person2));
}
}
open System
open System.Text.RegularExpressions
type Person(lastName, ssn) =
let mutable lastName = lastName
let ssn =
if Regex.IsMatch(ssn, @"\d{9}") then
$"{ssn.Substring(0, 3)}-{ssn.Substring(3, 2)}-{ssn.Substring(5, 4)}"
elif Regex.IsMatch(ssn, @"\d{3}-\d{2}-\d{4}") then
ssn
else
raise (FormatException "The social security number has an invalid format.")
member _.SSN =
ssn
member _.LastName
with get () = lastName
and set (value) =
if String.IsNullOrEmpty value then
invalidArg (nameof value) "The last name cannot be null or empty."
else
lastName <- value
static member op_Equality (person1: Person, person2: Person) =
if box person1 |> isNull || box person2 |> isNull then
Object.Equals(person1, person2)
else
person1.Equals person2
static member op_Inequality (person1: Person, person2: Person) =
if box person1 |> isNull || box person2 |> isNull then
Object.Equals(person1, person2) |> not
else
person1.Equals person2 |> not
override _.GetHashCode() =
ssn.GetHashCode()
override this.Equals(obj: obj) =
match obj with
| :? Person as personObj ->
(this :> IEquatable<_>).Equals personObj
| _ -> false
interface IEquatable<Person> with
member this.Equals(other: Person) =
match box other with
| null -> false
| _ ->
this.SSN = other.SSN
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)
If Regex.IsMatch(ssn, "\d{9}") Then
uniqueSsn = $"{ssn.Substring(0, 3)}-{ssn.Substring(3, 2)}-{ssn.Substring(5, 4)}"
ElseIf Regex.IsMatch(ssn, "\d{3}-\d{2}-\d{4}") Then
uniqueSsn = ssn
Else
Throw New FormatException("The social security number has an invalid format.")
End If
Me.LastName = lastName
End Sub
Public ReadOnly Property SSN As String
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<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>();
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))
Console.WriteLine("Found {0} (SSN {1}).",
candidate.LastName, candidate.SSN);
else
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}.",
applicant2.LastName,
applicant2.SSN,
Person.Equals(applicant2, candidate));
}
}
// The example displays the following output:
// Found Jones (SSN 199-29-3999).
// Jones(199-29-3999) already on file: True.
// Create a Person object for each job applicant.
let applicant1 = Person("Jones", "099-29-4999")
let applicant2 = Person("Jones", "199-29-3999")
let applicant3 = Person("Jones", "299-49-6999")
// Add applicants to a List object.
let applicants = ResizeArray()
applicants.Add applicant1
applicants.Add applicant2
applicants.Add applicant3
// Create a Person object for the final candidate.
let candidate = Person("Jones", "199-29-3999")
if applicants.Contains candidate then
printfn $"Found {candidate.LastName} (SSN {candidate.SSN})."
else
printfn $"Applicant {candidate.SSN} not found."
// Call the shared inherited Equals(Object, Object) method.
// It will in turn call the IEquatable<T>.Equals implementation.
printfn $"{applicant2.LastName}({applicant2.SSN}) already on file: {Person.Equals(applicant2, candidate)}."
// The example displays the following output:
// Found Jones (SSN 199-29-3999).
// Jones(199-29-3999) already on file: True.
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.
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(T) method is called in the following circumstances:
When the
Equalsmethod is called and theotherargument is a strongly-typed object of typeT. (Ifotheris not of typeT, the base Object.Equals(Object) method is called. Of the two methods, IEquatable<T>.Equals offers slightly better performance.)When the search methods of a number of generic collection objects are called. Some of 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.
The search methods of the generic LinkedList<T> class, including LinkedList<T>.Contains 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.
Notes to Implementers
If you implement Equals(T), you should also override the base class implementations of Equals(Object) and GetHashCode() so that their behavior is consistent with that of the Equals(T) method. If you do override 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.
