System.Object.Equals method
This article provides supplementary remarks to the reference documentation for this API.
This article pertains to the Object.Equals(Object) method.
The type of comparison between the current instance and the obj parameter depends on whether the current instance is a reference type or a value type.
If the current instance is a reference type, the Equals(Object) method tests for reference equality, and a call to the Equals(Object) method is equivalent to a call to the ReferenceEquals method. Reference equality means that the object variables that are compared refer to the same object. The following example illustrates the result of such a comparison. It defines a
Personclass, which is a reference type, and calls thePersonclass constructor to instantiate two newPersonobjects,person1aandperson2, which have the same value. It also assignsperson1ato another object variable,person1b. As the output from the example shows,person1aandperson1bare equal because they reference the same object. However,person1aandperson2are not equal, although they have the same value.using System; // Define a reference type that does not override Equals. public class Person { private string personName; public Person(string name) { this.personName = name; } public override string ToString() { return this.personName; } } public class Example1 { public static void Main() { Person person1a = new Person("John"); Person person1b = person1a; Person person2 = new Person(person1a.ToString()); Console.WriteLine("Calling Equals:"); Console.WriteLine("person1a and person1b: {0}", person1a.Equals(person1b)); Console.WriteLine("person1a and person2: {0}", person1a.Equals(person2)); Console.WriteLine("\nCasting to an Object and calling Equals:"); Console.WriteLine("person1a and person1b: {0}", ((object) person1a).Equals((object) person1b)); Console.WriteLine("person1a and person2: {0}", ((object) person1a).Equals((object) person2)); } } // The example displays the following output: // person1a and person1b: True // person1a and person2: False // // Casting to an Object and calling Equals: // person1a and person1b: True // person1a and person2: False// Define a reference type that does not override Equals. type Person(name) = override _.ToString() = name let person1a = Person "John" let person1b = person1a let person2 = Person(string person1a) printfn "Calling Equals:" printfn $"person1a and person1b: {person1a.Equals person1b}" printfn $"person1a and person2: {person1a.Equals person2}" printfn "\nCasting to an Object and calling Equals:" printfn $"person1a and person1b: {(person1a :> obj).Equals(person1b :> obj)}" printfn $"person1a and person2: {(person1a :> obj).Equals(person2 :> obj)}" // The example displays the following output: // person1a and person1b: True // person1a and person2: False // // Casting to an Object and calling Equals: // person1a and person1b: True // person1a and person2: False' Define a reference type that does not override Equals. Public Class Person1 Private personName As String Public Sub New(name As String) Me.personName = name End Sub Public Overrides Function ToString() As String Return Me.personName End Function End Class Module Example0 Public Sub Main() Dim person1a As New Person1("John") Dim person1b As Person1 = person1a Dim person2 As New Person1(person1a.ToString()) Console.WriteLine("Calling Equals:") Console.WriteLine("person1a and person1b: {0}", person1a.Equals(person1b)) Console.WriteLine("person1a and person2: {0}", person1a.Equals(person2)) Console.WriteLine() Console.WriteLine("Casting to an Object and calling Equals:") Console.WriteLine("person1a and person1b: {0}", CObj(person1a).Equals(CObj(person1b))) Console.WriteLine("person1a and person2: {0}", CObj(person1a).Equals(CObj(person2))) End Sub End Module ' The example displays the following output: ' Calling Equals: ' person1a and person1b: True ' person1a and person2: False ' ' Casting to an Object and calling Equals: ' person1a and person1b: True ' person1a and person2: FalseIf the current instance is a value type, the Equals(Object) method tests for value equality. Value equality means the following:
The two objects are of the same type. As the following example shows, a Byte object that has a value of 12 does not equal an Int32 object that has a value of 12, because the two objects have different run-time types.
byte value1 = 12; int value2 = 12; object object1 = value1; object object2 = value2; Console.WriteLine("{0} ({1}) = {2} ({3}): {4}", object1, object1.GetType().Name, object2, object2.GetType().Name, object1.Equals(object2)); // The example displays the following output: // 12 (Byte) = 12 (Int32): Falselet value1 = 12uy let value2 = 12 let object1 = value1 :> obj let object2 = value2 :> obj printfn $"{object1} ({object1.GetType().Name}) = {object2} ({object2.GetType().Name}): {object1.Equals object2}" // The example displays the following output: // 12 (Byte) = 12 (Int32): FalseModule Example2 Public Sub Main() Dim value1 As Byte = 12 Dim value2 As Integer = 12 Dim object1 As Object = value1 Dim object2 As Object = value2 Console.WriteLine("{0} ({1}) = {2} ({3}): {4}", object1, object1.GetType().Name, object2, object2.GetType().Name, object1.Equals(object2)) End Sub End Module ' The example displays the following output: ' 12 (Byte) = 12 (Int32): FalseThe values of the public and private fields of the two objects are equal. The following example tests for value equality. It defines a
Personstructure, which is a value type, and calls thePersonclass constructor to instantiate two newPersonobjects,person1andperson2, which have the same value. As the output from the example shows, although the two object variables refer to different objects,person1andperson2are equal because they have the same value for the privatepersonNamefield.using System; // Define a value type that does not override Equals. public struct Person3 { private string personName; public Person3(string name) { this.personName = name; } public override string ToString() { return this.personName; } } public struct Example3 { public static void Main() { Person3 person1 = new Person3("John"); Person3 person2 = new Person3("John"); Console.WriteLine("Calling Equals:"); Console.WriteLine(person1.Equals(person2)); Console.WriteLine("\nCasting to an Object and calling Equals:"); Console.WriteLine(((object) person1).Equals((object) person2)); } } // The example displays the following output: // Calling Equals: // True // // Casting to an Object and calling Equals: // True// Define a value type that does not override Equals. [<Struct>] type Person(personName: string) = override _.ToString() = personName let person1 = Person "John" let person2 = Person "John" printfn "Calling Equals:" printfn $"{person1.Equals person2}" printfn $"\nCasting to an Object and calling Equals:" printfn $"{(person1 :> obj).Equals(person2 :> obj)}" // The example displays the following output: // Calling Equals: // True // // Casting to an Object and calling Equals: // True' Define a value type that does not override Equals. Public Structure Person4 Private personName As String Public Sub New(name As String) Me.personName = name End Sub Public Overrides Function ToString() As String Return Me.personName End Function End Structure Module Example4 Public Sub Main() Dim p1 As New Person4("John") Dim p2 As New Person4("John") Console.WriteLine("Calling Equals:") Console.WriteLine(p1.Equals(p2)) Console.WriteLine() Console.WriteLine("Casting to an Object and calling Equals:") Console.WriteLine(CObj(p1).Equals(p2)) End Sub End Module ' The example displays the following output: ' Calling Equals: ' True ' ' Casting to an Object and calling Equals: ' True
Because the Object class is the base class for all types in .NET, the Object.Equals(Object) method provides the default equality comparison for all other types. However, types often override the Equals method to implement value equality. For more information, see the Notes for Callers and Notes for Inheritors sections.
Notes for the Windows Runtime
When you call the Equals(Object) method overload on a class in the Windows Runtime, it provides the default behavior for classes that don't override Equals(Object). This is part of the support that .NET provides for the Windows Runtime (see .NET Support for Windows Store Apps and Windows Runtime). Classes in the Windows Runtime don't inherit Object, and currently don't implement an Equals(Object) method. However, they appear to have ToString, Equals(Object), and GetHashCode methods when you use them in your C# or Visual Basic code, and .NET provides the default behavior for these methods.
Note
Windows Runtime classes that are written in C# or Visual Basic can override the Equals(Object) method overload.
Notes for callers
Derived classes frequently override the Object.Equals(Object) method to implement value equality. In addition, types also frequently provide an additional strongly typed overload to the Equals method, typically by implementing the IEquatable<T> interface. When you call the Equals method to test for equality, you should know whether the current instance overrides Object.Equals and understand how a particular call to an Equals method is resolved. Otherwise, you may be performing a test for equality that is different from what you intended, and the method may return an unexpected value.
The following example provides an illustration. It instantiates three StringBuilder objects with identical strings, and then makes four calls to Equals methods. The first method call returns true, and the remaining three return false.
using System;
using System.Text;
public class Example5
{
public static void Main()
{
StringBuilder sb1 = new StringBuilder("building a string...");
StringBuilder sb2 = new StringBuilder("building a string...");
Console.WriteLine("sb1.Equals(sb2): {0}", sb1.Equals(sb2));
Console.WriteLine("((Object) sb1).Equals(sb2): {0}",
((Object) sb1).Equals(sb2));
Console.WriteLine("Object.Equals(sb1, sb2): {0}",
Object.Equals(sb1, sb2));
Object sb3 = new StringBuilder("building a string...");
Console.WriteLine("\nsb3.Equals(sb2): {0}", sb3.Equals(sb2));
}
}
// The example displays the following output:
// sb1.Equals(sb2): True
// ((Object) sb1).Equals(sb2): False
// Object.Equals(sb1, sb2): False
//
// sb3.Equals(sb2): False
open System
open System.Text
let sb1 = StringBuilder "building a string..."
let sb2 = StringBuilder "building a string..."
printfn $"sb1.Equals(sb2): {sb1.Equals sb2}"
printfn $"((Object) sb1).Equals(sb2): {(sb1 :> obj).Equals sb2}"
printfn $"Object.Equals(sb1, sb2): {Object.Equals(sb1, sb2)}"
let sb3 = StringBuilder "building a string..."
printfn $"\nsb3.Equals(sb2): {sb3.Equals sb2}"
// The example displays the following output:
// sb1.Equals(sb2): True
// ((Object) sb1).Equals(sb2): False
// Object.Equals(sb1, sb2): False
//
// sb3.Equals(sb2): False
Imports System.Text
Module Example5
Public Sub Main()
Dim sb1 As New StringBuilder("building a string...")
Dim sb2 As New StringBuilder("building a string...")
Console.WriteLine("sb1.Equals(sb2): {0}", sb1.Equals(sb2))
Console.WriteLine("CObj(sb1).Equals(sb2): {0}",
CObj(sb1).Equals(sb2))
Console.WriteLine("Object.Equals(sb1, sb2): {0}",
Object.Equals(sb1, sb2))
Console.WriteLine()
Dim sb3 As Object = New StringBuilder("building a string...")
Console.WriteLine("sb3.Equals(sb2): {0}", sb3.Equals(sb2))
End Sub
End Module
' The example displays the following output:
' sb1.Equals(sb2): True
' CObj(sb1).Equals(sb2): False
' Object.Equals(sb1, sb2): False
'
' sb3.Equals(sb2): False
In the first case, the strongly typed StringBuilder.Equals(StringBuilder) method overload, which tests for value equality, is called. Because the strings assigned to the two StringBuilder objects are equal, the method returns true. However, StringBuilder does not override Object.Equals(Object). Because of this, when the StringBuilder object is cast to an Object, when a StringBuilder instance is assigned to a variable of type Object, and when the Object.Equals(Object, Object) method is passed two StringBuilder objects, the default Object.Equals(Object) method is called. Because StringBuilder is a reference type, this is equivalent to passing the two StringBuilder objects to the ReferenceEquals method. Although all three StringBuilder objects contain identical strings, they refer to three distinct objects. As a result, these three method calls return false.
You can compare the current object to another object for reference equality by calling the ReferenceEquals method. In Visual Basic, you can also use the is keyword (for example, If Me Is otherObject Then ...).
Notes for inheritors
When you define your own type, that type inherits the functionality defined by the Equals method of its base type. The following table lists the default implementation of the Equals method for the major categories of types in .NET.
| Type category | Equality defined by | Comments |
|---|---|---|
| Class derived directly from Object | Object.Equals(Object) | Reference equality; equivalent to calling Object.ReferenceEquals. |
| Structure | ValueType.Equals | Value equality; either direct byte-by-byte comparison or field-by-field comparison using reflection. |
| Enumeration | Enum.Equals | Values must have the same enumeration type and the same underlying value. |
| Delegate | MulticastDelegate.Equals | Delegates must have the same type with identical invocation lists. |
| Interface | Object.Equals(Object) | Reference equality. |
For a value type, you should always override Equals, because tests for equality that rely on reflection offer poor performance. You can also override the default implementation of Equals for reference types to test for value equality instead of reference equality and to define the precise meaning of value equality. Such implementations of Equals return true if the two objects have the same value, even if they are not the same instance. The type's implementer decides what constitutes an object's value, but it is typically some or all the data stored in the instance variables of the object. For example, the value of a String object is based on the characters of the string; the String.Equals(Object) method overrides the Object.Equals(Object) method to return true for any two string instances that contain the same characters in the same order.
The following example shows how to override the Object.Equals(Object) method to test for value equality. It overrides the Equals method for the Person class. If Person accepted its base class implementation of equality, two Person objects would be equal only if they referenced a single object. However, in this case, two Person objects are equal if they have the same value for the Person.Id property.
public class Person6
{
private string idNumber;
private string personName;
public Person6(string name, string id)
{
this.personName = name;
this.idNumber = id;
}
public override bool Equals(Object obj)
{
Person6 personObj = obj as Person6;
if (personObj == null)
return false;
else
return idNumber.Equals(personObj.idNumber);
}
public override int GetHashCode()
{
return this.idNumber.GetHashCode();
}
}
public class Example6
{
public static void Main()
{
Person6 p1 = new Person6("John", "63412895");
Person6 p2 = new Person6("Jack", "63412895");
Console.WriteLine(p1.Equals(p2));
Console.WriteLine(Object.Equals(p1, p2));
}
}
// The example displays the following output:
// True
// True
open System
type Person(name, id) =
member _.Name = name
member _.Id = id
override _.Equals(obj) =
match obj with
| :? Person as personObj ->
id.Equals personObj.Id
| _ ->
false
override _.GetHashCode() =
id.GetHashCode()
let p1 = Person("John", "63412895")
let p2 = Person("Jack", "63412895")
printfn $"{p1.Equals p2}"
printfn $"{Object.Equals(p1, p2)}"
// The example displays the following output:
// True
// True
Public Class Person
Private idNumber As String
Private personName As String
Public Sub New(name As String, id As String)
Me.personName = name
Me.idNumber = id
End Sub
Public Overrides Function Equals(obj As Object) As Boolean
Dim personObj As Person = TryCast(obj, Person)
If personObj Is Nothing Then
Return False
Else
Return idNumber.Equals(personObj.idNumber)
End If
End Function
Public Overrides Function GetHashCode() As Integer
Return Me.idNumber.GetHashCode()
End Function
End Class
Module Example6
Public Sub Main()
Dim p1 As New Person("John", "63412895")
Dim p2 As New Person("Jack", "63412895")
Console.WriteLine(p1.Equals(p2))
Console.WriteLine(Object.Equals(p1, p2))
End Sub
End Module
' The example displays the following output:
' True
' True
In addition to overriding Equals, you can implement the IEquatable<T> interface to provide a strongly typed test for equality.
The following statements must be true for all implementations of the Equals(Object) method. In the list, x, y, and z represent object references that are not null.
x.Equals(x)returnstrue.x.Equals(y)returns the same value asy.Equals(x).x.Equals(y)returnstrueif bothxandyareNaN.If
(x.Equals(y) && y.Equals(z))returnstrue, thenx.Equals(z)returnstrue.Successive calls to
x.Equals(y)return the same value as long as the objects referenced byxandyare not modified.x.Equals(null)returnsfalse.
Implementations of Equals must not throw exceptions; they should always return a value. For example, if obj is null, the Equals method should return false instead of throwing an ArgumentNullException.
Follow these guidelines when overriding Equals(Object):
Types that implement IComparable must override Equals(Object).
Types that override Equals(Object) must also override GetHashCode; otherwise, hash tables might not work correctly.
You should consider implementing the IEquatable<T> interface to support strongly typed tests for equality. Your IEquatable<T>.Equals implementation should return results that are consistent with Equals.
If your programming language supports operator overloading and you overload the equality operator for a given type, you must also override the Equals(Object) method to return the same result as the equality operator. This helps ensure that class library code that uses Equals (such as ArrayList and Hashtable) behaves in a manner that is consistent with the way the equality operator is used by application code.
Guidelines for reference types
The following guidelines apply to overriding Equals(Object) for a reference type:
Consider overriding Equals if the semantics of the type are based on the fact that the type represents some value(s).
Most reference types must not overload the equality operator, even if they override Equals. However, if you are implementing a reference type that is intended to have value semantics, such as a complex number type, you must override the equality operator.
You should not override Equals on a mutable reference type. This is because overriding Equals requires that you also override the GetHashCode method, as discussed in the previous section. This means that the hash code of an instance of a mutable reference type can change during its lifetime, which can cause the object to be lost in a hash table.
Guidelines for value types
The following guidelines apply to overriding Equals(Object) for a value type:
If you are defining a value type that includes one or more fields whose values are reference types, you should override Equals(Object). The Equals(Object) implementation provided by ValueType performs a byte-by-byte comparison for value types whose fields are all value types, but it uses reflection to perform a field-by-field comparison of value types whose fields include reference types.
If you override Equals and your development language supports operator overloading, you must overload the equality operator.
You should implement the IEquatable<T> interface. Calling the strongly typed IEquatable<T>.Equals method avoids boxing the
objargument.
Examples
The following example shows a Point class that overrides the Equals method to provide value equality, and a Point3D class that is derived from Point. Because Point overrides Object.Equals(Object) to test for value equality, the Object.Equals(Object) method is not called. However, Point3D.Equals calls Point.Equals because Point implements Object.Equals(Object) in a manner that provides value equality.
using System;
class Point2
{
protected int x, y;
public Point2() : this(0, 0)
{ }
public Point2(int x, int y)
{
this.x = x;
this.y = y;
}
public override bool Equals(Object obj)
{
//Check for null and compare run-time types.
if ((obj == null) || !this.GetType().Equals(obj.GetType()))
{
return false;
}
else
{
Point2 p = (Point2)obj;
return (x == p.x) && (y == p.y);
}
}
public override int GetHashCode()
{
return (x << 2) ^ y;
}
public override string ToString()
{
return String.Format("Point2({0}, {1})", x, y);
}
}
sealed class Point3D : Point2
{
int z;
public Point3D(int x, int y, int z) : base(x, y)
{
this.z = z;
}
public override bool Equals(Object obj)
{
Point3D pt3 = obj as Point3D;
if (pt3 == null)
return false;
else
return base.Equals((Point2)obj) && z == pt3.z;
}
public override int GetHashCode()
{
return (base.GetHashCode() << 2) ^ z;
}
public override String ToString()
{
return String.Format("Point2({0}, {1}, {2})", x, y, z);
}
}
class Example7
{
public static void Main()
{
Point2 point2D = new Point2(5, 5);
Point3D point3Da = new Point3D(5, 5, 2);
Point3D point3Db = new Point3D(5, 5, 2);
Point3D point3Dc = new Point3D(5, 5, -1);
Console.WriteLine("{0} = {1}: {2}",
point2D, point3Da, point2D.Equals(point3Da));
Console.WriteLine("{0} = {1}: {2}",
point2D, point3Db, point2D.Equals(point3Db));
Console.WriteLine("{0} = {1}: {2}",
point3Da, point3Db, point3Da.Equals(point3Db));
Console.WriteLine("{0} = {1}: {2}",
point3Da, point3Dc, point3Da.Equals(point3Dc));
}
}
// The example displays the following output:
// Point2(5, 5) = Point2(5, 5, 2): False
// Point2(5, 5) = Point2(5, 5, 2): False
// Point2(5, 5, 2) = Point2(5, 5, 2): True
// Point2(5, 5, 2) = Point2(5, 5, -1): False
type Point(x, y) =
new () = Point(0, 0)
member _.X = x
member _.Y = y
override _.Equals(obj) =
//Check for null and compare run-time types.
match obj with
| :? Point as p ->
x = p.X && y = p.Y
| _ ->
false
override _.GetHashCode() =
(x <<< 2) ^^^ y
override _.ToString() =
$"Point({x}, {y})"
type Point3D(x, y, z) =
inherit Point(x, y)
member _.Z = z
override _.Equals(obj) =
match obj with
| :? Point3D as pt3 ->
base.Equals(pt3 :> Point) && z = pt3.Z
| _ ->
false
override _.GetHashCode() =
(base.GetHashCode() <<< 2) ^^^ z
override _.ToString() =
$"Point({x}, {y}, {z})"
let point2D = Point(5, 5)
let point3Da = Point3D(5, 5, 2)
let point3Db = Point3D(5, 5, 2)
let point3Dc = Point3D(5, 5, -1)
printfn $"{point2D} = {point3Da}: {point2D.Equals point3Da}"
printfn $"{point2D} = {point3Db}: {point2D.Equals point3Db}"
printfn $"{point3Da} = {point3Db}: {point3Da.Equals point3Db}"
printfn $"{point3Da} = {point3Dc}: {point3Da.Equals point3Dc}"
// The example displays the following output:
// Point(5, 5) = Point(5, 5, 2): False
// Point(5, 5) = Point(5, 5, 2): False
// Point(5, 5, 2) = Point(5, 5, 2): True
// Point(5, 5, 2) = Point(5, 5, -1): False
Class Point1
Protected x, y As Integer
Public Sub New()
Me.x = 0
Me.y = 0
End Sub
Public Sub New(x As Integer, y As Integer)
Me.x = x
Me.y = y
End Sub
Public Overrides Function Equals(obj As Object) As Boolean
' Check for null and compare run-time types.
If obj Is Nothing OrElse Not Me.GetType().Equals(obj.GetType()) Then
Return False
Else
Dim p As Point1 = DirectCast(obj, Point1)
Return x = p.x AndAlso y = p.y
End If
End Function
Public Overrides Function GetHashCode() As Integer
Return (x << 2) Xor y
End Function
Public Overrides Function ToString() As String
Return String.Format("Point1({0}, {1})", x, y)
End Function
End Class
Class Point3D : Inherits Point1
Private z As Integer
Public Sub New(ByVal x As Integer, ByVal y As Integer, ByVal z As Integer)
MyBase.New(x, y)
Me.z = z
End Sub
Public Overrides Function Equals(ByVal obj As Object) As Boolean
Dim pt3 As Point3D = TryCast(obj, Point3D)
If pt3 Is Nothing Then
Return False
Else
Return MyBase.Equals(CType(pt3, Point1)) AndAlso z = pt3.z
End If
End Function
Public Overrides Function GetHashCode() As Integer
Return (MyBase.GetHashCode() << 2) Xor z
End Function
Public Overrides Function ToString() As String
Return String.Format("Point1({0}, {1}, {2})", x, y, z)
End Function
End Class
Module Example1
Public Sub Main()
Dim point2D As New Point1(5, 5)
Dim point3Da As New Point3D(5, 5, 2)
Dim point3Db As New Point3D(5, 5, 2)
Dim point3Dc As New Point3D(5, 5, -1)
Console.WriteLine("{0} = {1}: {2}",
point2D, point3Da, point2D.Equals(point3Da))
Console.WriteLine("{0} = {1}: {2}",
point2D, point3Db, point2D.Equals(point3Db))
Console.WriteLine("{0} = {1}: {2}",
point3Da, point3Db, point3Da.Equals(point3Db))
Console.WriteLine("{0} = {1}: {2}",
point3Da, point3Dc, point3Da.Equals(point3Dc))
End Sub
End Module
' The example displays the following output
' Point1(5, 5) = Point1(5, 5, 2): False
' Point1(5, 5) = Point1(5, 5, 2): False
' Point1(5, 5, 2) = Point1(5, 5, 2): True
' Point1(5, 5, 2) = Point1(5, 5, -1): False
The Point.Equals method checks to make sure that the obj argument is not null and that it references an instance of the same type as this object. If either check fails, the method returns false.
The Point.Equals method calls the GetType method to determine whether the run-time types of the two objects are identical. If the method used a check of the form obj is Point in C# or TryCast(obj, Point) in Visual Basic, the check would return true in cases where obj is an instance of a derived class of Point, even though obj and the current instance are not of the same run-time type. Having verified that both objects are of the same type, the method casts obj to type Point and returns the result of comparing the instance fields of the two objects.
In Point3D.Equals, the inherited Point.Equals method, which overrides Object.Equals(Object), is invoked before anything else is done. Because Point3D is a sealed class (NotInheritable in Visual Basic), a check in the form obj is Point in C# or TryCast(obj, Point) in Visual Basic is adequate to ensure that obj is a Point3D object. If it is a Point3D object, it is cast to a Point object and passed to the base class implementation of Equals. Only when the inherited Point.Equals method returns true does the method compare the z instance fields introduced in the derived class.
The following example defines a Rectangle class that internally implements a rectangle as two Point objects. The Rectangle class also overrides Object.Equals(Object) to provide for value equality.
using System;
class Rectangle
{
private Point a, b;
public Rectangle(int upLeftX, int upLeftY, int downRightX, int downRightY)
{
this.a = new Point(upLeftX, upLeftY);
this.b = new Point(downRightX, downRightY);
}
public override bool Equals(Object obj)
{
// Perform an equality check on two rectangles (Point object pairs).
if (obj == null || GetType() != obj.GetType())
return false;
Rectangle r = (Rectangle)obj;
return a.Equals(r.a) && b.Equals(r.b);
}
public override int GetHashCode()
{
return Tuple.Create(a, b).GetHashCode();
}
public override String ToString()
{
return String.Format("Rectangle({0}, {1}, {2}, {3})",
a.x, a.y, b.x, b.y);
}
}
class Point
{
internal int x;
internal int y;
public Point(int X, int Y)
{
this.x = X;
this.y = Y;
}
public override bool Equals (Object obj)
{
// Performs an equality check on two points (integer pairs).
if (obj == null || GetType() != obj.GetType()) return false;
Point p = (Point)obj;
return (x == p.x) && (y == p.y);
}
public override int GetHashCode()
{
return Tuple.Create(x, y).GetHashCode();
}
}
class Example
{
public static void Main()
{
Rectangle r1 = new Rectangle(0, 0, 100, 200);
Rectangle r2 = new Rectangle(0, 0, 100, 200);
Rectangle r3 = new Rectangle(0, 0, 150, 200);
Console.WriteLine("{0} = {1}: {2}", r1, r2, r1.Equals(r2));
Console.WriteLine("{0} = {1}: {2}", r1, r3, r1.Equals(r3));
Console.WriteLine("{0} = {1}: {2}", r2, r3, r2.Equals(r3));
}
}
// The example displays the following output:
// Rectangle(0, 0, 100, 200) = Rectangle(0, 0, 100, 200): True
// Rectangle(0, 0, 100, 200) = Rectangle(0, 0, 150, 200): False
// Rectangle(0, 0, 100, 200) = Rectangle(0, 0, 150, 200): False
type Point(x, y) =
member _.X = x
member _.Y = y
override _.Equals(obj) =
// Performs an equality check on two points (integer pairs).
match obj with
| :? Point as p ->
x = p.X && y = p.Y
| _ ->
false
override _.GetHashCode() =
(x, y).GetHashCode()
type Rectangle(upLeftX, upLeftY, downRightX, downRightY) =
let a = Point(upLeftX, upLeftY)
let b = Point(downRightX, downRightY)
member _.UpLeft = a
member _.DownRight = b
override _.Equals(obj) =
// Perform an equality check on two rectangles (Point object pairs).
match obj with
| :? Rectangle as r ->
a.Equals(r.UpLeft) && b.Equals(r.DownRight)
| _ ->
false
override _.GetHashCode() =
(a, b).GetHashCode()
override _.ToString() =
$"Rectangle({a.X}, {a.Y}, {b.X}, {b.Y})"
let r1 = Rectangle(0, 0, 100, 200)
let r2 = Rectangle(0, 0, 100, 200)
let r3 = Rectangle(0, 0, 150, 200)
printfn $"{r1} = {r2}: {r1.Equals r2}"
printfn $"{r1} = {r3}: {r1.Equals r3}"
printfn $"{r2} = {r3}: {r2.Equals r3}"
// The example displays the following output:
// Rectangle(0, 0, 100, 200) = Rectangle(0, 0, 100, 200): True
// Rectangle(0, 0, 100, 200) = Rectangle(0, 0, 150, 200): False
// Rectangle(0, 0, 100, 200) = Rectangle(0, 0, 150, 200): False
Class Rectangle
Private a, b As Point
Public Sub New(ByVal upLeftX As Integer, ByVal upLeftY As Integer, _
ByVal downRightX As Integer, ByVal downRightY As Integer)
Me.a = New Point(upLeftX, upLeftY)
Me.b = New Point(downRightX, downRightY)
End Sub
Public Overrides Function Equals(ByVal obj As [Object]) As Boolean
' Performs an equality check on two rectangles (Point object pairs).
If obj Is Nothing OrElse Not [GetType]().Equals(obj.GetType()) Then
Return False
End If
Dim r As Rectangle = CType(obj, Rectangle)
Return a.Equals(r.a) AndAlso b.Equals(r.b)
End Function
Public Overrides Function GetHashCode() As Integer
Return Tuple.Create(a, b).GetHashCode()
End Function
Public Overrides Function ToString() As String
Return String.Format("Rectangle({0}, {1}, {2}, {3})",
a.x, a.y, b.x, b.y)
End Function
End Class
Class Point
Friend x As Integer
Friend y As Integer
Public Sub New(ByVal X As Integer, ByVal Y As Integer)
Me.x = X
Me.y = Y
End Sub
Public Overrides Function Equals(ByVal obj As [Object]) As Boolean
' Performs an equality check on two points (integer pairs).
If obj Is Nothing OrElse Not [GetType]().Equals(obj.GetType()) Then
Return False
Else
Dim p As Point = CType(obj, Point)
Return x = p.x AndAlso y = p.y
End If
End Function
Public Overrides Function GetHashCode() As Integer
Return Tuple.Create(x, y).GetHashCode()
End Function
End Class
Class Example
Public Shared Sub Main()
Dim r1 As New Rectangle(0, 0, 100, 200)
Dim r2 As New Rectangle(0, 0, 100, 200)
Dim r3 As New Rectangle(0, 0, 150, 200)
Console.WriteLine("{0} = {1}: {2}", r1, r2, r1.Equals(r2))
Console.WriteLine("{0} = {1}: {2}", r1, r3, r1.Equals(r3))
Console.WriteLine("{0} = {1}: {2}", r2, r3, r2.Equals(r3))
End Sub
End Class
' The example displays the following output:
' Rectangle(0, 0, 100, 200) = Rectangle(0, 0, 100, 200): True
' Rectangle(0, 0, 100, 200) = Rectangle(0, 0, 150, 200): False
' Rectangle(0, 0, 100, 200) = Rectangle(0, 0, 150, 200): False
Some languages such as C# and Visual Basic support operator overloading. When a type overloads the equality operator, it must also override the Equals(Object) method to provide the same functionality. This is typically accomplished by writing the Equals(Object) method in terms of the overloaded equality operator, as in the following example.
using System;
public struct Complex
{
public double re, im;
public override bool Equals(Object obj)
{
return obj is Complex && this == (Complex)obj;
}
public override int GetHashCode()
{
return Tuple.Create(re, im).GetHashCode();
}
public static bool operator ==(Complex x, Complex y)
{
return x.re == y.re && x.im == y.im;
}
public static bool operator !=(Complex x, Complex y)
{
return !(x == y);
}
public override String ToString()
{
return String.Format("({0}, {1})", re, im);
}
}
class MyClass
{
public static void Main()
{
Complex cmplx1, cmplx2;
cmplx1.re = 4.0;
cmplx1.im = 1.0;
cmplx2.re = 2.0;
cmplx2.im = 1.0;
Console.WriteLine("{0} <> {1}: {2}", cmplx1, cmplx2, cmplx1 != cmplx2);
Console.WriteLine("{0} = {1}: {2}", cmplx1, cmplx2, cmplx1.Equals(cmplx2));
cmplx2.re = 4.0;
Console.WriteLine("{0} = {1}: {2}", cmplx1, cmplx2, cmplx1 == cmplx2);
Console.WriteLine("{0} = {1}: {2}", cmplx1, cmplx2, cmplx1.Equals(cmplx2));
}
}
// The example displays the following output:
// (4, 1) <> (2, 1): True
// (4, 1) = (2, 1): False
// (4, 1) = (4, 1): True
// (4, 1) = (4, 1): True
[<Struct; CustomEquality; NoComparison>]
type Complex =
val mutable re: double
val mutable im: double
override this.Equals(obj) =
match obj with
| :? Complex as c when c = this -> true
| _ -> false
override this.GetHashCode() =
(this.re, this.im).GetHashCode()
override this.ToString() =
$"({this.re}, {this.im})"
static member op_Equality (x: Complex, y: Complex) =
x.re = y.re && x.im = y.im
static member op_Inequality (x: Complex, y: Complex) =
x = y |> not
let mutable cmplx1 = Complex()
let mutable cmplx2 = Complex()
cmplx1.re <- 4.0
cmplx1.im <- 1.0
cmplx2.re <- 2.0
cmplx2.im <- 1.0
printfn $"{cmplx1} <> {cmplx2}: {cmplx1 <> cmplx2}"
printfn $"{cmplx1} = {cmplx2}: {cmplx1.Equals cmplx2}"
cmplx2.re <- 4.0
printfn $"{cmplx1} = {cmplx2}: {cmplx1 = cmplx2}"
printfn $"{cmplx1} = {cmplx2}: {cmplx1.Equals cmplx2}"
// The example displays the following output:
// (4, 1) <> (2, 1): True
// (4, 1) = (2, 1): False
// (4, 1) = (4, 1): True
// (4, 1) = (4, 1): True
Public Structure Complex
Public re, im As Double
Public Overrides Function Equals(ByVal obj As [Object]) As Boolean
Return TypeOf obj Is Complex AndAlso Me = CType(obj, Complex)
End Function
Public Overrides Function GetHashCode() As Integer
Return Tuple.Create(re, im).GetHashCode()
End Function
Public Shared Operator = (x As Complex, y As Complex) As Boolean
Return x.re = y.re AndAlso x.im = y.im
End Operator
Public Shared Operator <> (x As Complex, y As Complex) As Boolean
Return Not (x = y)
End Operator
Public Overrides Function ToString() As String
Return String.Format("({0}, {1})", re, im)
End Function
End Structure
Class Example8
Public Shared Sub Main()
Dim cmplx1, cmplx2 As Complex
cmplx1.re = 4.0
cmplx1.im = 1.0
cmplx2.re = 2.0
cmplx2.im = 1.0
Console.WriteLine("{0} <> {1}: {2}", cmplx1, cmplx2, cmplx1 <> cmplx2)
Console.WriteLine("{0} = {1}: {2}", cmplx1, cmplx2, cmplx1.Equals(cmplx2))
cmplx2.re = 4.0
Console.WriteLine("{0} = {1}: {2}", cmplx1, cmplx2, cmplx1 = cmplx2)
Console.WriteLine("{0} = {1}: {2}", cmplx1, cmplx2, cmplx1.Equals(cmplx2))
End Sub
End Class
' The example displays the following output:
' (4, 1) <> (2, 1): True
' (4, 1) = (2, 1): False
' (4, 1) = (4, 1): True
' (4, 1) = (4, 1): True
Because Complex is a value type, it cannot be derived from. Therefore, the override to Equals(Object) method need not call GetType to determine the precise run-time type of each object, but can instead use the is operator in C# or the TypeOf operator in Visual Basic to check the type of the obj parameter.
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