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Object.Equals Method (Object)

Updated: October 2010

Determines whether the specified Object is equal to the current Object.

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

'Declaration
Public Overridable Function Equals ( _
	obj As Object _
) As Boolean
'Usage
Dim instance As Object 
Dim obj As Object 
Dim returnValue As Boolean 

returnValue = instance.Equals(obj)

Parameters

obj
Type: System.Object

The Object to compare with the current Object.

Return Value

Type: System.Boolean
true if the specified Object is equal to the current Object; otherwise, false.

The default implementation of Equals supports reference equality for reference types, and bitwise equality for value types. Reference equality means the object references that are compared refer to the same object. Bitwise equality means the objects that are compared have the same binary representation.

Note that a derived type might override the Equals method to implement value equality. Value equality means the compared objects have the same value even though they have different binary representations. For example, consider two Decimal objects that represent the numbers 1.10 and 1.1000. The Decimal objects do not have bitwise equality because they have different binary representations to account for the different number of trailing zeroes. However, the objects have value equality because the numbers 1.10 and 1.1000 are considered equal for comparison purposes since the trailing zeroes are insignificant.

Notes to Implementers:

This method can be overridden by a derived class. For example, many of the base data types return true if both objects represent the same value; otherwise, false.

This method only compares primitives and objects. It must be overridden to compare more complex structures, such as arrays of objects.

The following statements must be true for all implementations of the Equals method. In the list, x, y, and z represent object references that are not Nothing.

  • x.Equals(x) returns true, except in cases that involve floating-point types. See IEC 60559:1989, Binary Floating-point Arithmetic for Microprocessor Systems.

  • x.Equals(y) returns the same value as y.Equals(x).

  • x.Equals(y) returns true if both x and y are NaN.

  • If (x.Equals(y) && y.Equals(z)) returns true, then x.Equals(z) returns true.

  • Successive calls to x.Equals(y) return the same value as long as the objects referenced by x and y are not modified.

  • x.Equals(Nothing) returns false.

See GetHashCode for additional required behaviors pertaining to the Equals method.

Implementations of Equals must not throw exceptions.

You can compare the current object to another object for reference equality by using the this keyword in C# (for example, if (this == otherObj)) or the Me keyword in Visual Basic (for example, If Me Is otherObject Then …).

For some kinds of objects, it is desirable to have Equals test for value equality instead of referential 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 is based on the characters of the string; the Equals method of the String class returns true for any two string instances that contain exactly the same characters in the same order.

Types that implement IComparable must override Equals.

Types that override Equals must also override GetHashCode; otherwise, Hashtable might not work correctly.

If your programming language supports operator overloading and if you choose to overload the equality operator for a given type, that type must override the Equals method. Such implementations of the Equals method must return the same results as the equality operator. Following this guideline will help ensure that class library code using Equals (such as ArrayList and Hashtable) behaves in a manner that is consistent with the way the equality operator is used by application code.

The following guidelines are for implementing a value type:

  • Consider overriding Equals to gain increased performance over that provided by the default implementation of Equals on ValueType.

  • If you override Equals and the language supports operator overloading, you must overload the equality operator for your value type.

The following guidelines are for implementing a reference type:

  • Consider overriding Equals on a reference type 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.

The following code example compares the current instance with another object.

Imports System
Imports System.IO

Class Program
    Public Shared Sub Main(ByVal args As String())
        Dim Obj1 As New Object()
        Dim Obj2 As New Object()
        Console.WriteLine(Obj1.Equals(Obj2))
        Obj2 = Obj1
        Console.WriteLine(Obj1.Equals(Obj2))
    End Sub 
End Class 

'This example produces the following output: 
'False 
'True

The following example shows a Point class that overrides the Equals method to provide value equality and a class Point3D, which is derived from Point. Because Point 's override of Equals is the first in the inheritance chain to introduce value equality, the Equals method of the base class (which is inherited from Object and checks for referential equality) is not invoked. However, Point3D.Equals invokes Point.Equals because Point implements Equals in a manner that provides value equality.

Imports System

Class Point
    Inherits Object
    Protected x, y As Integer 

    Public Sub New() 
        Me.x = 0
        Me.y = 0
    End Sub 'New 

    Public Sub New(ByVal X As Integer, ByVal Y As Integer) 
        Me.x = X
        Me.y = Y
    End Sub 'New 

    Public Overrides Function Equals(ByVal obj As Object) As Boolean  
        'Check for null and compare run-time types. 
        If obj Is Nothing OrElse Not [GetType]().Equals(obj.GetType()) Then 
            Return False 
        End If 
        Dim p As Point = CType(obj, Point)
        Return x = p.x AndAlso y = p.y
    End Function 'Equals

    Public Overrides Function GetHashCode() As Integer  
        Return x ^ y
    End Function 'GetHashCode
End Class 'Point

Class Point3D
    Inherits Point
    Private z As Integer 

    Public Sub New(ByVal X As Integer, ByVal Y As Integer, ByVal Z As Integer) 
        Me.x = X
        Me.y = Y
        Me.z = Z
    End Sub 'New 

    Public Overrides Function Equals(ByVal obj As Object) As Boolean  
        Return MyBase.Equals(obj) AndAlso z = CType(obj, Point3D).z
    End Function 'Equals

    Public Overrides Function GetHashCode() As Integer  
        Return MyBase.GetHashCode() ^ z
    End Function 'GetHashCode
End Class 'Point3D

Class [MyClass]
    Public Shared Sub Main() 
        Dim point2D As New Point(5, 5)
        Dim point3Da As New Point3D(5, 5, 2)
        Dim point3Db As New Point3D(5, 5, 2)

        If Not point2D.Equals(point3Da) Then
            Console.WriteLine("point2D does not equal point3Da.")
        End If 
        If Not point3Db.Equals(point2D) Then
            Console.WriteLine("Likewise, point3Db does not equal point2D.")
        End If 
        If point3Da.Equals(point3Db) Then
            Console.WriteLine("However, point3Da equals point3Db.")
        End If 

    End Sub 'Main 
End Class '[MyClass]
' ---------------------------------- 
' Output should be: 
'  
' point2D does not equal point3Da. 
' Likewise, point3Db does not equal point2D. 
' However, point3Da equals point3Db.

The Point.Equals method checks that the obj argument is not Nothing and that it references an instance of the same type as this object. If either of those checks fail, the method returns false.

The Equals method uses GetType to determine whether the run-time types of the two objects are identical. (Note that typeof is not used here because it returns the static type.) If the method used a check of the form obj is Point, 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 runtime 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 variables of the two objects.

In Point3D.Equals, the inherited Equals method is invoked before anything else is done; the inherited Equals method checks to see that obj is not Nothing, that obj is an instance of the same class as this object and that the inherited instance variables match. Only when the inherited Equals returns true does the method compare the instance variables introduced in the derived class. Specifically, the cast to Point3D is not executed unless obj has been determined to be of type Point3D or a derived class of Point3D.

In the previous example, the equality operator is used to compare the individual instance variables. In some cases, it is appropriate to use the Equals method to compare instance variables in an Equals implementation, as shown in the following code example.

Imports System

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 'New 

    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)
        'Uses Equals to compare variables. 
        Return a.Equals(r.a) AndAlso b.Equals(r.b)
    End Function 'Equals

    Public Overrides Function GetHashCode() As Integer  
        Return a.GetHashCode() ^ b.GetHashCode()
    End Function 'GetHashCode
End Class 'Rectangle

' Class Point added for clean compile 

Class Point
    Private x As Integer 
    Private y As Integer 

    Public Sub New(ByVal X As Integer, ByVal Y As Integer) 
        Me.x = X
        Me.y = Y
    End Sub 'New 

    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 
        End If 
        Dim p As Point = CType(obj, Point)
        Return x = p.x AndAlso y = p.y

    End Function 'Equals

    Public Overrides Function GetHashCode() As Integer  
        Return x.GetHashCode() ^ y.GetHashCode()
    End Function 'GetHashCode
End Class 'Point 


Class [MyClass]
    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)

        If r1.Equals(r2) Then
            Console.WriteLine("Rectangle r1 equals rectangle r2!")
        End If 
        If Not r2.Equals(r3) Then
            Console.WriteLine("But rectangle r2 does not equal rectangle r3.")
        End If 
    End Sub 'Main
End Class '[MyClass]
' ------------------------------ 
' Output should be: 
' Rectangle r1 equals rectangle r2! 
' But rectangle r2 does not equal rectangle r3.

In some languages, such as C# and Visual Basic, operator overloading is supported. When a type overloads the equality operator, it must also override the Equals method to provide the same functionality. This is typically accomplished by writing the Equals method in terms of the overloaded equality, as in the following code example.

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 re.GetHashCode() ^ 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  
End Structure 

Class Example
   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

      If cmplx1 <> cmplx2 Then
         Console.WriteLine("The two objects are not equal.")
      End If 
      If Not cmplx1.Equals(cmplx2) Then
         Console.WriteLine("The two objects are not equal.")
      End If

      cmplx2.re = 4.0

      If cmplx1.Equals(cmplx2) Then
         Console.WriteLine("The two objects are now equal!")
      End If 
      If cmplx1 = cmplx2 Then
         Console.WriteLine("The two objects are now equal!")
      End If 
   End Sub 
End Class  
' The example displays the following output:

Because Complex is a value type, it cannot be derived from; therefore, the Equals method need not compare the GetType results for each object, but can instead use the is operator to check the type of the obj parameter.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

October 2010

Revised the last example.

Customer feedback.

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