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.NET Framework Class Library

Object.ReferenceEquals Method

Determines whether the specified Object instances are the same instance.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)
Public Shared Function ReferenceEquals ( _
	objA As Object, _
	objB As Object _
) As Boolean

Parameters

objA
Type: System.Object

The first object to compare.

objB
Type: System.Object

The second object to compare.

Return Value

Type: System.Boolean
true if objA is the same instance as objB or if both are null; otherwise, false.

Unlike the Equals method and the equality operator, the ReferenceEquals method cannot be overridden. Because of this, if you want to test two object references for equality and you are unsure about the implementation of the Equals method, you can call the ReferenceEquals method.

However, the return value of the ReferenceEquals method may appear to be anomalous in these two scenarios:

  • When comparing value types. If objA and objB are value types, they are boxed before they are passed to the ReferenceEquals method. This means that if both objA and objB represent the same instance of a value type, the ReferenceEquals method nevertheless returns false, as the following example shows.

    Public Module Example
       Public Sub Main
          Dim int1 As Integer = 3
          Console.WriteLine(Object.ReferenceEquals(int1, int1))
          Console.WriteLine(int1.GetType().IsValueType)
       End Sub 
    End Module 
    ' The example displays the following output: 
    '       False 
    '       True
    
  • When comparing strings. If objA and objB are strings, the ReferenceEquals method returns true if the string is interned. It does not perform a test for value equality. In the following example, s1 and s2 are equal because they are two instances of a single interned string. However, s3 and s4 are not equal, because although they are have identical string values, that string is not interned.

    Module Example
       Public Sub Main()
          Dim s1 As String = "String1" 
          Dim s2 As String = "String1"
          Console.WriteLine("s1 = s2: {0}", Object.ReferenceEquals(s1, s2))
          Console.WriteLine("{0} interned: {1}", s1, 
                            If(String.IsNullOrEmpty(String.IsInterned(s1)), "No", "Yes"))
    
          Dim suffix As String = "A" 
          Dim s3 = "String" + suffix
          Dim s4 = "String" + suffix
          Console.WriteLine("s3 = s4: {0}", Object.ReferenceEquals(s3, s4))
          Console.WriteLine("{0} interned: {1}", s3, 
                            If(String.IsNullOrEmpty(String.IsInterned(s3)), "No", "Yes"))
       End Sub 
    End Module 
    ' The example displays the following output: 
    '       s1 = s2: True 
    '       String1 interned: Yes 
    '       s3 = s4: False 
    '       StringA interned: No
    

    For more information about string interning, see String.IsInterned.

The following example uses ReferenceEquals to determine if two objects are the same instance.

Imports System

Public Class App
    Public Shared Sub Main() 
        Dim o As Object = Nothing 
        Dim p As Object = Nothing 
        Dim q As New Object
        Console.WriteLine(Object.ReferenceEquals(o, p))
        p = q
        Console.WriteLine(Object.ReferenceEquals(p, q))
        Console.WriteLine(Object.ReferenceEquals(o, p))
    End Sub  
End Class  
' This code produces the following output: 

' True 
' True 
' False 
'

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library