Object.Equals Method (Object)
Determines whether the specified object is equal to the current object.
Assembly: mscorlib (in mscorlib.dll)
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 method tests for reference equality, and a call to the 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 Person class, which is a reference type, and calls the Person class constructor to instantiate two new Person objects, person1a and person2, which have the same value. It also assigns person1a to another object variable, person1b. As the output from the example shows, person1a and person1b are equal because they reference the same object. However, person1a and person2 are not equal, although they have the same value.
If the current instance is a value type, the 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.
The values of the public and private fields of the two objects are equal. The following example tests for value equality. It defines a Person structure, which is a value type, and calls the Person class constructor to instantiate two new Person objects, person1 and person2, which have the same value. As the output from the example shows, although the two object variables refer to different objects, person1 and person2 are equal because they have the same value for the private personName field.
Because the Object class is the base class for all types in the .NET Framework, the 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 method overload on a class in the Windows Runtime, it provides the default behavior for classes that don’t override . This is part of the support that the .NET Framework provides for the Windows Runtime (see .NET Framework Support for Windows Store Apps and Windows Runtime). Classes in the Windows Runtime don’t inherit Object, and currently don’t implement an method. However, they appear to have ToString, , and GetHashCode methods when you use them in your C# or Visual Basic code, and the .NET Framework provides the default behavior for these methods.
Windows Runtime classes that are written in C# or Visual Basic can override the method overload.
Notes for Callers
Derived classes frequently override the 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.
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 . 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 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 the .NET Framework.
Equality defined by
Class derived directly from Object
Reference equality; equivalent to calling Object.ReferenceEquals.
Value equality; either direct byte-by-byte comparison or field-by-field comparison using reflection.
Values must have the same enumeration type and the same underlying value.
Delegates must have the same type with identical invocation lists.
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 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 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.
The following statements must be true for all implementations of the method. In the list, x, y, and z represent object references that are not null.
x.Equals(x) returns true, except in cases that involve floating-point types. See ISO/IEC/IEEE 60559:2011, Information technology -- Microprocessor Systems -- Floating-Point arithmetic.
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(null) returns false.
Implementations of Equals must not throw exceptions; they should always return a value. For example, if obj is a null reference (Nothing in Visual Basic), the Equals method should return false instead of throwing an ArgumentNullException.
Follow these guidelines when overriding :
Types that implement IComparable must override .
Types that override 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 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.
The following guidelines apply to overriding 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.
The following guidelines apply to overriding 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 . The 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.
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 to test for value equality, the method is not called. However, Point3D.Equals calls Point.Equals because Point implements in a manner that provides value equality.
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 , 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 to provide for value equality.
Some languages such as C# and Visual Basic support operator overloading. When a type overloads the equality operator, it must also override the method to provide the same functionality. This is typically accomplished by writing the method in terms of the overloaded equality operator, as in the following example.
Because Complex is a value type, it cannot be derived from. Therefore, the override to 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.