System.Object Class

public class Object

Assembly

mscorlib

Library

BCL

Summary

Provides support for classes. This class is the root of the object hierarchy.

Description

[Note: Classes derived from Object can override the following methods of the Object class:

]

See Also

System Namespace

Members

Object Constructors

Object Constructor

Object Methods

Object.Equals(System.Object) Method
Object.Equals(System.Object, System.Object) Method
Object.Finalize Method
Object.GetHashCode Method
Object.GetType Method
Object.MemberwiseClone Method
Object.ReferenceEquals Method
Object.ToString Method

Object Constructor

public Object();

Summary

Constructs a new instance of the Object class.

Description

[Usage: This constructor is called by constructors in derived classes, but it can also be used to directly create an instance of the Object class. This might be useful, for example, if you need to obtain a reference to an object so that you can synchronize on it, as might be the case when using the C# lock statement.]

See Also

System.Object Class, System Namespace

Object.Equals(System.Object) Method

public virtual bool Equals(object obj);

Summary

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

Parameters

obj
The Object to compare with the current instance.

Return Value

true if obj is equal to the current instance; otherwise, false .

Description

[Behaviors: The statements listed below are required to be true for all implementations of the System.Object.Equals(System.Object) method. In the list, x, y, and z represent non-null object references.

See System.Object.GetHashCode for additional required behaviors pertaining to the System.Object.Equals(System.Object) method.

[Note: Implementations of System.Object.Equals(System.Object) should not throw exceptions.]

]

[Default: The System.Object.Equals(System.Object) method tests for referential equality , which means that System.Object.Equals(System.Object) returns true if the specified instance of Object and the current instance are the same instance; otherwise, it returns false .

[Note: An implementation of the System.Object.Equals(System.Object) method is shown in the following C# code:

public virtual bool Equals(Object obj) {

return this == obj;

}

]

]

[Overrides: For some kinds of objects, it is desirable to have System.Object.Equals(System.Object) 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 definition of what constitutes an object's "value" is up to the implementer of the type, but it is typically some or all of 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.

When the Equals method of a base class provides value equality, an override of Equals in a class derived from that base class should invoke the inherited implementation of Equals .

All implementations of System.Object.GetHashCode are required to ensure that for any two object references x and y, if x.Equals( y) == true , then x.GetHashCode() == y.GetHashCode() .

If your programming language supports operator overloading, and if you choose to overload the equality operator for a given type, that type should override the Equals method. Such implementations of the Equals method should 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.

If you are implementing a value type, you should follow these guidelines:

For reference types, the guidelines are as follows:

If you implement IComparable on a given type, you should override Equals on that type.

]

[Usage: The System.Object.Equals(System.Object) method is called by methods in collections classes that perform search operations, including the System.Array.IndexOf(System.Array,System.Object) method and the System.Collections.ArrayList.Contains(System.Object) method.]

Example

Example 1:

The following example contains two calls to the default implementation of System.Object.Equals(System.Object) .

using System;
class MyClass {
   static void Main() {
      Object obj1 = new Object();
      Object obj2 = new Object();
      Console.WriteLine(obj1.Equals(obj2));
      obj1 = obj2; 
      Console.WriteLine(obj1.Equals(obj2)); 
   }
}
The output is

False

True

Example 2:

The following example shows a Point class that overrides the System.Object.Equals(System.Object) method to provide value equality and a class Point3D , which is derived from Point . Because Point's override of System.Object.Equals(System.Object) 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.

using System;
public class Point: object {
 int x, y;
 public override bool Equals(Object obj) {
 //Check for null and compare run-time types.
 if (obj == null || GetType() != obj.GetType()) return false;
 Point p = (Point)obj;
 return (x == p.x) && (y == p.y);
 }
 public override int GetHashCode() {
 return x ^ y;
 }
}

class Point3D: Point {
 int z;
 public override bool Equals(Object obj) {
 return base.Equals(obj) && z == ((Point3D)obj).z;
 }
 public override int GetHashCode() {
 return base.GetHashCode() ^ z;
 }
}
The Point.Equals method checks that the obj argument is non-null and that it references an instance of the same type as this object. If either of those checks fail, the method returns false. The System.Object.Equals(System.Object) method uses System.Object.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 instead the method had used a check of the form obj is Point , the check would return true in cases where obj is an instance of a subclass 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 non-null, 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 subclass. Specifically, the cast to Point3D is not executed unless obj has been determined to be of type Point3D or a subclass of Point3D .

Example 3:

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

using System;
class Rectangle {
 Point a, b;
 public override bool Equals(Object obj) {
 if (obj == null || GetType() != obj.GetType()) return false;
 Rectangle r = (Rectangle)obj;
 //Use Equals to compare instance variables
 return a.Equals(r.a) && b.Equals(r.b);
 }
 public override int GetHashCode() {
 return a.GetHashCode() ^ b.GetHashCode();
 }
}
Example 4:

In some languages, such as C#, operator overloading is supported. When a type overloads operator ==, it should also override the System.Object.Equals(System.Object) method to provide the same functionality. This is typically accomplished by writing the Equals method in terms of the overloaded operator ==. For example:

using System;
public struct Complex {
 double re, im;
 public override bool Equals(Object obj) {
 return obj is Complex && this == (Complex)obj;
 }
 public override int GetHashCode() {
 return re.GetHashCode() ^ 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);
 }
}
Because Complex is a C# struct (a value type), it is known that there will be no subclasses of Complex . Therefore, the System.Object.Equals(System.Object) 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.

See Also

System.Object Class, System Namespace

Object.Equals(System.Object, System.Object) Method

public static bool Equals(object objA, object objB);

Summary

Determines whether two object references are equal.

Parameters

objA
First object to compare.
objB
Second object to compare.

Return Value

true if one or more of the following statements is true:

otherwise returns false .

Description

This static method checks for null references before it calls objA.Equals(objB ) and returns false if either objA or objB is null. If the Equals(object obj) implementation throws an exception, this method throws an exception.

Example

The following example demonstrates the System.Object.Equals(System.Object) method.

using System;

public class MyClass {
   public static void Main() {
   string s1 = "Tom";
   string s2 = "Carol";
   Console.WriteLine("Object.Equals(\"{0}\", \"{1}\") => {2}", 
      s1, s2, Object.Equals(s1, s2));

   s1 = "Tom";
   s2 = "Tom";
   Console.WriteLine("Object.Equals(\"{0}\", \"{1}\") => {2}", 
      s1, s2, Object.Equals(s1, s2));

   s1 = null;
   s2 = "Tom";
   Console.WriteLine("Object.Equals(null, \"{1}\") => {2}",
       s1, s2, Object.Equals(s1, s2));

   s1 = "Carol";
   s2 = null;
   Console.WriteLine("Object.Equals(\"{0}\", null) => {2}", 
       s1, s2, Object.Equals(s1, s2));

   s1 = null;
   s2 = null;
   Console.WriteLine("Object.Equals(null, null) => {2}", 
       s1, s2, Object.Equals(s1, s2));
   }
}
The output is

Object.Equals("Tom", "Carol") => False

Object.Equals("Tom", "Tom") => True

Object.Equals(null, "Tom") => False

Object.Equals("Carol", null) => False

Object.Equals(null, null) => True

See Also

System.Object Class, System Namespace

Object.Finalize Method

~Object();

Summary

Allows a Object to perform cleanup operations before the memory allocated for the Object is automatically reclaimed.

Description

[Behaviors: During execution, System.Object.Finalize is automatically called after an object becomes inaccessible, unless the object has been exempted from finalization by a call to System.GC.SuppressFinalize(System.Object). During shutdown of an application domain, System.Object.Finalize is automatically called on objects that are not exempt from finalization, even those that are still accessible. System.Object.Finalize is automatically called only once on a given instance, unless the object is re-registered using a mechanism such as System.GC.ReRegisterForFinalize(System.Object) and System.GC.SuppressFinalize(System.Object) has not been subsequently called.

Conforming implementations of the CLI are required to make every effort to ensure that for every object that has not been exempted from finalization, the System.Object.Finalize method is called after the object becomes inaccessible. However, there might be some circumstances under which Finalize is not called. Conforming CLI implementations are required to explicitly specify the conditions under which Finalize is not guaranteed to be called. [Note: For example, Finalize might not be guaranteed to be called in the event of equipment failure, power failure, or other catastrophic system failures.]

In addition to System.GC.ReRegisterForFinalize(System.Object) and System.GC.SuppressFinalize(System.Object), conforming implementations of the CLI are allowed to provide other mechanisms that affect the behavior of System.Object.Finalize . Any mechanisms provided are required to be specified by the CLI implementation.

The order in which the Finalize methods of two objects are run is unspecified, even if one object refers to the other.

The thread on which Finalize is run is unspecified.

Every implementation of System.Object.Finalize in a derived type is required to call its base type's implementation of Finalize . This is the only case in which application code calls System.Object.Finalize .

]

[Default: The System.Object.Finalize implementation does nothing.

]

[Overrides: A type should implement Finalize when it uses unmanaged resources such as file handles or database connections that must be released when the managed object that uses them is reclaimed. Because Finalize methods can be invoked in any order (including from multiple threads), synchronization can be necessary if the Finalize method can interact with other objects, whether accessible or not. Furthermore, since the order in which Finalize is called is unspecified, implementers of Finalize (or of destructors implemented through overriding Finalize) must take care to correctly handle references to other objects, as their Finalize method might already have been invoked. In general, referenced objects should not be considered valid during finalization.

See the IDisposable interface for an alternate means of disposing of resources.

]

[Usage: For C# developers: Destructors are the C# mechanism for performing cleanup operations. Destructors provide appropriate safeguards, such as automatically calling the base type's destructor. In C# code, System.Object.Finalize cannot be called or overridden.]

See Also

System.Object Class, System Namespace

Object.GetHashCode Method

public virtual int GetHashCode();

Summary

Generates a hash code for the current instance.

Return Value

A Int32 containing the hash code for the current instance.

Description

System.Object.GetHashCode serves as a hash function for a specific type. [Note: A hash function is used to quickly generate a number (a hash code) corresponding to the value of an object. Hash functions are used with hashtables . A good hash function algorithm rarely generates hash codes that collide. For more information about hash functions, see The Art of Computer Programming , Vol. 3, by Donald E. Knuth.]

[Behaviors: All implementations of System.Object.GetHashCode are required to ensure that for any two object references x and y, if x.Equals(y) == true, then x.GetHashCode() == y.GetHashCode().

Hash codes generated by System.Object.GetHashCode need not be unique.

Implementations of System.Object.GetHashCode are not permitted to throw exceptions.

]

[Default: The System.Object.GetHashCode implementation attempts to produce a unique hash code for every object, but the hash codes generated by this method are not guaranteed to be unique. Therefore, System.Object.GetHashCode can generate the same hash code for two different instances.]

[Overrides: It is recommended (but not required) that types overriding System.Object.GetHashCode also override System.Object.Equals(System.Object) . Hashtables cannot be relied on to work correctly if this recommendation is not followed.]

[Usage: Use this method to obtain the hash code of an object. Hash codes should not be persisted (i.e. in a database or file) as they are allowed to change from run to run.]

Example

Example 1

In some cases, System.Object.GetHashCode is implemented to simply return an integer value. The following example illustrates an implementation of System.Int32.GetHashCode , which returns an integer value:

using System;
public struct Int32 {
 int value;
 //other methods...

 public override int GetHashCode() {
 return value;
 }
}
Example 2

Frequently, a type has multiple data members that can participate in generating the hash code. One way to generate a hash code is to combine these fields using an xor (exclusive or) operation, as shown in the following example:

using System;
public struct Point {
 int x;
 int y; 
 //other methods
 
 public override int GetHashCode() {
 return x ^ y;
 }
}
Example 3

The following example illustrates another case where the type's fields are combined using xor (exclusive or) to generate the hash code. Notice that in this example, the fields represent user-defined types, each of which implements System.Object.GetHashCode (and should implement System.Object.Equals(System.Object) as well):

using System;
public class SomeType {
 public override int GetHashCode() {
 return 0;
 }
}

public class AnotherType {
 public override int GetHashCode() {
 return 1;
 }
}

public class LastType {
 public override int GetHashCode() {
 return 2;
 }
}
public class MyClass {
 SomeType a = new SomeType();
 AnotherType b = new AnotherType();
 LastType c = new LastType();

 public override int GetHashCode () {
 return a.GetHashCode() ^ b.GetHashCode() ^ c.GetHashCode();
 }
}
Avoid implementing System.Object.GetHashCode in a manner that results in circular references. In other words, if AClass.GetHashCode calls BClass.GetHashCode, it should not be the case that BClass.GetHashCode calls AClass.GetHashCode.

Example 4

In some cases, the data member of the class in which you are implementing System.Object.GetHashCode is bigger than a Int32. In such cases, you could combine the high order bits of the value with the low order bits using an XOR operation, as shown in the following example:

using System;
public struct Int64 {
 long value;
 //other methods...

 public override int GetHashCode() {
 return ((int)value ^ (int)(value >> 32));
 }
}

See Also

System.Object Class, System Namespace

Object.GetType Method

public Type GetType();

Summary

Gets the type of the current instance.

Return Value

The instance of Type that represents the run-time type (the exact type) of the current instance.

Description

For two objects x and y that have identical run-time types, System.Object.ReferenceEquals(System.Object,System.Object)(x.GetType(),y.GetType()) returns true .

Example

The following example demonstrates the fact that System.Object.GetType returns the run-time type of the current instance:

using System;
public class MyBaseClass: Object {
}
public class MyDerivedClass: MyBaseClass {
}
public class Test {
   public static void Main() {
   MyBaseClass myBase = new MyBaseClass();
   MyDerivedClass myDerived = new MyDerivedClass();

   object o = myDerived;
   MyBaseClass b = myDerived;

   Console.WriteLine("mybase: Type is {0}", myBase.GetType());
   Console.WriteLine("myDerived: Type is {0}", myDerived.GetType());
   Console.WriteLine("object o = myDerived: Type is {0}", o.GetType());
   Console.WriteLine("MyBaseClass b = myDerived: Type is {0}", b.GetType());
   }
}
The output is

mybase: Type is MyBaseClass

myDerived: Type is MyDerivedClass

object o = myDerived: Type is MyDerivedClass

MyBaseClass b = myDerived: Type is MyDerivedClass

See Also

System.Object Class, System Namespace

Object.MemberwiseClone Method

protected object MemberwiseClone();

Summary

Creates a shallow copy of the current instance.

Return Value

A shallow copy of the current instance. The run-time type (the exact type) of the returned object is the same as the run-time type of the object that was copied.

Description

System.Object.MemberwiseClone creates a new instance of the same type as the current instance and then copies each of the object's non-static fields in a manner that depends on whether the field is a value type or a reference type. If the field is a value type, a bit-by-bit copy of all the field's bits is performed. If the field is a reference type, only the reference is copied. The algorithm for performing a shallow copy is as follows (in pseudo-code):

for each instance field f in this instance

if (f is a value type)

bitwise copy the field

if (f is a reference type)

copy the reference

end for loop

[Note: This mechanism is referred to as a shallow copy because it copies rather than clones the non-static fields.]

Because System.Object.MemberwiseClone implements the above algorithm, for any object, a, the following statements are required to be true:

System.Object.MemberwiseClone does not call any of the type's constructors.

[Note: If System.Object.Equals(System.Object) has been overridden, a.MemberwiseClone().Equals(a) might return false .]

[Usage: For an alternate copying mechanism, see ICloneable .

System.Object.MemberwiseClone is protected (rather than public) to ensure that from verifiable code it is only possible to clone objects of the same class as the one performing the operation (or one of its subclasses). Although cloning an object does not directly open security holes, it does allow an object to be created without running any of its constructors. Since these constructors might establish important invariants, objects created by cloning might not have these invariants established, and this can lead to incorrect program behavior. For example, a constructor might add the new object to a linked list of all objects of this class, and cloning the object would not add the new object to that list -- thus operations that relied on the list to locate all instances would fail to notice the cloned object. By making the method protected, only objects of the same class (or a subclass) can produce a clone and implementers of those classes are (presumably) aware of the appropriate invariants and can arrange for them to be true without necessarily calling a constructor.

]

Example

The following example shows a class called MyClass as well as a representation of the instance of MyClass returned by System.Object.MemberwiseClone .

using System;
class MyBaseClass {
   public static string CompanyName = "My Company";
   public int age;
   public string name;
}

class MyDerivedClass: MyBaseClass {

   static void Main() {
   
   //Create an instance of MyDerivedClass
   //and assign values to its fields.
   MyDerivedClass m1 = new MyDerivedClass();
   m1.age = 42;
   m1.name = "Sam";

   //Do a shallow copy of m1
   //and assign it to m2.
   MyDerivedClass m2 = (MyDerivedClass) m1.MemberwiseClone();
   }
}
A graphical representation of m1 and m2 might look like this

+---------------+

|     42        |                           m1 

+---------------+

|     +---------|-----------------> "Sam" 

+---------------+                    /|\ 

                                      | 

+---------------+                     | 

|     42        |                     |      m2 

+---------------+                     | 

|      +--------|---------------------| 

+---------------+

See Also

System.Object Class, System Namespace

Object.ReferenceEquals Method

public static bool ReferenceEquals(object objA, object objB);

Summary

Determines whether two object references are identical.

Parameters

objA
First object to compare.
objB
Second object to compare.

Return Value

True if a and b refer to the same object or are both null references; otherwise, false .

Description

This static method provides a way to compare two objects for reference equality. It does not call any user-defined code, including overrides of System.Object.Equals(System.Object) .

Example

using System;
class MyClass {
   static void Main() {
   object o = null;
   object p = null;
   object q = new Object();
   Console.WriteLine(Object.ReferenceEquals(o, p));
   p = q;
   Console.WriteLine(Object.ReferenceEquals(p, q));
   Console.WriteLine(Object.ReferenceEquals(o, p));
   }
}
The output is

True

True

False

See Also

System.Object Class, System Namespace

Object.ToString Method

public virtual string ToString();

Summary

Creates and returns a String representation of the current instance.

Return Value

A String representation of the current instance.

Description

[Behaviors: System.Object.ToString returns a string whose content is intended to be understood by humans. Where the object contains culture-sensitive data, the string representation returned by System.Object.ToString takes into account the current system culture. For example, for an instance of the Double class whose value is zero, the implementation of System.Double.ToString might return "0.00" or "0,00" depending on the current UI culture. [Note: Although there are no exact requirements for the format of the returned string, it should as much as possible reflect the value of the object as perceived by the user.]

]

[Default: System.Object.ToString is equivalent to calling System.Object.GetType to obtain the Type object for the current instance and then returning the result of calling the System.Object.ToString implementation for that type. [Note: The value returned includes the full name of the type.]

]

[Overrides: It is recommended, but not required, that System.Object.ToString be overridden in a derived class to return values that are meaningful for that type. For example, the base data types, such as Int32, implement System.Object.ToString so that it returns the string form of the value the object represents.

Subclasses that require more control over the formatting of strings than System.Object.ToString provides should implement IFormattable, whose System.Object.ToString method uses the culture of the current thread.

]

Example

The following example outputs the textual description of the value of an object of type Object to the console.

using System;

class MyClass {
   static void Main() {
      object o = new object();
      Console.WriteLine (o.ToString());
   }
}
The output is

System.Object

See Also

System.Object Class, System Namespace