.NET 1.1+

The Object Class (2)

The Object class is a special type that is the base class for all other classes and types, including the value types. It defines a set of methods that are therefore inherited by every other type that is defined within the .NET framework class library.

GetHashCode Method

The GetHashCode method provides an algorithm to generate a hash code for an object. Hash codes are used when creating hash tables to permit objects to be found quickly in large sets of data. The GetHashCode method is used by the Hashtable collection class for this purpose.

The GetHashCode method returns an integer containing the hash code for an object. The value is not unique and should not be used as an identifier or for any purposes other than when using a hashing function. This is particularly relevant when using multiple versions of the .NET framework as the hashing algorithms for classes vary between versions, leading to different results for identical objects.

You can see examples of the return values by executing the following code. The results shown are generated using version 3.5 of the .NET framework and may differ from those you see.

int i = 10;
float f = 10;
string s = "Hello";
int result;

result = i.GetHashCode();           // result = 10
result = f.GetHashCode();           // result = 1092616192
result = s.GetHashCode();           // result = -694847

The GetHashCode method can be overridden. When doing so, the following guidelines should be followed:

• If GetHashCode is overridden, the Equals method must also be overridden for compatibility. Otherwise, Hashtables may function incorrectly.
• The value returned from the hashing algorithm must be appropriate for value types. Two values that would be considered equal when using the Equals method must return the same hash code.
• The hash codes generated by the algorithm should be well distributed amongst the available range of integer return values. If the algorithm produces many duplicates or similar values, the performance of Hashtables will be impacted.
• The hashing algorithm should be as fast and efficient as possible to avoid performance issues with Hashtables.
• The GetHashCode method must not throw exceptions.

GetType Method

The GetType method simply returns the type of the object that invokes it. This is useful when using polymorphism techniques as the type of the underlying object can be identified, even if held in a variable declared as another type. For example, if "Dog" is a subclass of "Animal" and a Dog object is being held in an Animal variable, the type returned will still be Dog. The method is also used for reflection.

The type is returned in a System.Type object. A detailed description of the System.Type class is beyond the scope of this article. For demonstration purposes we will simply output a string representation of the type to the console.

string s = "Hello";
Console.WriteLine(s.GetType());     // Outputs "System.String"

object o = s;
Console.WriteLine(o.GetType());     // Outputs "System.String"

ReferenceEquals Method

The ReferenceEquals method is a static member of the Object class. It is used with reference types to determine if two instances of a class contain the same reference. If the references are the same, the method returns true. If the references are different, the method returns false, even if the values of the two instances match. If the two items to be compared are both null, the resultant value is true. If they are two value types, the result is always false.

The method is called with two parameters, each holding one of the references to be compared.

object o1 = new object();
object o2 = new object();
object o3 = o1;
bool result;

result = object.ReferenceEquals(o1, o2);    // result = false
result = object.ReferenceEquals(o1, o3);    // result = true

int i1 = 1;
int i2 = 1;

result = object.ReferenceEquals(i1, i2);    // result = false