Namespace and Assemblies
The first line of the “Hello, C# World!” program was this:
using System;
This line adds a reference to the System namespace to the program. After adding a reference to a namespace, you can access any member of the namespace. As mentioned, in .NET library references documentation, each class belongs to a namespace. But what exactly is a namespace?
To define .NET classes in a category so they’d be easy to recognize, Microsoft used the C++ class-packaging concept know as namespaces. A namespace is simply a grouping of related classes. The root of all namespaces is the System namespace. If you see namespaces in the .NET library, each class is defined in a group of similar category. For example, The System.Data namespace only possesses data-related classes, and System.Multithreading contains only multithreading classes. When you create a new application using visual C#, you see that each application is defined as a namespace and that all classes belong to that namespace. You can access these classes from other application by referencing their namespaces.
Standard Input and Output Streams
The System.Console class provides the capability to read streams from and write streams to the System console. It also defines functionality for error streams. The Read operation reads data from the console to the standard input stream, and the Write operation writes data to the standard output stream. The standard error stream is responsible for storing error data. These streams are the automatically associated with the system console.
The error, in, and out properties of the Console class represents standard error output, standard input and standard output streams. In the standard output stream, the Read method reads the next character, and the ReadLine method reads the next line. The Write and WriteLine methods write the data to the standard output stream. Table 1 describes some of the console class methods.
The Object Class
As described, in the .NET framework, all types are represented as objects and are derived from the Object class. The Object class defines five methods: Equals,
ReferenceEquals GetHashCode, GetType and ToString. Table 2 describes these methods, which are available to all types in the .NET library.
Table 2. Object class methods
The Equals and ReferenceEqual Methods
The Equals method in the Object class can compare two objects. The ReferenceEqual method can compare the two objects’ instances. For example:
Console.WriteLine(Object.Equals(cls1, cls2));
Console.WriteLine(Object.Equals(str1, str2));
The ToString Method and String Conversion
The ToString method of the Object class converts a type to a string type
The GetHashCode method
A hashtable (also commonly known as a map or dictionary) is a data structure that stores one or more key- value pairs of data. Hashtables are useful when you want fast access to a list of data through a key (which can be a number, letter, string, or any object). In .NET the HashTable class represents a hashtable, which is implemented based on a hashing algorithm. This class also provides methods and constructors to define the size of the hash table. You can use the Add and Remove methods to add and remove items from a hashtable. The Count property of the HashTable class returns the number of items in a hashtable.
The GetHashCode method returns the hash code of an object. To return a hash code for a type, you must override the GetHashCode method. An integer value is returned, which represents whether an object is available in a hashtable.Two other useful methods of the object class are MemberWiseClone and Finalize
methods. The MemberWiseClone method creates a shallow copy of an object, which can be used as a clone of an object. The Finalize method acts as a destructor and can clean up the resources before the garbage collector calls the object. You need to override this method and write your own code to clean up the resources. The garbage collector automatically calls the Finalize method if an object is no longer in use.