In C#, a structure is a value type data type. It helps you to make a single variable hold related data of various data types. The struct keyword is used for creating a structure. Structures are used to represent a record. Suppose you want to keep track of your books in a library. You might want to track the following attributes about each book −
  • Title
  • Author
  • Subject
  • Book ID

Defining a Structure

To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member for your program. For example, here is the way you can declare the Book structure − 
struct Books {
   public string title;
   public string author;
   public string subject;
   public int book_id;
};
The following program shows the use of the structure − 
using System;

struct Books {
   public string title;
   public string author;
   public string subject;
   public int book_id;
};  

public class testStructure {
   public static void Main(string[] args) {
      Books Book1;   /* Declare Book1 of type Book */
      Books Book2;   /* Declare Book2 of type Book */

      /* book 1 specification */
      Book1.title = "C Programming";
      Book1.author = "Nuha Ali"; 
      Book1.subject = "C Programming Tutorial";
      Book1.book_id = 6495407;

      /* book 2 specification */
      Book2.title = "Telecom Billing";
      Book2.author = "Zara Ali";
      Book2.subject =  "Telecom Billing Tutorial";
      Book2.book_id = 6495700;

      /* print Book1 info */
      Console.WriteLine( "Book 1 title : {0}", Book1.title);
      Console.WriteLine("Book 1 author : {0}", Book1.author);
      Console.WriteLine("Book 1 subject : {0}", Book1.subject);
      Console.WriteLine("Book 1 book_id :{0}", Book1.book_id);

      /* print Book2 info */
      Console.WriteLine("Book 2 title : {0}", Book2.title);
      Console.WriteLine("Book 2 author : {0}", Book2.author);
      Console.WriteLine("Book 2 subject : {0}", Book2.subject);
      Console.WriteLine("Book 2 book_id : {0}", Book2.book_id);       

      Console.ReadKey();
   }
}
When the above code is compiled and executed, it produces the following result − 
Book 1 title : C Programming
Book 1 author : Nuha Ali
Book 1 subject : C Programming Tutorial
Book 1 book_id : 6495407
Book 2 title : Telecom Billing
Book 2 author : Zara Ali
Book 2 subject : Telecom Billing Tutorial
Book 2 book_id : 6495700

Features of C# Structures

You have already used a simple structure named Books. Structures in C# are quite different from that in traditional C or C++. The C# structures have the following features −
  • Structures can have methods, fields, indexers, properties, operator methods, and events.
  • Structures can have defined constructors, but not destructors. However, you cannot define a default constructor for a structure. The default constructor is automatically defined and cannot be changed.
  • Unlike classes, structures cannot inherit other structures or classes.
  • Structures cannot be used as a base for other structures or classes.
  • A structure can implement one or more interfaces.
  • Structure members cannot be specified as abstract, virtual, or protected.
  • When you create a struct object using the New operator, it gets created and the appropriate constructor is called. Unlike classes, structs can be instantiated without using the New operator.
  • If the New operator is not used, the fields remain unassigned and the object cannot be used until all the fields are initialized.

Class versus Structure

Classes and Structures have the following basic differences −
  • classes are reference types and structs are value types
  • structures do not support inheritance
  • structures cannot have default constructor
In the light of the above discussions, let us rewrite the previous example − 
using System;

struct Books {
   private string title;
   private string author;
   private string subject;
   private int book_id;
   
   public void getValues(string t, string a, string s, int id) {
      title = t;
      author = a;
      subject = s;
      book_id = id;
   }
   
   public void display() {
      Console.WriteLine("Title : {0}", title);
      Console.WriteLine("Author : {0}", author);
      Console.WriteLine("Subject : {0}", subject);
      Console.WriteLine("Book_id :{0}", book_id);
   }
};  

public class testStructure {

   public static void Main(string[] args) {
      Books Book1 = new Books();   /* Declare Book1 of type Book */
      Books Book2 = new Books();   /* Declare Book2 of type Book */

      /* book 1 specification */
      Book1.getValues("C Programming",
      "Nuha Ali", "C Programming Tutorial",6495407);

      /* book 2 specification */
      Book2.getValues("Telecom Billing",
      "Zara Ali", "Telecom Billing Tutorial", 6495700);

      /* print Book1 info */
      Book1.display();

      /* print Book2 info */
      Book2.display(); 

      Console.ReadKey();
   }
}
When the above code is compiled and executed, it produces the following result − 
Title : C Programming
Author : Nuha Ali
Subject : C Programming Tutorial
Book_id : 6495407
Title : Telecom Billing
Author : Zara Ali
Subject : Telecom Billing Tutorial
Book_id : 6495700