C# 0 The Complete Reference

PART I

C h a p t e r   1 2 :  

I n t e r f a c e s ,   S t r u c t u r e s ,   a n d   E n u m e r a t i o n s  

329

PART IPART I

  static void Main() {

    MyStruct a;

    MyStruct b;

    a.x = 10;

    b.x = 20;

    Console.WriteLine("a.x {0}, b.x {1}", a.x, b.x);

    a = b;

    b.x = 30;

    Console.WriteLine("a.x {0}, b.x {1}", a.x, b.x);

  }

}

The output is shown here:

a.x 10, b.x 20

a.x 20, b.x 30

As the output shows, after the assignment

a = b;

the structure variables 

a

 and 

b

 are still separate and distinct. That is, 

a

 does not refer to or 

relate to 

b

 in any way other than containing a copy of 

b

’s value. This would not be the case 

if

a

 and 

b

 were class references. For example, here is the 

class

 version of the preceding 

program:

// Use a class.

using System;

// Now a class.

class MyClass {

  public int x;

}

// Now show a class object assignment.

class ClassAssignment {

  static void Main() {

    MyClass a = new MyClass();

    MyClass b = new MyClass();

    a.x = 10;

    b.x = 20;

    Console.WriteLine("a.x {0}, b.x {1}", a.x, b.x);

    a = b;

    b.x = 30;

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T h e   C #   L a n g u a g e

    Console.WriteLine("a.x {0}, b.x {1}", a.x, b.x);

  }

}

The output from this version is shown here:

a.x 10, b.x 20

a.x 30, b.x 30

As you can see, after the assignment of 

b

 to 

a

, both variables refer to the same object—the 

one originally referred to by 

b

.

Why Structures?

At this point, you might be wondering why C# includes the 

struct

 since it seems to be a 

less-capable version of a 

class

. The answer lies in efficiency and performance. Because 

structures are value types, they are operated on directly rather than through a reference. 

Thus, a 

struct

 does not require a separate reference variable. This means that less memory 

is used in some cases. Furthermore, because a 

struct

 is accessed directly, it does not suffer 

from the performance loss that is inherent in accessing a class object. Because classes are 

reference types, all access to class objects is through a reference. This indirection adds 

overhead to every access. Structures do not incur this overhead. In general, if you need 

to simply store a group of related data, but don’t need inheritance and don’t need to operate 

on that data through a reference, then a 

struct

 can be a more efficient choice.

Here is another example that shows how a structure might be used in practice. It 

simulates an e-commerce transaction record. Each transaction includes a packet header that 

contains the packet number and the length of the packet. This is followed by the account 

number and the amount of the transaction. Because the packet header is a self-contained 

unit of information, it is organized as a structure. This structure can then be used to create 

a transaction record, or any other type of information packet.

// Structures are good when grouping small amounts of data.

using System;

// Define a packet structure.

struct PacketHeader {

  public uint PackNum; // packet number

  public ushort PackLen; // length of packet

}

// Use PacketHeader to create an e-commerce transaction record.

class Transaction {

  static uint transacNum = 0;

  PacketHeader ph;  // incorporate PacketHeader into Transaction

  string accountNum;

  double amount;

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