Chapter 1: .Net 2.0 Framework 基础之使用值类型

这是系列文章，是读"MS Press - MCTS Self-Paced Training Kit"的笔记，并把主要部分翻译成中文。
作者地址：http://www.cnblogs.com/nevernet （转载请保留）

使用值类型

通常有三种值类型:

.Net 框架内建的类型
用户自定义类型
枚举类型

上述每个类型都继承于 System.Value . 后续部分将显示如何使用这些不同的类型.

内建的值类型

所有的数值类型都是值类型.

Table 1-1: Built-in Value Types
Type (Visual Basic/C# alias)	Bytes	Range	Use for
System.SByte (SByte/sbyte)	1	-128 to 127	Signed byte values
System.Byte (Byte/byte)	1	0 to 255	Unsigned bytes
System.Int16 (Short/short)	2	-32768 to 32767	Interoperation and other specialized uses
System.Int32 (Integer/int)	4	-2147483648 to 2147483647	Whole numbers and counters
System.UInt32 (UInteger/uint)	4	0 to 4294967295	Positive whole numbers and counters
System.Int64 (Long/long)	8	-9223372036854775808 to 9223372036854775807	Large whole numbers
System.Single (Single/float)	4	-3.402823E+38 to 3.402823E+38	Floating point numbers
System.Double (Double/double)	8	-1.79769313486232E+308 to 1.79769313486232E+308	Precise or large floating point numbers
System.Decimal (Decimal/decimal)	16	-79228162514264337593543950335 to 79228162514264337593543950335	Financial and scientific calculations requiring great precision

Best Practices—Optimizing performance with built-in types

The runtime optimizes the performance of 32-bit integer types (Int32 and UInt32), so use those types for counters and other frequently accessed integral variables. For floating-point operations, Double is the most efficient type because those operations are optimized by hardware.

以下是最常用的类型，(C#或者VB对以下类型做了别名)

Table 1-2: Other Value Types
Type (Visual Basic/C# alias)	Bytes	Range	Use for
System.Char (Char/char)	2	N/A	Single Unicode characters
System.Boolean (Boolean/bool)	4	N/A	True/False values
System.IntPtr (none)	Platform-dependent	N/A	Pointer to a memory address
System.DateTime (Date/date)	8	1/1/0001 12:00:00 AM to 12/31/9999 11:59:59 PM	Moments in time

在.Net里面有近300个值类型

我们经常使用ToString方法来显示function或者objects的simple values。ToString is overridden from the fundamental System.Object type.

The Object base class

In the .NET Framework, all types are derived from System.Object. That relationship helps establish the common type system used throughout the Framework.

怎样定义值类型？

Keyword differences in Visual Basic and C#

Code
' VB
Dim b As Boolean = False

// C#
bool b = false;

Variable capitalizations in Visual Basic and C#

注意C#区分大小写，但是VB不区分

定义nullable

' VB
Dim b As Nullable(Of Boolean) = Nothing

// C#
Nullable<bool> b = null;

// Shorthand notation, only for C#
bool? b = null;

NET 2.0

The Nullable type is new in .NET 2.0.

Declaring a variable as nullable enables the HasValue and Value members. Use HasValue to detect whether or not a value has been set:

' VB
If b.HasValue Then Console.WriteLine("b is {0}.", b.Value) _
  Else Console.WriteLine("b is not set.")

// C#
if (b.HasValue)Console.WriteLine("b is {0}.", b.Value);
  else Console.WriteLine("b is not set.");

如何创建自定义值类型

' VB - Requires reference to System.Drawing

' Create point
Dim p As New System.Drawing.Point(20, 30)

' Move point diagonally
p.Offset(-1, -1)
Console.WriteLine("Point X {0}, Y {1}", p.X, p.Y)

// C# - Requires reference to System.Drawing
// Create point
System.Drawing.Point p = new System.Drawing.Point(20, 30);

// Move point diagonally
p.Offset(-1, -1);
Console.WriteLine("Point X {0}, Y {1}", p.X, p.Y);

通过Structure 关键字，可以创建自定义struct

' VB
Structure Cycle
    ' Private fields
    Dim _val, _min, _max As Integer

    ' Constructor
    Public Sub New(ByVal min As Integer, ByVal max As Integer)
        _val = min : _min = min : _max = max
    End Sub

    ' Public members
    Public Property Value() As Integer
        Get
            Return _val
        End Get
        Set(ByVal value As Integer)
            ' Ensure new setting is between _min and _max.
            If value > _max Then _val = _min _
              Else If value < _min Then _val = _max _
              Else _val = value
        End Set
    End Property

    Public Overrides Function ToString() As String
        Return Value.ToString
    End Function

    Public Function ToInteger() As Integer
        Return Value
    End Function

    ' Operators (new in 2.0)
    Public Shared Operator +(ByVal arg1 As Cycle, _
      ByVal arg2 As Integer) As Cycle
        arg1.Value += arg2
        Return arg1
    End Operator

    Public Shared Operator -(ByVal arg1 As Cycle, _
      ByVal arg2 As Integer) As Cycle
        arg1.Value -= arg2
        Return arg1
    End Operator
End Structure

// C#
struct Cycle
{
    // Private fields
    int _val, _min, _max;

    // Constructor
    public Cycle(int min, int max)
    {
        _val = min;
        _min = min;
        _max = max;
    }

    public int Value
    {
        get { return _val; }
        set
        {
            if (value > _max)
                _val = _min;
            else
            {
                if (value < _min)
                    _val = _max;
                else
                    _val = value;
            }
        }
    }

    public override string ToString()
    {
        return Value.ToString();
    }

    public int ToInteger()
    {
        return Value;
    }

    // Operators (new in .NET 2.0)
    public static Cycle operator +(Cycle arg1, int arg2)
    {
        arg1.Value += arg2;
        return arg1;
    }

    public static Cycle operator -(Cycle arg1, int arg2)
    {
        arg1.Value -= arg2;
        return arg1;
    }
}

.NET 2.0

The Operator keyword is new in .NET 2.0.

如何创建枚举类型

' VB
Enum Titles As Integer
    Mr
    Ms
    Mrs
    Dr
End Enum

// C#
enum Titles : int { Mr, Ms, Mrs, Dr };

在创建Titles 类型的实例后，你就可以把值赋给一个变量了，如下：

' VB
Dim t As Titles = Titles.Dr
Console.WriteLine("{0}.", t) ' Displays "Dr."

// C#
Titles t = Titles.Dr;
Console.WriteLine("{0}.", t); // Displays "Dr."

定义和使用值类型

下面是练习：

用Visual Studio, 创建console application 项目. 命名为：CreateStruct.
Create a new structure named Person, as the following code demonstrates:
```
' VB
Structure Person
End Structure

// C#
struct Person
{
}
```

Within the Person structure, define three public members:

firstName (a String)
lastName (a String)
age (an Integer)

The following code demonstrates this:

' VB
Public firstName As String
Public lastName As String
Public age As Integer

// C#
public string firstName;
public string lastName;
public int age;

Create a constructor that defines all three member variables, as the following code demonstrates:

' VB
Public Sub New(ByVal _firstName As String, ByVal _lastName As String, ByVal _age As
Integer)
    firstName = _firstName
    lastName = _lastName
    age = _age
End Sub

// C#
public Person(string _firstName, string _lastName, int _age)
{
    firstName = _firstName;
    lastName = _lastName;
    age = _age;
}

Override the ToString method to display the person's first name, last name, and age. The following code demonstrates this:

' VB
Public Overloads Overrides Function ToString() As String
    Return firstName + " " + lastName + ", age " + age.ToString
End Function

// C#
public override string ToString()
{
    return firstName + " " + lastName + ", age " + age;
}

Within the Main method of the console application, write code to create an instance of the structure and pass the instance to the Console.WriteLine method, as the following code demonstrates:
```
' VB
Dim p As Person = New Person("Tony", "Allen", 32)
Console.WriteLine(p)

// C#
Person p = new Person("Tony", "Allen", 32);
Console.WriteLine(p);
```
Run the console application to verify that it works correctly.

Exercise 2: Add an Enumeration to a Structure

In this exercise, you will extend the structure you created in Exercise 1 by adding an enumeration.

Open the project you created in Exercise 1.
Declare a new enumeration in the Person structure. Name the enumeration Genders, and specify two possible values: Male and Female. The following code sample demonstrates this:
```
' VB
Enum Genders
    Male
    Female
End Enum

// C#
public enum Genders : int { Male, Female };
```

Add a public member of type Genders, and modify the Person constructor to accept an instance of Gender. The following code demonstrates this:

' VB
Public firstName As String
Public lastName As String
Public age As Integer
Public gender As Genders

Public Sub New(ByVal _firstName As String, ByVal _lastName As String, _
ByVal _age As Integer, ByVal _gender As Genders)
    firstName = _firstName
    lastName = _lastName
    age = _age
    gender = _gender
End Sub

// C#
public string firstName;
public string lastName;
public int age;
public Genders gender;

public Person(string _firstName, string _lastName, int _age, Genders _gender)
{
    firstName = _firstName;
    lastName = _lastName;
    age = _age;
    gender = _gender;
}

Modify the Person.ToString method to also display the gender, as the following code sample demonstrates:

' VB
Public Overloads Overrides Function ToString() As String
    Return firstName + " " + lastName + " (" + gender.ToString() + "), age " +
age.ToString
End Function

// C#
public override string ToString()
{
    return firstName + " " + lastName + " (" + gender + "), age " + age;
}

Modify your Main code to properly construct an instance of the Person class, as the following code sample demonstrates:

' VB
Sub Main()
    Dim p As Person = New Person("Tony", "Allen", 32, Person.Genders.Male)
    Console.WriteLine(p)
End Sub

// C#
static void Main(string[] args)
{
    Person p = new Person("Tony", "Allen", 32, Person.Genders.Male);
    Console.WriteLine(p.ToString());
}

Run the console application to verify that it works correctly.

Lesson Summary

The .NET Framework includes a large number of built-in types that you can use directly or use to build your own custom types.
Value types directly contain their data, offering excellent performance. However, value types are limited to types that store very small pieces of data. In the .NET Framework, all value types are 16 bytes or shorter.
You can create user-defined types that store multiple values and methods. In object-oriented development environments, a large portion of your application logic will be stored in user-defined types.
Enumerations improve code readability by providing symbols for a set of values.

Lesson Review

You can use the following questions to test your knowledge of the information in Lesson 1, "Using Value Types." The questions are also available on the companion CD if you prefer to review them in electronic form.

Answers

Answers to these questions and explanations of why each answer choice is right or wrong are located in the "Answers" section at the end of the book.

1.	Which of the following are value types? (Choose all that apply.) Decimal String System.Drawing.Point Integer
2.	You pass a value-type variable into a procedure as an argument. The procedure changes the variable; however, when the procedure returns, the variable has not changed. What happened? (Choose one.) The variable was not initialized before it was passed in. Passing a value type into a procedure creates a copy of the data. The variable was redeclared within the procedure level. The procedure handled the variable as a reference.
3.	Which is the correct declaration for a nullable integer? ' VB Dim i As Nullable<Of Integer> = Nothing // C# Nullable(int) i = null; ' VB Dim i As Nullable(Of Integer) = Nothing // C# Nullable<int> i = null; ' VB Dim i As Integer = Nothing // C# int i = null; ' VB Dim i As Integer(Nullable) = Nothing // C# int<Nullable> i = null;
4.	You need to create a simple class or structure that contains only value types. You must create the class or structure so that it runs as efficiently as possible. You must be able to pass the class or structure to a procedure without concern that the procedure will modify it. Which of the following should you create? A reference class A reference structure A value class A value structure