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VB.Net - 声明

2022-05-24 15:36 更新

statement 声明是Visual Basic程序中的完整指令。它可以包含关键字，运算符，变量，字面值，常量和表达式。

语句可以分类为：

1、Declaration statements 声明语句- 这些语句是您命名变量，常量或过程的语句，也可以指定数据类型。

2、Executable statements 可执行语句 - 这些是启动动作的语句。这些语句可以调用方法或函数，通过代码块循环或分支，或者将值或表达式赋值给变量或常量。在最后一种情况下，它被称为Assignment语句。

声明语句

声明语句用于命名和定义过程、变量、属性、数组和常量。当你声明的编程元素时，您还可以定义其数据类型、访问级别和范围。

您可以声明的编程元素包括变量、常数、枚举、类、结构、模块、接口、程序、过程参数、函数返回值、外部过程的引用、运算符、属性、事件和委托。

以下是在 VB.Net 中的声明语句︰


Dim number As Integer
Dim quantity As Integer = 100
Dim message As String = "Hello!"
Const maximum As Long = 1000
Const naturalLogBase As Object 
= CDec(2.7182818284)
Enum CoffeeMugSize
    Jumbo
    ExtraLarge
    Large
    Medium
    Small
End Enum 
Class Box
Public length As Double
Public breadth As Double   
Public height As Double
End Class
Structure Box
Public length As Double           
Public breadth As Double   
Public height As Double
End Structure
Public Module myModule
Sub Main()
Dim user As String = 
InputBox("What is your name?") 
MsgBox("User name is" & user)
End Sub 
End Module
Public Interface MyInterface
    Sub doSomething()
End Interface 
Function myFunction
(ByVal n As Integer) As Double 
    Return 5.87 * n
End Function
Sub mySub(ByVal s As String)
    Return
End Sub 
Declare Function getUserName
Lib "advapi32.dll" 
Alias "GetUserNameA" 
(
  ByVal lpBuffer As String, 
  ByRef nSize As Integer) As Integer 
Public Shared Operator +
(ByVal x As obj, ByVal y As obj) As obj
        Dim r As New obj
' implemention code for r = x + y
        Return r
    End Operator 
ReadOnly Property quote() As String 
    Get 
        Return quoteString
    End Get 
End Property
Public Event Finished()
Delegate Function MathOperator( 
    ByVal x As Double, 
    ByVal y As Double 
) As Double 

S.N	声明和说明	示例
1	Dim语句声明和分配一个或多个变量的存储空间。	Dim number As Integer Dim quantity As Integer = 100 Dim message As String = "Hello!"
2	Const语句声明和定义一个或多个常量。	Const maximum As Long = 1000 Const naturalLogBase As Object = CDec(2.7182818284)
3	枚举语句声明一个枚举并定义其成员的值。	Enum CoffeeMugSize Jumbo ExtraLarge Large Medium Small End Enum
4	类语句声明类的名称，并引入该类包含的变量，属性，事件和过程的定义。	Class Box Public length As Double Public breadth As Double Public height As Double End Class
5	结构声明声明结构的名称，并引入结构所包含的变量，属性，事件和过程的定义。	Structure Box Public length As Double Public breadth As Double Public height As Double End Structure
6	模块语句声明模块的名称，并介绍模块包含的变量，属性，事件和过程的定义。	Public Module myModule Sub Main() Dim user As String = InputBox("What is your name?") MsgBox("User name is" & user) End Sub End Module
7	接口语句声明接口的名称，并介绍接口包含的成员的定义。	Public Interface MyInterface Sub doSomething() End Interface
8	函数语句声明定义函数过程的名称，参数和代码。	Function myFunction (ByVal n As Integer) As Double Return 5.87 * n End Function
9	子语句声明定义Sub过程的名称，参数和代码。	Sub mySub(ByVal s As String) Return End Sub
10	声明语句声明对在外部文件中实现的过程的引用。	Declare Function getUserName Lib "advapi32.dll" Alias "GetUserNameA" ( ByVal lpBuffer As String, ByRef nSize As Integer) As Integer
11	运算符声明声明的运算符符号、操作数和在类或结构定义一个运算符过程的代码。	Public Shared Operator + (ByVal x As obj, ByVal y As obj) As obj Dim r As New obj ' implemention code for r = x + y Return r End Operator
12	属性声明声明属性的名称，以及用于存储和检索属性值的属性过程。	ReadOnly Property quote() As String Get Return quoteString End Get End Property
13	事件声明声明用户定义的事件。	Public Event Finished()
14	委托声明用于声明一个委托。	Delegate Function MathOperator( ByVal x As Double, ByVal y As Double ) As Double

可执行语句

可执行语句执行操作。调用过程，分支到代码中的另一个地方，循环使用几个语句或评估表达式的语句是可执行语句。赋值语句是可执行语句的一种特殊情况。

示例

以下示例演示了一个决策语句：

Module decisions
   Sub Main()
      'local variable definition '
      Dim a As Integer = 10

      ' check the boolean condition using if statement '
      If (a < 20) Then
         ' if condition is true then print the following '
         Console.WriteLine("a is less than 20")
      End If
      Console.WriteLine("value of a is : {0}", a)
      Console.ReadLine()
   End Sub
End Module

当上述代码被编译和执行时，它产生了以下结果：

a is less than 20;
value of a is : 10

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