VB.Net - 語句

在 Visual Basic 程式中，語句是一個完整的指令。它可以包含關鍵字、運算子、變數、字面值、常量和表示式。

語句可以分為以下幾類：

宣告語句 - 這些語句用於命名變數、常量或過程，也可以指定資料型別。
可執行語句 - 這些語句啟動操作。這些語句可以呼叫方法或函式、迴圈或分支透過程式碼塊，或者將值或表示式賦值給變數或常量。在最後一種情況下，它被稱為賦值語句。

宣告語句

宣告語句用於命名和定義過程、變數、屬性、陣列和常量。宣告程式設計元素時，還可以定義其資料型別、訪問級別和作用域。

您可以宣告的程式設計元素包括變數、常量、列舉、類、結構、模組、介面、過程、過程引數、函式返回值、外部過程引用、運算子、屬性、事件和委託。

以下是 VB.Net 中的宣告語句：


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 

序號	語句和描述	示例
1	Dim 語句宣告並分配一個或多個變數的儲存空間。	Dim number As Integer Dim quantity As Integer = 100 Dim message As String = "Hello!"
2	Const 語句宣告並定義一個或多個常量。	Const maximum As Long = 1000 Const naturalLogBase As Object = CDec(2.7182818284)
3	Enum 語句宣告一個列舉並定義其成員的值。	Enum CoffeeMugSize Jumbo ExtraLarge Large Medium Small End Enum
4	Class 語句宣告類的名稱並介紹類包含的變數、屬性、事件和過程的定義。	Class Box Public length As Double Public breadth As Double Public height As Double End Class
5	Structure 語句宣告結構的名稱並介紹結構包含的變數、屬性、事件和過程的定義。	Structure Box Public length As Double Public breadth As Double Public height As Double End Structure
6	Module 語句宣告模組的名稱並介紹模組包含的變數、屬性、事件和過程的定義。	Public Module myModule Sub Main() Dim user As String = InputBox("What is your name?") MsgBox("User name is" & user) End Sub End Module
7	Interface 語句宣告介面的名稱並介紹介面包含的成員的定義。	Public Interface MyInterface Sub doSomething() End Interface
8	Function 語句宣告函式過程的名稱、引數和程式碼。	Function myFunction (ByVal n As Integer) As Double Return 5.87 * n End Function
9	Sub 語句宣告子過程的名稱、引數和程式碼。	Sub mySub(ByVal s As String) Return End Sub
10	Declare 語句宣告對外部檔案中實現的過程的引用。	Declare Function getUserName Lib "advapi32.dll" Alias "GetUserNameA" ( ByVal lpBuffer As String, ByRef nSize As Integer) As Integer
11	Operator 語句宣告運算子符號、運算元和定義類或結構上運算子過程的程式碼。	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	Property 語句宣告屬性的名稱以及用於儲存和檢索屬性值的屬性過程。	ReadOnly Property quote() As String Get Return quoteString End Get End Property
13	Event 語句宣告使用者定義的事件。	Public Event Finished()
14	Delegate 語句用於宣告委託。	Delegate Function MathOperator( ByVal x As Double, ByVal y As Double ) As Double

可執行語句

可執行語句執行操作。呼叫過程、分支到程式碼中的其他位置、迴圈遍歷多個語句或計算表示式的語句都是可執行語句。賦值語句是可執行語句的特例。

示例

以下示例演示了決策語句：

線上演示

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

編譯並執行以上程式碼後，將產生以下結果：

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

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