VB.Net 流程控制

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  决策

  决策结构要求程序员指定一个或多个要由程序评估或测试的条件，以及确定该条件为真的情况下要执行的一条或多条语句，以及指定该条件时要执行的其他语句（可选）确定为假。下面显示的是大多数编程语言中常见的典型决策结构的一般形式-

  

  VB.Net 提供以下类型的决策语句。

  声明	描述
  If ... Then 声明	一个 If ... Then 语句包含一个布尔表达式后跟一个或多个语句。
  If...Then...Else 声明	一个If...Then... 语句可以跟着一个可选的Else语句，当布尔表达式是假的，其执行。
  嵌套If 语句	您可以在另一个If或Else if语句中使用一个If或Else if语句。
  Select Case 语句	Select Case 语句允许一个变量来针对值的列表平等进行测试。
  嵌套的Select Case语句	您可以在另一个Select Case语句中使用 Select Case 语句。

  范例 - If...Then 声明：

  

  
  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
  

  尝试一下

  范例 - if...else声明：

  

  
  Module decisions
     Sub Main()
         'local variable definition '
        Dim a As Integer = 100
  
        ' 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")
        Else
            ' if condition is false then print the following 
            Console.WriteLine("a is not less than 20")
        End If
        Console.WriteLine("value of a is : {0}", a)
        Console.ReadLine()
     End Sub
  End Module
  

  尝试一下

  If...Else If...Else

  
  Module decisions
     Sub Main()
        'local variable definition '
        Dim a As Integer = 100
        ' check the boolean condition '
        If (a = 10) Then
            ' if condition is true then print the following '
            Console.WriteLine("Value of a is 10") '
        ElseIf (a = 20) Then
            'if else if condition is true '
            Console.WriteLine("Value of a is 20") '
        ElseIf (a = 30) Then
            'if else if condition is true  
            Console.WriteLine("Value of a is 30")
        Else
            'if none of the conditions is true 
            Console.WriteLine("None of the values is matching")
        End If
        Console.WriteLine("Exact value of a is: {0}", a)
        Console.ReadLine()
     End Sub
  End Module
  

  尝试一下

  范例 - 嵌套if语句：

  
  Module decisions
     Sub Main()
        'local variable definition
        Dim a As Integer = 100
        Dim b As Integer = 200
        ' check the boolean condition 
        
        If (a = 100) Then
           ' if condition is true then check the following 
           If (b = 200) Then
              ' if condition is true then print the following 
              Console.WriteLine("Value of a is 100 and b is 200")
           End If
        End If
        Console.WriteLine("Exact value of a is : {0}", a)
        Console.WriteLine("Exact value of b is : {0}", b)
        Console.ReadLine()
     End Sub
  End Module
  

  尝试一下

  范例 - Select Case 语句：

  

  
  Module decisions
     Sub Main()
        'local variable definition
        Dim grade As Char
        grade = "B"
        Select grade
            Case "A"
                Console.WriteLine("Excellent!")
            Case "B", "C"
                Console.WriteLine("Well done")
            Case "D"
                Console.WriteLine("You passed")
            Case "F"
                Console.WriteLine("Better try again")
            Case Else
                Console.WriteLine("Invalid grade")
        End Select
        Console.WriteLine("Your grade is  {0}", grade)
        Console.ReadLine()
     End Sub
  End Module
  

  尝试一下

  范例 - 嵌套的Select Case语句：

  
  Module decisions
     Sub Main()
        'local variable definition
        Dim a As Integer = 100
        Dim b As Integer = 200
        Select a
           Case 100
              Console.WriteLine("This is part of outer case ")
              Select Case b
                 Case 200
                    Console.WriteLine("This is part of inner case ")
              End Select
           End Select
        Console.WriteLine("Exact value of a is : {0}", a)
        Console.WriteLine("Exact value of b is : {0}", b)
        Console.ReadLine()
     End Sub
  End Module
  

  尝试一下