VB.Net 流程控制

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