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2. 学习VB.Net
3. If...Then...Else statement

If...Then...Else statement

If语句后面可以跟一个可选的Else语句，该语句在布尔表达式为false时执行。

语法 (Syntax)

VB.Net中If ... Then ... Else语句的语法如下 -

If(boolean_expression)Then
   'statement(s) will execute if the Boolean expression is true 
Else
  'statement(s) will execute if the Boolean expression is false 
End If

如果布尔表达式的计算结果为true ，那么将执行if代码块，否则将执行代码块。

流程图 (Flow Diagram)

例子 (Example)

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

编译并执行上述代码时，会产生以下结果 -

a is not less than 20
value of a is : 100

If ... Else If ... Else Statement

If语句后面可以跟一个可选的Else if...Else语句，这对于使用单个If ... Else If语句测试各种条件非常有用。

当使用If ... Else If ... Else语句时，要记住几点。

• 一个If可以有零个或一个Else，它必须在Else If之后。

• 一个If可以有零到多个Else If并且它们必须在Else之前。

• 一旦Else成功，剩余的Else If或Else将不会被测试。

语法 (Syntax)

VB.Net中if ... else if语句的语法如下 -

If(boolean_expression 1)Then
   ' Executes when the boolean expression 1 is true 
ElseIf( boolean_expression 2)Then
   ' Executes when the boolean expression 2 is true 
ElseIf( boolean_expression 3)Then
   ' Executes when the boolean expression 3 is true 
Else 
   ' executes when the none of the above condition is true 
End If

例子 (Example)

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

编译并执行上述代码时，会产生以下结果 -

None of the values is matching
Exact value of a is: 100