面试题：Visual Basic 可空类型在方法参数及返回值中的应用

Question

编写一个 Visual Basic 方法 `Calculate`，它接受两个可空的双精度浮点数作为参数。如果其中任何一个参数为 `Nothing`，则返回 `Nothing`；否则返回两个数的乘积。在主程序中调用该方法，传入不同的可空值进行测试，并处理返回的可空结果，展示如何确保程序不会因空值而引发异常。

Answer 1

Module Module1
    Function Calculate(ByVal num1 As Double?, ByVal num2 As Double?) As Double?
        If num1 Is Nothing OrElse num2 Is Nothing Then
            Return Nothing
        Else
            Return num1.Value * num2.Value
        End If
    End Function

    Sub Main()
        '测试传入不同的可空值
        Dim result1 As Double? = Calculate(5.5, 3.2)
        Dim result2 As Double? = Calculate(Nothing, 3.2)
        Dim result3 As Double? = Calculate(5.5, Nothing)
        Dim result4 As Double? = Calculate(Nothing, Nothing)

        '处理返回的可空结果，确保程序不会因空值而引发异常
        If result1.HasValue Then
            Console.WriteLine("结果1: " & result1.Value)
        Else
            Console.WriteLine("结果1为Nothing")
        End If

        If result2.HasValue Then
            Console.WriteLine("结果2: " & result2.Value)
        Else
            Console.WriteLine("结果2为Nothing")
        End If

        If result3.HasValue Then
            Console.WriteLine("结果3: " & result3.Value)
        Else
            Console.WriteLine("结果3为Nothing")
        End If

        If result4.HasValue Then
            Console.WriteLine("结果4: " & result4.Value)
        Else
            Console.WriteLine("结果4为Nothing")
        End If

        Console.ReadLine()
    End Sub
End Module