Visual Basic 无效转换运行时错误答案

【问题标题】：Visual Basic invalid cast runtime errorVisual Basic 无效转换运行时错误
【发布时间】：2015-01-24 08:29:16
【问题描述】：

我是 Visual Basic 的新手，现在正在做作业。这是我的代码

Sub Main()
    Console.Write("Please let me know your nickname: ")
    Dim name As String = Console.ReadLine()
    Console.WriteLine("Thank you " + name + "!")
    Console.WriteLine()
    Console.Write("How many litres " + name + "<only whole litres please>? ")
    Dim litres As Integer = Console.ReadLine()
    Console.Write("Premium quality? <y/n>: ")
    Dim ans As Char = Console.ReadLine()
    Dim prem As Boolean
    If ans = "y" Then
        prem = True
    ElseIf ans = "n" Then
        prem = False
    End If

    Console.WriteLine()
    Console.WriteLine("WELCOME TO APU'S GAS STATION")
    Console.Write("Quality:     ")
    If prem = True Then
        Console.Write("Premium")
    Else : Console.Write("Regular")
    End If

    Dim price As Double = 12.44
    Console.WriteLine("Quantity <l>:    " + litres)
    Console.WriteLine("Price per l:     " + price)
    Console.WriteLine("Sum to pay: " + litres * price)

    Console.ReadLine()
End Sub

在运行时，我可以毫无问题地输入所有数据，但随后我收到一个错误，我可以粗略地将其翻译为“字符串“数量：”的无效转换以键入“双”。我不太清楚发生了什么，希望指点。

【问题讨论】：

将选项严格设置为on。使用& 进行字符串连接。问题解决了。
您不能将双精度加到字符串中，这是类型不匹配的，您可以使用 & 将双精度链接到字符串，只需将所有 + 替换为 &

标签： vb.net

【解决方案1】：

为了避免这个问题，你可以使用

Console.WriteLine("Quantity <l>:    " + litres.ToString)

或者你可以使用

Console.WriteLine("Quantity <l>:    " + CStr(price))

【讨论】：

【解决方案2】：

我修复了你的代码，这应该可以工作：

 Sub Main()
    Console.Write("Please let me know your nickname: ")
    Dim name As String = Console.ReadLine()
    Console.WriteLine("Thank you " & name & "!")
    Console.WriteLine()
    Console.Write("How many litres " & name & "<only whole litres please>? ")
    Dim litres As Integer = Console.ReadLine()
    Console.Write("Premium quality? <y/n>: ")
    Dim ans As Char = Console.ReadLine()
    Dim prem As Boolean
    If ans = "y" Then
        prem = True
    ElseIf ans = "n" Then
        prem = False
    End If

    Console.WriteLine()
    Console.WriteLine("WELCOME TO APU'S GAS STATION")
    Console.Write("Quality:     ")
    If prem = True Then
        Console.Write("Premium")
    Else : Console.Write("Regular")
    End If

    Dim price As Double = 12.44
    Console.WriteLine("Quantity <l>:    " & litres)
    Console.WriteLine("Price per l:     " & price)
    Console.WriteLine("Sum to pay: " & (litres * price))
    Console.ReadLine()
End Sub

【讨论】：