VB.NET中0 < CInt(Num(x)) < 10比较语句失效问题求助

核心错误原因 #

你写的If 0 < CInt(Num(x)) < 10这种连续比较写法，VB是不支持的。它会被拆解成两步计算：

1. 先计算0 < CInt(Num(x))，不管输入的是90还是9000000，这个表达式都会返回True
2. 再把第一步的结果（True）和10比较，VB里布尔值True会被转换成数值-1，-1 < 10显然成立，所以整个条件永远返回True

修复方案 #

把连续比较拆成两个独立条件，用And连接：

If CInt(Num(x)) > 0 And CInt(Num(x)) < 10 Then

额外的代码优化建议 #

1. 变量声明问题：Dim Num(2), INPUT As String中，只有INPUT是String类型，Num数组默认是Variant类型，建议明确声明类型：

   Dim Num(2) As String, INPUT As String
   

2. 输入方式混用：代码里同时用了MsgBox和Console.ReadLine，建议统一成一种，比如都用控制台输入输出：

   Console.WriteLine("Please enter a number")
   Num(x) = Console.ReadLine()
   

3. 无限循环问题：外层While Cond = False循环里，Cond始终是False，会导致程序一直重复弹出"All three numbers are valid"，可以在循环内设置Cond = True来终止循环，或者直接去掉外层循环。

修复后的完整代码示例 #

Dim Num(2) As String, INPUT As String
Dim Cond As Boolean = False
Dim Valid As Boolean = False
Dim Isnum As Boolean
Dim x, Number, IsTen As Integer

While Cond = False
    Do Until IsTen = 3 And Number = 3
        IsTen = 0
        Number = 0
        For x = 0 To 2
            Console.WriteLine("Please enter a number")
            Num(x) = Console.ReadLine()
            Isnum = IsNumeric(Num(x))
            If Isnum = True Then
                IsTen = IsTen + 1
            End If
            If Isnum = True Then
                ' 修复后的条件判断
                If CInt(Num(x)) > 0 And CInt(Num(x)) < 10 Then
                    Number = Number + 1
                End If
            End If
        Next
        If Number <> 3 Then
            Console.WriteLine("Sorry all three numbers werent between 0 and 10, Please enter again")
        End If
        If IsTen <> 3 Then
            Console.WriteLine("Sorry all three inputs werent numbers, Please enter again")
        End If
    Loop
    Console.WriteLine("All three numbers are valid")
    ' 添加终止循环的条件
    Cond = True
End While

内容的提问来源于stack exchange，提问作者Noe Topalian