Question

    Private Const TOKEN_LENGTH As Integer = 8 ' this may come from app.config at startup
    Private Const TOKEN_MIN As Integer = 10 ^ (TOKEN_LENGTH - 1)
    Private Const TOKEN_MAX As Integer = 10 ^ TOKEN_LENGTH - 1

    'how do I make TOKEN_FORMAT a CONST?
    Private Const TOKEN_FORMAT As String = "0".PadRight(TOKEN_LENGTH)

    'sample usage
    Dim TokenCode As String = New Random().Next(TOKEN_MIN, TOKEN_MAX).ToString(TOKEN_FORMAT)

以下代码给出了此错误：需要常量表达式。

Private Const TOKEN_FORMAT As String = "0".PadRight(TOKEN_LENGTH)

一旦定义，TOKEN_FORMAT将永远不会改变，其定义仅取决于另一个常数TOKEN_LENGTH。那么为什么它也不能被编译时评估呢？

感谢。

Answer 1

你想要太花哨了。这些是常量，因此给它们保持不变的值。你的TOKEN_LENGTH常数毫无意义。

Const TOKEN_FORMAT As String = "00000000"
Const TOKEN_MIN As Integer = 10000000
Const TOKEN_MAX As Integer = 99999999

这就是你所需要的一切。

Answer 2

你需要什么方式更好？没有setter的财产怎么样，例如：

Public Class ConstantTest
    Private Const TOKEN_LENGTH As Integer = 6
    Private ReadOnly Property TOKEN_FORMAT() As String
        Get
            Return "0".PadRight(TOKEN_LENGTH, "0"c)
        End Get
    End Property
    Private ReadOnly ANOTHER_TOKEN_FORMAT As String = "0".PadRight(TOKEN_LENGTH, "0"c)

    Public Sub New()
        'you can change readonly field in constructor...'
        ANOTHER_TOKEN_FORMAT = "test"
        'but setting "get-only" property is not allowed even in constructor'
        TOKEN_FORMAT = "test"   '<- compile error here'
    End Sub
End Class

您可以在构造函数中更改readonly字段的值，使其与Const不太相似，但是相同的技巧不能应用于没有setter的属性，如上例所示。

相关讨论：const vs. readonly，What is the difference between const and readonly?

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