VB.Net结构具有预定义字符串列表中的变量

Question

The title probably isn't very accurate from a developer slang standpoint, but this is what I'm trying to achieve:

I have a structure z with one variable x and upon creating an instance of the structure z, I want x to be constrained to a list of types of x... so zx = xType.1 or zx = xType.2 where xType 1 and 2 are strings and x is also a string when it comes down to it.

Dim a As z
a.x = xType.1
Print(a.x)      'outputs "abc" because xType.1 = "abc"

EDIT 1:

Structure Z
    Dim X as String
End Structure

Sub Main()
    Dim a As Z
    a.X = "abc"
    Print(a.X) 'outputs "abc"
End Sub

This would be the simplest way where I can assign aX any value... I want to achieve something like this:

Structure Z
    Dim X as ???
End Structure

Sub Main()
    Dim a As Z
    a.X = XType.abc
    Print(a.X) 'outputs "abc"
    a.X = XType.Zebra
    Print(a.X) 'outputs "Melons"
End Sub

So I have to make another structure or define those XTypes somewhere somehow.

Answer 1

I think what you may be wanting to use is an enum. You could do something like this:

Structure Z
    Public Enum XType
        abc
        bcd
    End Enum
    Dim X As XType
End Structure

Sub Main()
    Dim a As Z
    a.X = Z.XType.abc
End Sub

This way you could create predefined values for X, and it would be similar to how a MsgBox has different MsgBoxStyles you can choose from.

EDIT:

If you wanted to make it so that you didn't have to call ToString() to get the string value of X, you could try something like this (probably not the most effective, but it works):

Structure Z
    Public Enum XType
        abc
        bcd
    End Enum
    Private Xt As XType
    Public Property X As Object
        Get
            Return Xt.ToString()
        End Get
        Set(ByVal value As Object)
            If value.GetType().Name = "XType" Then Xt = value
        End Set
    End Property
End Structure

Or you could have two properties like this:

Public X as XType
Public Property XString as String
    Get
        Return X.ToString()
    End Get
End Property

This link may also be helpful: http://msdn.microsoft.com/en-us/library/essfb559(v=vs.90).aspx?cs-save-lang=1&cs-lang=vb#code-snippet-1