Enum with string index or alternative - vb.net

Question

Is it possible to return a value from enum with a string index? For example I can use:
Enum test
firstval
secondval
thirdval
End Enum
Dim index As Integer = 1
CType(index, test).ToString()
to return firstval but is there a way to do something similar where index is a string value? For example:
Enum test
firstval = "one"
secondval = "two"
thirdval = "three"
End Enum
Dim index As string = "one"
CType(index, test).ToString()

Answer 1

It's not possible using an Enum, but you could easily create a type that can do what you want, using the Narrowing operator.
simple example:
Class Test
Private Shared _lookup As Dictionary(Of String, Test)
Private Key As String
Private Name As String
Public Shared ReadOnly firstval As Test = New Test("one", "firstval")
Public Shared ReadOnly secondval As Test = New Test("two", "secondval")
Public Shared ReadOnly thirdval As Test = New Test("three", "thirdval")
Private Sub New(key As String, name As String)
Me.Key = key
Me.Name = name
If _lookup Is Nothing Then _
_lookup = New Dictionary(Of String, Test)
_lookup.Add(key, Me)
End Sub
Public Overrides Function ToString() As String
Return Me.Name ' or whatever you want '
End Function
Public Shared Widening Operator CType(obj As Test) As String
Return obj.Key
End Operator
Public Shared Narrowing Operator CType(key As String) As Test
Return _lookup(key)
End Operator
End Class
usage:
Dim index As string = "one"
' returns firstval '
CType(index, Test).ToString()

Answer 2

There are several other alternatives.
One is to get the names used in the enum. For instance:
Friend Enum ImgFormat
Bitmap
GIF
JPeg
TIFF
PNG
End Enum
Dim ImgNames() As String
...
ImgNames = [Enum].GetNames(GetType(ImgFormat))
If your names are not friendly enough, decorate them with Descriptions:
Imports System.ComponentModel
Friend Enum ImgFormat
<Description("Bitmap (BMP)")> Bitmap
<Description("Graphic Interchange (GIF)")> GIF
<Description("Jpg/JPeg (JPG)")> JPeg
<Description("Tagged Image (TIFF)")> TIFF
<Description("Portable Graphics (PNG)")> PNG
End Enum
To get the descriptions, requires reflection which gets involved:
Imports System.Reflection
Imports System.ComponentModel
Public Class EnumConverter
' gets a single enum description
Public Shared Function GetEnumDescription(ByVal EnumConstant As [Enum]) As String
Dim fi As FieldInfo = EnumConstant.GetType().GetField(EnumConstant.ToString())
Dim attr() As DescriptionAttribute = _
DirectCast( _
fi.GetCustomAttributes(GetType(DescriptionAttribute), False), _
DescriptionAttribute() )
If attr.Length > 0 Then
Return attr(0).Description
Else
Return EnumConstant.ToString()
End If
End Function
' get all the enum descriptions:
Public Shared Function GetEnumDescriptions(ByVal type As Type) As String()
Dim n As Integer = 0
Dim enumValues As Array = [Enum].GetValues(type)
Dim Descr(enumValues.Length - 1) As String
For Each value As [Enum] In enumValues
Descr(n) = GetEnumDescription(value)
n += 1
Next
Return Descr
End Function
End Class
To use:
Dim ImgNames() As String = EnumConverter.GetEnumDescriptions(ImgFormat)
ImgNames(ImgFormat.GIF) would be 'Graphic Interchange (GIF)'
This will break if the Enum values are not the default 0, 1, 2 ... IF that is an issue (and it really is), then build a class around it to store the Name or Description with the Enum Value. Rather than building a class to create a pseudo enum, make one to create a list of name-value pairs consisting of the Descriptions and Enum Value.