我可以在VB.NET中给枚举一个属性（就像我可以用Java做的那样）吗？

Question

In Java I can do something like this:

enum Country {
    IRELAND("Europe"),
    FRANCE("Europe"),
    NIGERIA("Africa"),
    THAILAND("Asia");

    private String continent;

    Country(String continent) {
        this.continent = continent;
    }

    public String getContinent() {
        return continent;
    }
}

which allows me to do something like:

Country country1 = getCountryFromSomewhere();
Country country2 = Country.FRANCE;
System.out.print("country1 is in " + country1.getContinent());
System.out.print("country2 is in " + country2.getContinent());

Is it possible to do the same thing in VB.NET ie add the continent attribute to the country enum?

Answer 1

(Apologies for using C# throughout - I believe the concepts are more about .NET than the language you happen to use; hopefully you're better at reading C# than I am at writing VB.)

Not directly - enums in .NET are just integer types with names for some of the values.

The closest you can come in .NET is to create a type with a fixed set of values. For example, in your case:

public sealed class Country
{
    public static readonly Country Ireland = new Country("Europe");
    public static readonly Country France = new Country("Europe");
    public static readonly Country Nigeria = new Country("Africa");
    public static readonly Country Thailand = new Country("Asia");

    private readonly string continent;

    public string Continent { get { return continent; } }

    private Country(string continent)
    {
        this.continent = continent;
    }
}

(I assume the VB.NET would be very similar.)

Note that this doesn't let you switch on the enum values.

If you want polymorphism, you can create nested subclasses which can still call the private constructor, which prevents any other subclasses being created.

One alternative to this is to use attributes on normal enums:

[AttributeUsageAttribute(AttributeTargets.Field)]
public class ContinentAttribute : Attribute
{
    // etc
}

public enum Country
{
    [Continent("Europe")] Ireland = 1,
    [Continent("Europe")] France = 2,
    ...
}

You'd then need to use reflection to get at the ContinentAttribute and retrieve the string.

Note that here there isn't really a fixed set of values - you could write:

Country country = (Country) 15;

At that point you can't get the continent for it, and if you pass it to any methods which expect it to be a real country, you've got problems. That isn't the case with the earlier solution, where you really are restricted to those few values (and null).

Answer 2

Here is the code:

Imports System.ComponentModel

Imports System.Reflection

Public Enum enumOrderStatus
    <Description("None")>
    None
    <Description("Sent")>
    Sent
    <Description("Accepted")>
    Accepted
    <Description("Cancelled")>
    Cancelled
    <Description("Declined")>
    Declined
End Enum


Public Function GetEnumDescription(ByVal EnumConstant As [Enum]) As String
    Dim fi As FieldInfo = EnumConstant.GetType().GetField(EnumConstant.ToString())
    Dim aattr() As DescriptionAttribute = DirectCast(fi.GetCustomAttributes(GetType(DescriptionAttribute), False), DescriptionAttribute())
    If aattr.Length > 0 Then
        Return aattr(0).Description
    Else
        Return EnumConstant.ToString()
    End If
End Function

Answer 3

I used this solution instead:

Declare enum:

Private Enum Country
    IRELAND
    FRANCE
    THAILAND
End Enum

Declare and initialise Dictionary (aka a map):

Dim countryContinentMap As IDictionary(Of Country, String) = New Dictionary(Of Country, String)
countryContinentMap.add(Country.IRELAND, "Europe")
countryContinentMap.add(Country.FRANCE, "Europe")
countryContinentMap.add(Country.THAILAND, "Asia")

which allows me to get the continent like this:

Dim franceContinent As String = countryContinentMap(Country.FRANCE)

Answer 4

Here is how I solved this in my application. Still looking for something even easier.

What do you think about it?

Public Sub Init()
    Dim values() As Integer = CType([Enum].GetValues(GetType(MyEnum)), Integer())
    For i As Integer = 0 To values.Count - 1
        Me.contextMenuInGUI.Items.Add(Me.GetEnumDescription(i))
    Next
End Sub

Private Function GetEnumDescription(ByVal i As Integer) As String
    Select Case i
        Case MyEnum.Comment
            Return "Description for Comment"

        Case MyEnum.SomeEnumValueInCamelCase
            Return "Value without camel case (€)(%)(@)"
    End Select
    Return "Add a case in Class:GetEnumDescription"
End Function

Answer 5

Create an extension method for your Enum

Usage example:

dim description = TableTag.Important.GetDescription()

Definition example:

Imports System.ComponentModel
Imports System.Reflection
Imports System.Runtime.CompilerServices

Namespace Foo

  Public Enum TableTag

    <Description("Identifies tables that should be availible for writing as table or view to the model database")>
    Important

    <Description("Example for a table group that helps to select disctinct tables")>
    CustomGroup

  End Enum

  Public Module TableTagExtensions

    <Extension>
    Public Function GetDescription(enumValue As TableTag) As String

      Dim fieldInfo As FieldInfo = enumValue.GetType().GetField(enumValue.ToString())
      Dim attributes = DirectCast(fieldInfo.GetCustomAttributes(GetType(DescriptionAttribute), False), DescriptionAttribute())

      If attributes.Length > 0 Then
        Return attributes(0).Description
      Else
        Return enumValue.ToString()
      End If

    End Function

  End Module

End Namespace