[AdamSpeight2008]

Enums

A Enum is simply a type-safe constant.

Public Enum CardSuit
  Spades
  Clubs
  Diamonds
  Hearts
End Enum

Public Enum CardFace
  Two
  Three
  Four
  Five
  Six
  Seven
  Eight
  Nine
  Ten
  Jack
  Queen
  King
  Ace
End Enum

Let's define a simple structure that utilises the two enums (CardSuit & CardFace).

Spoiler

Public Structure PlayingCard

  Private _Face As CardFace
  Private _Suit As CardSuit

  Public ReadOnly Property Face() As CardFace
    Get
      Return _Face
    End Get
  End Property

  Public ReadOnly Property Suit() As CardSuit
    Get
      Return _Suit
    End Get
  End Property

  Public Overrides Function ToString() As String
    Return String.Format("{0} of {1}", _Face, _Suit)
  End Function

  Public Shared Function CreateNew(ByVal Face As CardFace, ByVal Suit As CardSuit) As PlayingCard
    If Not [Enum].IsDefined(GetType(CardFace), Face) Then Throw New ArgumentOutOfRangeException
    If Not [Enum].IsDefined(GetType(CardSuit), Suit) Then Throw New ArgumentOutOfRangeException
    Return New PlayingCard With {._Face = Face, ._Suit = Suit}
  End Function
End Structure

The Good

The good thing about enums it makes understand your easier.

Dim Card_A = PlayingCard.CreateNew(CardFace.Ace, CardSuit.Spades)

You can easily till I'm a creating a new instance of the Playing Card Structure with the value Ace Of Spades.


]The Bad
This is valid.

  Dim Card_B = PlayingCard.CreateNew(CardSuit.Diamonds, CardFace.Two)

Which is the Four of Spades.
This because enums are that they are evaluated as the underlying type, which my case it is the default Integer.

This also explains the need for the follow section of code in CreateNew

 If Not [Enum].IsDefined(GetType(CardFace), Face) Then Throw New ArgumentOutOfRangeException
 If Not [Enum].IsDefined(GetType(CardSuit), Suit) Then Throw New ArgumentOutOfRangeException

If it was for this, it could be any valid integer.

What would be better if the enum type information was preserved and respected.
e.g. I defined that it should an CardFace and not an Integer.


[b]Edit:{/b}

Forgot to mention you can define an Enum which is treated like a sets of bits.

<Flags>
Enum 
 LowBit
 MiddleBit
 OtherBit
 HighBit
End Enum

Example

Dim myFlag = LowBit + HighBit + OtherBit

Note: A word of warning though. Don't change the ordering if you save a value containing an enum as it will change its meaning.

This post has been edited by AdamSpeight2008: 08 November 2010 - 01:00 PM

[AdamSpeight2008]

Joe Enos, on 08 November 2010 - 06:47 PM, said:

I believe

Option Strict On

will prevent your code from compiling if you try to pass the wrong type in.

Not always true.

  Dim Card_c = PlayingCard.CreateNew(0, 0)

Is allowed, no type casting.

[jtenos]

Good point - funny, I never realized that you can do this in C# either without explicit casting until I just tried it. I guess I've never had a scenario where I wanted to pass a regular integer into a method expecting an enum - just seems like a bad idea most of the time. Maybe it would work in the data layer to store an integer in the DB while treating it as an enum in code - but I think I'd still put the explicit casting in the code just to keep it readable.

But at least "Option Strict On" or C# will still prevent you from accidentally passing in the wrong enum. Definitely better than nothing.

AdamSpeight2008, on 08 November 2010 - 11:40 AM, said:

Joe Enos, on 08 November 2010 - 06:47 PM, said:

I believe

Option Strict On

will prevent your code from compiling if you try to pass the wrong type in.

Not always true.

  Dim Card_c = PlayingCard.CreateNew(0, 0)

Is allowed, no type casting.

[mangel]

Couldn't you create a card structure having two properties typed using the two enums. Then a deck of cards would be created
with generics as below.

Dim CardDeck as List of Type Card

The card deck will be type safe. Then the CardDeck's Add method would ensure type safety.