FlagsAttribute what for?

Question

What is the difference between the code bellow

' no Flags'
Public Enum MyEnum
  Monday = 1
  Tuesday = 2
  Wednesday = 4
  Thursday = 8
End Enum

and

<Flags()> _ 
Public Enum MyEnum
  Monday = 1
  Tuesday = 2
  Wednesday = 4
  Thursday = 8
End Enum

I do the

Dim days As MyEnum = MyEnum.Monday Or MyEnum.Tuesday Or MyEnum.Wednesday 

If (days And MyEnum.Tuesday) = MyEnum.Tuesday Then
  Console.WriteLine("Tuesday OK") ' here'
Else
  Console.WriteLine("Tuesday NOK")
End If

If (days And MyEnum.Thursday ) = MyEnum.Thursday Then
  Console.WriteLine("Thursday OK")
Else
  Console.WriteLine("Thursday NOK") ' here'
End If

and obtain the same result in both cases(with or without FlagAttribute).

Answer 1

Basically, it tells the CLR that the values of the enum can be combined. Without this attribute, combining the values would result in an unknown value (but it would still be valid). With the attribute, the combination is correctly interpreted

Without the Flags attributes :

' Gives "Monday, Tuesday" '
Dim s As String = (MyEnum.Monday Or MyEnum.Tuesday).ToString() 

Without the Flags attributes :

' Gives "3" '
Dim s As String = (MyEnum.Monday Or MyEnum.Tuesday).ToString() 

Answer 2

See Thomas Levesque answer. For example you then can do:

switch (test.day)
{
    case MyEnum.Monday:
        {
            //something when its monday
        }
        break;
    case MyEnum.Tuesday:
        {
            //something when its tuesday
        }
        break;
    case MyEnum.Monday | MyEnum.Tuesday:
        {
            //something when its monday and tuesday (oh the irony)
        }
        break;
}       

Answer 3

Also it impacts ToString() output.

Answer 4

Serhio,

I can't answer with VB code but flags are very useful in certain circumstances. The example you have demonstrated isn't so good. Image you've got something more like:

enum Format
{
  Bold = 1,
  Italic = 2,
  Underlined = 4
}

Then you could specify the Format to be:

Format format = Format.Bold | Format.Italic;
// Then a check to see if the format is bold or italic should both pass.

Now it is both both and italic (equal to 3). You wouldn't be able to set this without the flags attribute though. It kind of prevents options being mutally exclusive. To do the above without flags you'd have to do:

enum Format
{
  Bold,
  BoldUnderlined,
  BoldItalic,
  BoldUnderlinedItalic,  
  Underlined,  
  Italic,
  ItalicUnderlined
}

No way near as nice.