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()