How do I test if an enum is contained in a number? - vb.net

Question

In VB6 I combine several DT_DRAW_FLAG values like this:
dim lMyFlags As DT_DRAW_FLAG
lMyFlags = DT_CENTER OR DT_VCENTER OR DT_RTLREADING
This would result in lMyFlags = 131077
Now to test if a certain flag is contained in such a combine flags value, I would do the following:
If (131077 And DT_RTLREADING) = DT_RTLREADING Then
'DT_RTLREADING is contained.
Else
'DT_RTLREADING is NOT contained.
End Enum
How would I do this in VB.NET?
Do I still have to use this "pure math" approach, or is there a method like...
lMyFlags.ContainsEnum(DT_RTLREADING)
... which I have not found yet?
Thank you!

Answer 1

If you have an enum declaration like this
<Flags()> Public Enum DT_DRAW_FLAG As Integer
DT_CENTER = 1
DT_VCENTER = 2
DT_RTLREADING = 4
End Enum
Then you can use HasFlag to do your logic
Dim lMyFlags As DT_DRAW_FLAG = DT_DRAW_FLAG.DT_CENTER Or DT_DRAW_FLAG.DT_RTLREADING
lMyFlags.HasFlag(DT_DRAW_FLAG.DT_RTLREADING) ' => true
The important point here is that the single enum values are powers of two.

Answer 2

Accepted answer is good, it works. In your case, your Enum might already have the flags attribute, because it is a combination of three powers of 2: 2^0=1, 2^2=4, and 2^17=131072
Your enum may look like this
<Flags>
Public Enum DT_DRAW_FLAG As Long
''' <summary>
''' 2 ^ 0
''' </summary>
DT_CENTER = 1
' 2 ^ 1 in here
''' <summary>
''' 2 ^ 4
''' </summary>
DT_VCENTER = 4
' 2 ^ 3 through 2 ^ 16 in here
''' <summary>
''' 2 ^ 17
''' </summary>
DT_RTLREADING = 131072
End Enum
The Flags Attribute
Indicates that an enumeration can be treated as a bit field; that is, a set of flags.
However, whether or not it has the Flags attribute, you can treat it the same way using bitwise And. I believe the HasFlags function is just shorthand for the bitwise logic:
' Bitwise logic
If (lMyFlags And DT_DRAW_FLAG.DT_RTLREADING) = DT_DRAW_FLAG.DT_RTLREADING Then
' Reading is contained
Else
' Reading is not contained
End If
' HasFlags
If lMyFlags.HasFlag(DT_DRAW_FLAG.DT_RTLREADING) Then
' Reading is contained
Else
' Reading is not contained
End If
It is certainly less code.
Note, you don't combine enums in the way that you have shown, without some additional conversion. Use bitwise Or to do that
Dim lMyFlags = DT_DRAW_FLAG.DT_CENTER Or DT_DRAW_FLAG.DT_VCENTER Or DT_DRAW_FLAG.DT_RTLREADING
Also, you can use Enum.HasFlag regardless of whether you used the Flags Attribute. As far as I know, the attribute is just used to signal to the consumer that the values are distinct powers of two, and bitwise logic can be performed on them. There is nothing strictly going on with the flags attribute so there's some trust in the consumer to know what to do with it (and we assume the original author knew about it, too)