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The question What does the [Flags] Enum Attribute mean in C#? doesn't ask what I'm asking and also doesn't answer it. If there is such answer in that question, please, state which one (link) in the comments rather than mindlessly and erroneously flagging as a duplicate.

There is this note on MSDN's HasFlag documentation:

The HasFlag method is designed to be used with enumeration types that are marked with the FlagsAttribute attribute and can be used to determine whether multiple bit fields are set. For enumeration types that are not marked with the FlagsAttribute attribute, call either the Equals method or the CompareTo method.

But I've tested it and the method worked fine despite the enum being marked or not with the FlagsAttribute.

Maybe the note on the docs is just an attempt to enforce some sort of *good practice* by using the attribute (and it doesn't matter in the end at all)?

I took a peek here and it indeed seems that there is no restriction on the enum whatsoever (it's just a simple bitwise AND):

public Boolean HasFlag(Enum flag) {
    if (!this.GetType().IsEquivalentTo(flag.GetType())) {
        throw new ArgumentException(Environment.GetResourceString("Argument_EnumTypeDoesNotMatch", flag.GetType(), this.GetType()));
    }

    ulong uFlag = ToUInt64(flag.GetValue());
    ulong uThis = ToUInt64(GetValue());
    return ((uThis & uFlag) == uFlag);
}

So, why on earth does it say that it's *designed* to work with the FlagsAttribute when in reality it doesn't rely on it?

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talles
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    The whole Flags idea is unsupported by constraints or validations. All that `[Flags]` seems to do is enable round-tripping with ToString and Parse. So: Yes, it's a best-practice idea. – H H Jan 07 '14 at 12:35
  • @HenkHolterman, it seems so; wanted to confirm here. Shame on the poor word choice ("designed"). – talles Jan 07 '14 at 12:39
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    HasFlag() is excessively expensive, at least an order of magnitude slower than comparing the value. The code you quoted isn't the way it works anymore in .NET 4.5, it was moved inside the CLR to make it a bit cheaper. Don't write unnecessarily slow code. – Hans Passant Jan 07 '14 at 13:03
  • Duplicated? For gods sake, that question **does not** answer what I'm questioning. – talles Jan 07 '14 at 13:08
  • It is *designed* to work with flags, but it can also work in other cases. I think this is pretty logical (do you disagree?). I do not really see a question here. Can you distill your question into one sentence? – usr Jan 10 '14 at 16:46
  • @usr, yes, of course: **Why MSDN state that HasFlag is \*designed\* to work with FlagsAttribute?**. Note the emphasis on *designed*. – talles Jan 10 '14 at 16:55
  • HasFlags only makes sense on Flags enums. You don't have to declare an enum as Flags but that is recommended. It is documentation, and it tuns on flags ToString formatting. MSDN recommends that you either create a flags enum, mark it Flags and use HasFlags, or do none of that. Do not mix. Is that a useful explanation to you? (This is an honest question). – usr Jan 10 '14 at 17:12
  • @usr yes, that would be a good **answer** to my **question**. So far the best answer is the HenkHolterman one, confirming what I've suspected (a poor choice of words to encourage the practice). Shame all that being done here in the comment section, rather than with proper answers – talles Jan 10 '14 at 17:31
  • I've been in your place. This question is just not a good fit for Stack Overflow because there's no definitive answer. When you answer lots of questions you tend to become aggressive with closing because there's so much crap coming in. You having been shut down is a false positive. Take it as feedback to ask more clearly next time. – usr Jan 10 '14 at 17:40

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HasFlags only makes sense on Flags enums. You don't have to declare an enum as Flags but that is recommended. It is documentation, and it tuns on flags ToString formatting.

MSDN recommends that you either create a flags enum, mark it Flags and use HasFlags, or do none of that. Do not mix.

That's all this sentence is trying to convey. It pushes you to best-practices. Ideally, there would be a link to a page explaining flags but that is not how MSDN is typically handled (alas). They mostly tell you what you can do, not what you should do.

usr
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