What is Enum.HasFlag supposed to return?

Question

I have the enum:

[Flags]
enum Editions
{
    Educational,
    Basic,
    Pro,
    Ultra
}

why do I get this behavior?

var x = Editions.Basic;
var y = Editions.Educational;
var test =x.HasFlag(y); // why is this true!?
// and!!!
var test2 = y.HasFlag(x); // this is false!

Answer 1

When using the [Flags] attribute you should explicitly map the enum values to integer that contain non overlapping bit patterns. That is, each enum value should be mapped to a power of two:

[Flags]
enum Editions
{
    Educational = 1,
    Basic = 2,
    Pro = 4,
    Ultra = 8
}

Without the explicit numbering, Educational will be mapped to 0 and Basic to 1.

Enum.HasFlags checks if the bit field or bit fields that are set in the parameter are also all set in the tested enum. In your case, x is 1 and y is 0. That means that x contains all of the bits set in 0 (that is no bits at all). However 0 does not contain the bits set in 1 when testing the other way around.

Answer 2

It returns true because you implemented Editions incorrectly. You must explicitly set the values when you use the [Flags] attribute.

[Flags]
enum Editions
{                         //binary format
    Educational = 1 << 0, // 0001
    Basic  = 1 << 1,      // 0010
    Pro  = 1 << 2,        // 0100
    Ultra  = 1 << 3       // 1000
}

If you do not assign numbers it will auto assign them starting at 0 going up by 1 for each option after that (this is standard enum behavior, adding the [Flags] attribute does not change this behavior)

[Flags]
enum Editions
{                    //binary format
    Educational = 0, // 0000
    Basic  = 1,      // 0001
    Pro  = 2,        // 0010
    Ultra  = 3       // 0011
}

So the two tests you did was: 0001 has the flag 0000 set (which is true, you are not testing for any flags) and 0000 has the flag 0001 set (which is defiantly false)

---

However looking at your names I doubt you should be using [Flags] at all because I doubt whatever you are doing can be Basic and be Pro at the same time. If you are trying to use flags to make testing enum values easier just use a switch statement instead

Editions edition = GetEdition();

switch(edition)
{
    case Editions.Basic:
         DoSomthingSpecialForBasic();
         break;

    case Editions.Pro:
    case Editions.Ultra:
        DoSomthingSpecialForProAndUltra();
        break;

    //Does not do anything if Editions.Educational
}