我如何确定一个 Enum 值是否有一个或多个与之比较的值？

Question

I've got an Enum marked with the [Flags] attribute as follows:

[Flags]
public enum Tag : int
{
    None = 0,
    PrimaryNav = 1,
    HideChildPages = 2,
    HomePage = 4,
    FooterLink = 8
}

On sitemapnodes in my sitemap I store the int value for the tags combination as an attribute.

What I need to do is check if a node has any one of one or more tags, eg Tag.PrimaryNav | Tag.HomePage.

I'm struggling with the necessary boolean logic to determine if an Enum value has one or more of the values it's being compared with.

Apologies if this isn't clear. I can provide more information if necessary.

Answer 1

You can do that by combining values with | and checking via & .

To check if the value contains either of the tags:

if ((myValue & (Tag.PrimaryNav | Tag.HomePage)) != 0) { ... }

The | combines the enums you're testing (bitwise) and the & tests via bitwise masking -- if the result isn't zero, it has at least one of them set.

If you want to test whether it has both of them set, you can do that as well:

Tag desiredValue = Tag.PrimaryNav | Tag.HomePage;
if ((myValue & desiredValue) == desiredValue) { ... }

Here we're masking off anything we don't care about, and testing that the resulting value equals what we do care about (we can't use != 0 like before because that would match either value and here we're interested in both ).

Some links:

• The & Operator
• The | Operator

Answer 2

You can use the HasFlag Method to avoid the need for the boolean logic,

Tag Val = (Tag)9;

if (Val.HasFlag(Tag.PrimaryNav))
{
    Console.WriteLine("Primary Nav");
}

if(Val.HasFlag(Tag.HomePage))
{
    Console.WriteLine("Home Page");
}

Answer 3

For bitwise ( Flags ) enums, an "any of" test is != 0, so:

const Tag flagsToLookFor = Tag.PrimaryNav | Tag.HomePage;
if ((node.Tag & flagsToLookFor) != 0) {
    // has some cross-over with PrimaryNav or HomePage (and possibly others too) 
}

Answer 4

var someEnumValue = Tag.PrimaryNav | Tag.HomePage;

To test if the enum contains a given value:

if ((someEnumValue & Tag.PrimaryNav) == Tag.PrimaryNav)
{

}

Answer 5

var tag = Tag.HideChildPages | Tag.PrimaryNav;

If ((tag & Tag.PrimaryNav) == Tag.PrimaryNav) {
    // Tag.PrimaryNav set.
}

Answer 6

You could use Jon Skeet's Unconstrained Melody library:

var someEnumValue = Tag.PrimaryNav | Tag.HideChildPages;
someEnumValue.HasAny(Tag.PrimaryNav | Tag.HomePage); // Returns true

Answer 7

You can use this extension method on enum, for any type of enums:

public static bool IsSingle(this Enum value)
    {
        var items = Enum.GetValues(value.GetType());
        var counter = 0;
        foreach (var item in items)
        {
            if (value.HasFlag((Enum)item))
            {
                counter++;
            }
            if (counter > 1)
            {
                return false;
            }
        }
        return true;
    }

Answer 8

That's a classic Extension method:

    public static bool HasFlag(this Enum val, Enum t)
    {
            return (Convert.ToUInt64(val) & Convert.ToUInt64(t)) != 0;
    }