From time to time I see an enum like the following:
[Flags]
public enum Options
{
None = 0,
Option1 = 1,
Option2 = 2,
Option3 = 4,
Option4 = 8
}
I don't understand what exactly the [Flags]
attribute does.
Anyone have a good explanation or example they could post?
Best Answer
The
[Flags]
attribute should be used whenever the enumerable represents a collection of possible values, rather than a single value. Such collections are often used with bitwise operators, for example:Note that the
[Flags]
attribute doesn't enable this by itself - all it does is allow a nice representation by the.ToString()
method:It is also important to note that
[Flags]
does not automatically make the enum values powers of two. If you omit the numeric values, the enum will not work as one might expect in bitwise operations, because by default the values start with 0 and increment.Incorrect declaration:
The values, if declared this way, will be Yellow = 0, Green = 1, Red = 2, Blue = 3. This will render it useless as flags.
Here's an example of a correct declaration:
To retrieve the distinct values in your property, one can do this:
or prior to .NET 4:
Under the covers
This works because you used powers of two in your enumeration. Under the covers, your enumeration values look like this in binary ones and zeros:
Similarly, after you've set your property AllowedColors to Red, Green and Blue using the binary bitwise OR
|
operator, AllowedColors looks like this:So when you retrieve the value you are actually performing bitwise AND
&
on the values:The None = 0 value
And regarding the use of
0
in your enumeration, quoting from MSDN:You can find more info about the flags attribute and its usage at msdn and designing flags at msdn