Is flags attribute necessary?

Question

I found with or without flags attributes, I can do bit operation if I defined the following enum

enum TestType
{
    None = 0x0,
    Type1 = 0x1,
    Type2 = 0x2
}

I am wondering why we need flags attribute?

Answer 1

C# will treat them the same either way, but C# isn't the only consumer:

• PropertyGrid will render it differently to allow combinations
• XmlSerializer will accept / reject delimited combinations based on this flag
• Enum.Parse likewise (from string), and the enum's .ToString() will behave differently
• lots of other code that displays or processes the value will treat them differently

More importantly, though, it is an expression of intent to other developers (and code); this is meant to be treated as combinations, not exclusive values.

Answer 2

Sometimes bit combinations of enum values are meaningful (like FileAccess - read, write, read+write), sometimes they are not (usually). So [Flags] is descriptive way to store in metadata information that bit operations are meaningful on this enum type. There are several consumers of this attribute, for example ToString of that enum.