What happens if we declare [Flags] enum in order?

Question

I have came across this question: What does the [Flags] Enum Attribute mean in C#?

And one thing I have been wondering, using the accepted answer's example, what will happen if I declare:

[Flags]
public enum MyColors
{
    Yellow = 1,
    Green = 2,
    Red = 3,
    Blue = 4
}

Will the following steps in that example resulting an error? If no, how can I get to MyColors.Red?

Answer 1

The Flags attribute doesn't actually do anything to enum itself or affect how it's used in code. The one and only thing that the Flags attribute does affect is the output of the ToString method. The other purpose of the Flags attribute is to indicate to the developer how they should use the enum. Your example will not cause any error; it's just stupid code. The rules are very simple:

1. If only one value is supposed to be used at a time then don't apply the Flags attribute and use a singular name for the type.
2. If multiple values can be used together then apply the Flags attribute, use powers of 2 for the values and use a plural name for the type.

Answer 2

To elaborate of jdphenix's answer, remember that the [Flags] attribute doesn't actually do any magic. It just adds a purely cosmetic marker on the enum that lets other functions (by default, only an enum's ToString implementation) to treat the enum as a Flags enum.

Regardless of the attribute, an enum is still just a number, by default an Int32, with some syntactic sugar around it. So if you define your enum as you did, doing

MyColors newColor = MyColors.Yellow | MyColors.Green 

would just do a bitwise OR operation on 1 and 2. The magic that is bitwise enums isn't in the attribute, but in the fact that powers of 2 allows you to "switch" a certain bit on and off. Using ordinal numbers will switch off bits that are a part of the binary representation of more than one enum value, making the result of the bitwise operation meaningless.