Enum.IsDefined with flagged enums

Question

I'm currently reading the book C# 4.0 in a Nutshell, which by the way I think is an excellent book, even for advanced programmers to use as a good reference.

I was looking back on the chapters about the basics, and I came across a trick to tell if a certain value is defined in an Enum when using flagged enums.
The book states that using Enum.IsDefined doesn't work on flagged enums, and suggests a work-around like this :

static bool IsFlagDefined(Enum e)
{
    decimal d;
    return (!decimal.TryParse(e.ToString(), out d);
}

This should return true if a certain value is defined in an enum which is flagged.

Can someone please explain to me why this works ?

Thanks in advance :)

Answer 1

Basically, calling ToString on any enum value of a type declared with the [Flags] attribute will return something like this for any defined value:

SomeValue, SomeOtherValue

On the other hand, if the value is not defined within the enum type, then ToString will simply produce a string representation of that value's integer value, e.g.:

5

So what this means is that if you can parse the output of ToString as a number (not sure why the author chose decimal), it isn't defined within the type.

Here's an illustration:

[Flags]
enum SomeEnum
{
    SomeValue = 1,
    SomeOtherValue = 2,
    SomeFinalValue = 4
}

public class Program
{
    public static void Main()
    {
        // This is defined.
        SomeEnum x = SomeEnum.SomeOtherValue | SomeEnum.SomeFinalValue;

        Console.WriteLine(x);

        // This is not (no bitwise combination of 1, 2, and 4 will produce 8).
        x = (SomeEnum)8;

        Console.WriteLine(x);
    }
}

The output of the above program is:

SomeOtherValue, SomeFinalValue
8

So you can see how the suggested method works.