Question

What determines which name is selected when calling ToString() on an enum value which has multiple corresponding names?

Long explanation of question follows below.

I have determined that this not determined uniquely by any of: alphabetical order; declaration order; nor, name length.

For example, consider that I want to have an enum where the numeric values correspond directly to a practical use, (e.g. rgb values for color).

public enum RgbColor 
{
    Black   = 0x000000,
    Red     = 0xff0000,
    Green   = 0x00ff00,
    Blue    = 0x0000ff,
    White   = 0xffffff
}

Now, with this enum, calling default(RgbColor) will return the rgb value for black. Let’s say I don’t want the default value to be black, because I want UI designers to be able to use a call to “Default” when they don’t have specific instructions about what color to use. For now, the Default value for UI designers to use is actually “Blue”, but that could change. So, I add an additional TextDefault definition on the enum, and now it looks like:

public enum RgbColorWithTextDefaultFirst
{
    TextDefault = 0x0000ff,
    Black   = 0x000000,
    Red     = 0xff0000,
    Green   = 0x00ff00,
    Blue    = 0x0000ff,
    White   = 0xffffff
}

Now, I have tested this and I find that calling RgbColorWithTextDefaultFirst.TextDefault.ToString() and RgbColorWithTextDefaultFirst.Blue.ToString() both yield “Blue”. So, I figured that the name that is declared last will overwrite the name of the previous declarations. To test my assumption, I wrote:

public enum RgbColorWithTextDefaultLast
{
    Black   = 0x000000,
    Red     = 0xff0000,
    Green   = 0x00ff00,
    Blue    = 0x0000ff,
    White   = 0xffffff,
    TextDefault = 0x0000ff 
}

However, to my surprise, RgbColorWithTextDefaultLast.Blue.ToString() and RgbColorWithTextDefaultLast.TextDefault.ToString(). My next guess is that it sorts the names by alphabetical order and returns the first one. To test this I try:

public enum RgbColorWithATextDefaultFirst
{
    ATextDefault = 0x0000ff,
    Black   = 0x000000,
    Red     = 0xff0000,
    Green   = 0x00ff00,
    Blue    = 0x0000ff,
    White   = 0xffffff
}

public enum RgbColorWithATextDefaultLast
{
    Black   = 0x000000,
    Red     = 0xff0000,
    Green   = 0x00ff00,
    Blue    = 0x0000ff,
    White   = 0xffffff,
    ATextDefault = 0x0000ff
}

Now, for all four of RgbColorWithATextDefaultFirst.ATextDefault.ToString(), RgbColorWithATextDefaultFirst.Blue.ToString(), RgbColorWithATextDefaultLast.ATextDefault.ToString(), RgbColorWithATextDefaultLast.Blue.ToString(), I end up with “Blue”. I realize that there is another distinguishing factor, which is length of the string. My guess is now that the selected name is determined by the length of the name string. So, my test is to use these declarations:

public enum RgbColorWithALast
{
    Black   = 0x000000,
    Red     = 0xff0000,
    Green   = 0x00ff00,
    Blue    = 0x0000ff,
    White   = 0xffffff,
    A = 0x0000ff
}

public enum RgbColorWithAFirst
{
    A = 0x0000ff,
    Black   = 0x000000,
    Red     = 0xff0000,
    Green   = 0x00ff00,
    Blue    = 0x0000ff,
    White   = 0xffffff
}

Now, guess what value I got for all of: RgbColorWithAFirst.A.ToString(); RgbColorWithAFirst.Blue.ToString(); RgbColorWithALast.A.ToString(), RgbColorWithALast.Blue.ToString(). That’s right, “Blue”.

At this point, I’ve given up on trying to figure out what determines this by guessing. I opened up reflector, and I’m going to take a look and try to figure this out, but I figured I would ask a question here to see if anyone here already knows the answer, which is, again: What determines which name is selected when calling ToString() on an enum value which has multiple corresponding names?

Answer 1

I might be going too far here, but I think it’s decided by binary search of the sorted values, and so can depend on the parity of the total number of values. You can illustrate this with your last example (RgbColorWithAFirst and RgbColorWithALast) by defining another value in both – then you get A from all the ToString invocations.

I got here by decompiling mscorlib (4.0) and noting that eventually we get to a call to Array.BinarySearch on a sorted array of the declared values. Naturally, the binary search stops as soon as it gets a match, so to get it to switch between two identical values the easiest way is to alter the search tree, by adding an extra value.

Of course, this is an implementation detail and should not be relied on. It seems to me that in your case you would be best served by using DescriptionAttribute on enum values where you want to be explicit about the display value, and a helper method such as:

public static class EnumExtensions
{
    public static string Description(this Enum value)
    {
        var field = value.GetType().GetField(value.ToString());
        var attribute = Attribute.GetCustomAttribute(
                            field, 
                            typeof (DescriptionAttribute)) 
                        as DescriptionAttribute;

        return attribute == null ? value.ToString() : attribute.Description;
    }
}