Question

How does CLR determine which Color zero should be converted to?

internal static class Test
{
    private static void Main()
    {
        Console.WriteLine((Color)0);
    }

    private enum Color
    {
        Red,
        Green = Red
    }
}

Using this Color’s definition “Red” will be outputted.

If we use other definitions, results are really very interesting.

private enum Color
{
    Red,
    Green = Red,
    Blue = Red,
    Yellow = Red
}

The output is “Green”.

Another definition:

private enum Color
{
    Red,
    Green = Red,
    Blue,
    Yellow  = Red
}

The output is “Yellow”.

Answer 1

It just returns a Color value with an underlying value of 0. That’s the same value as Color.Red and Color.Green.

Fundamentally, your enum is broken in that Red and Green are the same value. You can’t distinguish between them at all.

Color red = Color.Red;
Color green = Color.Green;
Console.WriteLine(red == green); // True
Console.WriteLine(red.ToString() == green.ToString()); // True

I don’t know if there are any guarantees around whether ToString returns “Red” or “Green” – but if you get to the situation where that’s relevant, you should have described your enum differently.

EDIT: From the documentation:

If multiple enumeration members have the same underlying value and you attempt to retrieve the string representation of an enumeration member’s name based on its underlying value, your code should not make any assumptions about which name the method will return. For example, the following enumeration defines two members, Shade.Gray and Shade.Grey, that have the same underlying value.

…

The following method call attempts to retrieve the name of a member of the Shade enumeration whose underlying value is 1. The method can return either “Gray” or “Grey”, and your code should not make any assumptions about which string will be returned.