Enum Boxing and Equality

Question

Why does this return False

    public enum Directions { Up, Down, Left, Right }

    static void Main(string[] args)
    {
        bool matches = IsOneOf(Directions.Right, Directions.Left, Directions.Right);
        Console.WriteLine(matches);
        Console.Read();
    }

    public static bool IsOneOf(Enum self, params Enum[] values)
    {
        foreach (var value in values)
            if (self == value)
                return true;
        return false;
    }

while this returns True?

    public static bool IsOneOf(Enum self, params Enum[] values)
    {
        foreach (var value in values)
            if (self.Equals(value))
                return true;
        return false;
    }

score 27 · Accepted Answer · answered Mar 17 '09 at 00:56

27

Enum does not implement a == equality operator but it does override the Equals method.

Since it does not implement ==, the system performs a reference equality check. Note that System.Enum is a class not a structure. Hence, values such as Directions.Left are boxed. In this particular case, the boxed objects end up being separate objects, hence they fail a reference equality test.

The compiler understands == for concrete Enum types (such as Directions), but the compiler does not do this special processing against the System.Enum type.

answered Mar 17 '09 at 00:56

Jason Kresowaty

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Nice. "Boxing" isn't really a complete explanation, as yours is. – core Mar 17 '09 at 00:59
1

Thankyou for technical explanation! – Greg Mar 17 '09 at 01:01

score 4 · Answer 2 · answered Mar 17 '09 at 00:53

As JB says, boxing. You can see this by changing from Enum to Directions:

public static bool IsOneOf(Directions self, params Directions[] values)
{
    foreach (var value in values)
        if (self == value)
            return true;
    return false;
}

true is returned.

Enum Boxing and Equality

2 Answers2

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