Casting ints to enums in C#

Question

There is something that I cannot understand in C#. You can cast an out-of-range int into an enum and the compiler does not flinch. Imagine this enum:

enum Colour
{
    Red = 1,
    Green = 2,
    Blue = 3
}

Now, if you write:

Colour eco;
eco = (Colour)17;

The compiler thinks that’s fine. And the runtime, too. Uh?

Why did the C# team decide to make this possible? This decision misses the point of using enums, I think, in scenarios like this:

void DoSomethingWithColour(Colour eco)
{
    //do something to eco.
}

In a strong-typed language like C#, I would like to assume that eco will always hold a legal Colour value. But this is not the case. A programmer could call my method with a value of 17 assigned to eco (as in previous code snippet), so the code in my method must not assume that eco holds a legal Colour value. I need to test for it explicitly and handle the exceptional values as I please. Why is this?

In my humble opinion, it would be much nicer if the compiler issued an error (or even a warning) message when casting an out-of range int into an enum, if the int value is known at compile time. If not, the runtime should throw an exception at the assignment statement.

What do you think? Is there any reason why this is so?

(Note. This is a question I posted ages ago on my blog but got no informative response.)

Answer 1

Guessing about 'why' is always dangerous, but consider this:

enum Direction { North =1, East = 2, South = 4, West = 8 }
Direction ne = Direction.North | Direction.East;

int value = (int) ne; // value == 3
string text = ne.ToString();  // text == "3"

If the [Flags] attribute is put in front of the enum, that last line changes to

string text = ne.ToString();  // text == "North, East"

Answer 2

When you define an enum you are essentially giving names to values (syntatic sugar if you will). When you cast 17 to Colour, you are storing a value for a Colour that has no name. As you probably know in the end it is just an int field anyways.

This is akin to declaring an integer that would accept only values from 1 to 100; the only language I ever saw that supported this level of checking was CHILL.

Answer 3

You don't need to deal with exceptions. The precondition for the method is that callers should use the enum, not cast any willy nilly int to the said enum. That would be madness. Isn't the point of enums not to use the ints?

Any dev who would cast 17 to the Colour enum would need 17 kicks up their backside as far as I'm concerned.

Answer 4

That is one of the many reasons why you should never be assigning integer values to your enums. If they have important values that need to be used in other parts of the code turn the enum into an object.

Answer 5

It would throw an error by using the Enum.Parse();

Enum parsedColour = (Colour)Enum.Parse(typeof(Colour), "17");

Might be worth using to get a runtime error thrown, if you like.

Answer 6

Short version:

Don't do this.

Trying to change enums into ints with only allowing valid values (maybe a default fallback) requires helper methods. At that point you have don't have an enum--you reallly have a class.

Doubly so if the ints are improtant--like Bryan Rowe said.

Answer 7

Not sure about why, but I recently found this "feature" incredibly useful. I wrote something like this the other day

// a simple enum
public enum TransmissionStatus
{
    Success = 0,
    Failure = 1,
    Error = 2,
}
// a consumer of enum
public class MyClass 
{
    public void ProcessTransmissionStatus (TransmissionStatus status)
    {
        ...
        // an exhaustive switch statement, but only if
        // enum remains the same
        switch (status)
        {
            case TransmissionStatus.Success: ... break;
            case TransmissionStatus.Failure: ... break;
            case TransmissionStatus.Error: ... break;
            // should never be called, unless enum is 
            // extended - which is entirely possible!
            // remember, code defensively! future proof!
            default:
                throw new NotSupportedException ();
                break;
        }
        ...
    }
}

question is, how do I test that last case clause? It is completely reasonable to assume someone may extend TransmissionStatus and not update its consumers, like poor little MyClass above. Yet, I would still like to verify its behaviour in this scenario. One way is to use casting, such as

[Test]
[ExpectedException (typeof (NotSupportedException))]
public void Test_ProcessTransmissionStatus_ExtendedEnum ()
{
    MyClass myClass = new MyClass ();
    myClass.ProcessTransmissionStatus ((TransmissionStatus)(10));
}