Passing specific enum to Enum class

Question

I have the code below in which there are 2 enums and a function that takes base class Enum as parameter, casts the general Enum to SomeEnum and displays it. I would have expected that when you pass SomeOtherEnum in the function, a InvalidCastException to be thrown because I was suspecting that the compiler generates for every enum another type. From the behavior however, it seems that the compiler has a single generated class type and every instance has different parameters(the enums). Is this correct? If not, why is it possible to pass seemingly incompatible types and the compiler doesn't complain?

'

enum SomeEnum
{
    X1,
    X2, 
    X3
}

enum SomeOtherEnum
{
    X1,
    X2,
    X3,
    X4,
    X5
}

public static void SomeFunction(Enum someEnum)
{
    SomeEnum x = SomeEnum.X3; // some dummy init
    try
    {
        x = (SomeEnum) someEnum;
    }
    catch (InvalidCastException) 
    {
        Console.WriteLine("Exception"); // why no exception caught ? why legit cast ?
    }
    Console.WriteLine(x);
}
    private static void Main(string[] args)
    {
        SomeFunction(SomeOtherEnum.X5); // pass a different type than the one in the function
        Console.ReadKey();
    }

`

because in your example, your enums are ints! your method just assigns 4 to the value of x within SomeFunction — Ric
– Ric, Commented Oct 7, 2015 at 12:24
Enums have a base numerical type (the default is int). With that said why are you defining your method to take Enum instead of the more specific SomeEnum? — juharr
– juharr, Commented Oct 7, 2015 at 12:24
You're sidestepping the compiler's type safety by having SomeFunction take an Enum (or something that inherits from it like either SomeEnum or SomeOtherEnum) — Rowland Shaw
– Rowland Shaw, Commented Oct 7, 2015 at 12:25
FYI if you just want to determine if the value is valid for a specific enum use Enum.IsDefined — juharr
– juharr, Commented Oct 7, 2015 at 12:27

Jerren Saunders · Accepted Answer · 2015-10-07 12:25:56Z

2

Your SomeFunction signature is currently instructed to accept any "Enum" datatype, of which both SomeEnum and SomeOtherEnum are.

Change your signature from:

public static void SomeFunction(Enum someEnum)

to

public static void SomeFunction(SomeEnum someEnum)

and your call to SomeFunction(SomeOtherEnum.X5) will become an error.

answered Oct 7, 2015 at 12:25

Jerren Saunders

1,4602 gold badges13 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

user2436426 Over a year ago

I am not asking why no compiler error is generated by the compiler, I am asking why the cast is correct ?

SWeko · Accepted Answer · 2015-10-07 12:28:28Z

2

Enums are basically named integers, with some compiler time checks. Still, there are a lot of way to fool the compiler into accepting invalid values for enums, e.g.

public enum First {
   One = 1, 
   Two = 2
}

First first = (First) 3;

is completely valid code.

So the line x = (SomeEnum) someEnum works, because it essentially executes as:

 x = (SomeEnum) (Enum) SomeOtherEnum.X5;

Each of the two casts are valid by themselves, even if they produce a value that makes to sense.

IMHO, enums are the one place where the C# design team completely dropped the ball - they are sufficiently different from integers, that you have to handle them differently, yet not different enough to make sense on their own.

answered Oct 7, 2015 at 12:28

SWeko

30.9k11 gold badges74 silver badges107 bronze badges

2 Comments

user2436426 Over a year ago

Well, that's not quite clear, I thought also about integers but Enum is a class. How you say, something does not make sense, the signature of the function parameter is a class but you pass a integer. How works that?

SWeko Over a year ago

No, System.Enum is quite special, and the way defined enums inherit from it is baked into the language. Any enum can be cast to Enum, and an Enum can be cast to any enum type - similar as the integer example I gave - and that's why your code does not throw an exception. Indeed it functions, only not in the way you intended..

Community · Accepted Answer · 2017-05-23 12:23:34Z

1

Behind the scenes, Enums are essentially just named integers.

Because of this, when you have an Enum object (IE the base type of all enums) you can freely cast to any inherited type.

See this question for more information about why it's not an invalid cast.

edited May 23, 2017 at 12:23

CommunityBot

11 silver badge

answered Oct 7, 2015 at 12:26

Vlad274

6,9642 gold badges37 silver badges46 bronze badges

8 Comments

user2436426 Over a year ago

How can they be integers? I don't think so, Enum is a class. Sure , they behave like passing integers but you cannot pass a integer where a Enum is expected. To me , there seems to be more behind...

Vlad274 Over a year ago

You're correct, they are not integers. However, as mentioned in other answers, the backing data is an integer. Hence, you can cast and enum to/from int.

user2436426 Over a year ago

The fact that you can cast integers is probably syntactic sugar. I don't understand why the cast between enums works because apparently SomeEnum is SomeOtherEnum are different types(or not?). I worked until know in C++ and coming to C# I saw that it is much more type safe (stronger typed) and this cast left me wondering ... it kind off jumps over type safety and let you do stupid things passing enums that shouldn't be there.

Vlad274 Over a year ago

You're not casting from SomeEnum to SomeOtherEnum. Your function accepts type Enum, so SomeOtherEnum is cast to Enum which is then cast to SomeEnum. Which is a valid cast chain.

user2436426 Over a year ago

It is not. With every other type you will get a InvalidCastException which makes sense. You cannot cast from base type to derived type if the base type is not a pointer to a derived object.

|

shashank · Accepted Answer · 2015-10-07 12:34:57Z

0

enum SomeEnum {
    X1,
    X2, 
    X3
}

enum SomeOtherEnum
{
    X1,
    X2,
    X3,
    X4,
    X5
}

public static void SomeFunction(SomeEnum someEnum))

{

    SomeEnum x = SomeEnum.X3; // some dummy init
    try
    {
        x = (SomeEnum) someEnum;
    }
    catch (InvalidCastException) 
    {
        Console.WriteLine("Exception"); // why no exception caught ? why legit cast ?
    }
    Console.WriteLine(x);
}
    private static void Main(string[] args)
    {
        SomeFunction(SomeOtherEnum.X5); // pass a different type than the one in the function
        Console.ReadKey();
    }

answered Oct 7, 2015 at 12:34

shashank

1,1612 gold badges10 silver badges24 bronze badges

Collectives™ on Stack Overflow

Passing specific enum to Enum class

4 Answers 4

1 Comment

2 Comments

8 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

2 Comments

8 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related