Can I cast from a generic type to an enum in C#?

Question

I'm writing an utility function that gets a integer from the database and returns a typed enum to the application.

Here is what I tried to do (note I pass in a data reader and column name instead of the int in my real function):

public static T GetEnum<T>(int enumAsInt)
{
    Type enumType = typeof(T);

    Enum value = (Enum)Enum.ToObject(enumType, enumAsInt);
    if (Enum.IsDefined(enumType, value) == false)
    {
        throw new NotSupportedException("Unable to convert value from database to the type: " + enumType.ToString());
    }

    return (T)value;
}

But it won't let me cast (T)value saying:

Cannot convert type 'System.Enum' to 'T'.

Also I've read quite a bit of mixed reviews about using Enum.IsDefined. Performance wise it sounds very poor. How else can I guarantee a valid value?

Note that if the case had been other way around, say, you have to do `(Enum)value` and you get `Cannot convert type 'T' to 'System.Enum'`, you could merely do an `as` cast, like `value as Enum`. — nawfal, Jun 09 '13 at 13:07
Not true (at least in my case), you get the old 'The as operator must be used with a reference or nullable type ('Foo.bar' is a non-nullable value type) error. — keithl8041, Feb 10 '14 at 12:54

score 72 · Accepted Answer · answered Jul 15 '10 at 23:10

72

Like this:

return (T)(object)value;

answered Jul 15 '10 at 23:10

SLaks

868,454
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3

It would be better to initially declare `value` as an `object`. – João Angelo Jul 15 '10 at 23:13
Thanks that seems to compile. I don't understand the reason for the error then? Bonus points if you can explain why in the interest of learning:) – Justin Jul 15 '10 at 23:20
2

The compiler doesn't realize that `T` is a enum type. As far as it is aware, `T` might be `DateTime`. Therefore, you cannot cast directly from any type other than `object` to `T`. – SLaks Jul 15 '10 at 23:23
I did try to constrain T in the method signature with "where T : Enum". But the compiler tells me Constraint cannot be special class 'System.Enum'. – Justin Jul 15 '10 at 23:27
3

@Justin: [Yes; this is an annoying and unnecessary limitation](http://stackoverflow.com/questions/1331739/enum-type-constraints-in-c/1416660#1416660) – SLaks Jul 15 '10 at 23:51
1

Can't this be done without box-unbox? May be using this `__makeref` thing – AgentFire May 02 '15 at 20:57

score 20 · Answer 2 · answered Jul 15 '10 at 23:27

20

Change this:

Enum value = (Enum)Enum.ToObject(enumType, enumAsInt);

to this:

T value = (T)Enum.ToObject(enumType, enumAsInt);

and remove the cast :)

answered Jul 15 '10 at 23:27

porges

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114

I couldn't get Slaks solution to work, but using the Enum.ToObject cast worked great. – Tod Cunningham Jan 04 '19 at 16:46

score 2 · Answer 3 · answered May 14 '19 at 14:19

2

For information, using the generic constraint Enum is available from C# 7.3 and greater.

answered May 14 '19 at 14:19

Eli

414
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10

OMG, the best answer here (for my issue). Thanks a lot! – Uwe Keim Jul 18 '22 at 05:13

Can I cast from a generic type to an enum in C#?

3 Answers3