How to cast a variable of an enum generic type to int and back

Question

I am using C# to write a class that takes an enum type parameter, like this:

class EnumWrapper<T>
    where T: Enum
{
}

Inside the class, I have a method that takes an argument of type T, and I need to cast it to an int, like this:

void DoSomething(T arg)
{
    int a = (int)arg;
}

But the compiler issues a CS0030 error "Cannot convert type 'T' to 'int'. Something similar happens when I try to cast an int back into a T. However, this works with a fixed enum type that is not provided via a type parameter. For example, this works fine:

void DoSomething(DayOfWeek arg)
{
    int a = (int)arg;
}

Why is this happening, and how can I cast between a generic enum type and int?

Remember that enums can have any integer base type -- you can have `enum Foo : ulong` if you so wish. What do you want your code to do in that case? — canton7, Apr 08 '21 at 10:46
That's the "why is this happening" part. For the "How can I cast..." part, answer my question: What do you want your code to do in that case? — canton7, Apr 08 '21 at 10:55
I'm wondering why you need such a wrapper class. It sounds like you are possibly doing something 'unusual', hence your problem, when there may be a better way to solve your actual problem without a wrapper — Neil, Apr 08 '21 at 12:04
@Neil, I am developing a wrapper because I need two features: a string description of each enum value, and hierarchically organised values. — CesarGon, Apr 08 '21 at 14:11
@CesarGon Please check out the end of my answer :) I also had a very similar problem a couple of years ago — Hasan Emrah Süngü, Apr 08 '21 at 14:14
String descriptions should probably use the `[Description]` attribute (which already supports resx/translations etc). Hierarchy, could also come from a custom attribute, I guess `[Hierarchy("parent")]` ? — Neil, Apr 08 '21 at 16:02
Thanks for the ideas, @Neil. We are currently exploring alternative approaches, and attributes are one of them. — CesarGon, Apr 08 '21 at 16:12

Pavel Anikhouski · Accepted Answer · 2021-04-08T10:53:27.760

4

You may add IConvertible to generic constraint and use ToInt32 method, something like

class EnumWrapper<T>
    where T : Enum, IConvertible
{
    void DoSomething(T arg)
    {
        int a = arg.ToInt32(null);
    }
}

Have a look at the Enum constraint for details. Also, as mentioned in comments, it isn't necessary that your enum is based on int type

edited Apr 08 '21 at 10:53

answered Apr 08 '21 at 10:48

Pavel Anikhouski

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2

Alternatively, skip the constraint and call `Convert.ToInt32(arg)` (which internally tests for `IConvertible`). – canton7 Apr 08 '21 at 10:51
Lacks the "back" part of the title. But that is already explained here: https://stackoverflow.com/a/10387134/284240 – Tim Schmelter Apr 08 '21 at 11:06

How to cast a variable of an enum generic type to int and back

1 Answers1