Help in creating Enum extension helper

Question

Take this enum for example:

public enum PersonName : short
{
    Mike = 1,
    Robert= 2
}

I wnat to have en extension method like this:

 PersonName person = PersonName.Robert; short personId = person.GetId(); //Instead of: short personId = (short)person; 

Any idea how to implement this?

it need to be generic to int, short etc..., and to generic enums as well, not limited to PersonName.

Answer 1

Well, its sort of possible, but on the flip side you will have to specify what the underlying type is:

public static T GetId<T>(this Enum value) 
                        where T : struct, IComparable, IFormattable, IConvertible
{
    return (T)Convert.ChangeType(value, typeof(T));
}

since System.Enum is the base class of all enum types. Call it:

PersonName person = PersonName.Robert;
short personId = person.GetId<short>();

Since you can cast almost anything to long , you could even have this:

public static long GetId(this Enum value)
{
    return Convert.ToInt64(value);
}

and call:

PersonName person = PersonName.Robert;
long personId = person.GetId();

What is otherwise possible is to get an object instance returned, like this or so:

public static object GetId(this Enum value)
{
    return Convert.ChangeType(task, Enum.GetUnderlyingType(value.GetType()));
}

PersonName person = PersonName.Robert;
var personId = (short)person.GetId(); 
// but that sort of defeats the purpose isnt it?

All of which involves some sort of boxing and unboxing. The second approach looks better (you could even make it int , the standard type) and then the first.

Answer 2

This is completely impossible.

You cannot constrain a method based on an enum 's underlying type.

Explanation:

Here is how you might try to do this:

public static TNumber GetId<TEnum, TNumber>(this TEnum val) 
       where TEnum : Enum 
       where TEnum based on TNumber

The first constraint is unfortunately not allowed by C# , and the second constraint is not supported by the CLR.

Answer 3

I believe you should be able to define an extension method on the PersonName class:

public static class PersonExtensions // now that sounds weird...
{
    public static short GetID (this PersonName name)
    {
        return (short) name;
    }
}

Side node: I somehow hope that the code in your question is overly simplified, as it does not seem quite right to implement a person repository as an enum :)

Answer 4

Is there any reason you can't turn it into a class? It would probably make more sense in context of a class.

Answer 5

我只想使用像IEnumerable<KeyValuePair<string,short>>或Dictionary<string,short> ...你可以根据需要将你的扩展添加到IDictionary或IEnumerable。