如何向枚举添加扩展方法

Question

I have this Enum code:

enum Duration { Day, Week, Month };

Can I add a extension methods for this Enum?

Answer 1

According to this site :

Extension methods provide a way to write methods for existing classes in a way other people on your team might actually discover and use. Given that enums are classes like any other it shouldn't be too surprising that you can extend them, like:

enum Duration { Day, Week, Month };

static class DurationExtensions 
{
  public static DateTime From(this Duration duration, DateTime dateTime) 
  {
    switch (duration) 
    {
      case Day:   return dateTime.AddDays(1);
      case Week:  return dateTime.AddDays(7);
      case Month: return dateTime.AddMonths(1);
      default:    throw new ArgumentOutOfRangeException("duration");
    }
  }
}

I think enums are not the best choice in general but at least this lets you centralize some of the switch/if handling and abstract them away a bit until you can do something better. Remember to check the values are in range too.

You can read more here at Microsft MSDN.

Answer 2

You can also add an extension method to the Enum type rather than an instance of the Enum:

/// <summary> Enum Extension Methods </summary>
/// <typeparam name="T"> type of Enum </typeparam>
public class Enum<T> where T : struct, IConvertible
{
    public static int Count
    {
        get
        {
            if (!typeof(T).IsEnum)
                throw new ArgumentException("T must be an enumerated type");

            return Enum.GetNames(typeof(T)).Length;
        }
    }
}

You can invoke the extension method above by doing:

var result = Enum<Duration>.Count;

It's not a true extension method. It only works because Enum<> is a different type than System.Enum.

Answer 3

Of course you can, say for example, you want to use the DescriptionAttribue on your enum values:

using System.ComponentModel;

public enum Duration 
{ 
    [Description("Eight hours")]
    Day,

    [Description("Five days")]
    Week,

    [Description("Twenty-one days")] 
    Month 
}

Now you want to be able to do something like:

Duration duration = Duration.Week;
var description = duration.GetDescription(); // will return "Five days"

Your extension method GetDescription() can be written as follows:

using System.ComponentModel;
using System.Reflection;

public static string GetDescription(this Enum value)
{
    FieldInfo fieldInfo = value.GetType().GetField(value.ToString());
    if (fieldInfo == null) return null;
    var attribute = (DescriptionAttribute)fieldInfo.GetCustomAttribute(typeof(DescriptionAttribute));
    return attribute.Description;
}

Answer 4

All answers are great, but they are talking about adding extension method to a specific type of enum.

What if you want to add a method to all enums like returning an int of current value instead of explicit casting?

public static class EnumExtensions
{
    public static int ToInt<T>(this T soure) where T : IConvertible//enum
    {
        if (!typeof(T).IsEnum)
            throw new ArgumentException("T must be an enumerated type");

        return (int) (IConvertible) soure;
    }

    //ShawnFeatherly funtion (above answer) but as extention method
    public static int Count<T>(this T soure) where T : IConvertible//enum
    {
        if (!typeof(T).IsEnum)
            throw new ArgumentException("T must be an enumerated type");

        return Enum.GetNames(typeof(T)).Length;
    }
}

The trick behind IConvertible is its Inheritance Hierarchy see MDSN

Thanks to ShawnFeatherly for his answer

Answer 5

You can create an extension for anything, even object (although that's not considered best-practice ). Understand an extension method just as a public static method. You can use whatever parameter-type you like on methods.

public static class DurationExtensions
{
  public static int CalculateDistanceBetween(this Duration first, Duration last)
  {
    //Do something here
  }
}

Answer 6

A Simple workaround.

public static class EnumExtensions
{
    public static int ToInt(this Enum payLoad) {

        return ( int ) ( IConvertible ) payLoad;

    }
}

int num = YourEnum.AItem.ToInt();
Console.WriteLine("num : ", num);

Answer 7

See MSDN .

public static class Extensions
{
  public static string SomeMethod(this Duration enumValue)
  {
    //Do something here
    return enumValue.ToString("D"); 
  }
}

Answer 8

we have just made an enum extension for c# https://github.com/simonmau/enum_ext

It's just a implementation for the typesafeenum, but it works great so we made a package to share - have fun with it

public sealed class Weekday : TypeSafeNameEnum<Weekday, int>
{
    public static readonly Weekday Monday = new Weekday(1, "--Monday--");
    public static readonly Weekday Tuesday = new Weekday(2, "--Tuesday--");
    public static readonly Weekday Wednesday = new Weekday(3, "--Wednesday--");
    ....

    private Weekday(int id, string name) : base(id, name)
    {
    }

    public string AppendName(string input)
    {
        return $"{Name} {input}";
    }
}

I know the example is kind of useless, but you get the idea ;)