How to add extension methods to Enums
I have this Enum code:
enum Duration { Day, Week, Month };
Can I add a extension methods for this Enum?
Solution 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.
Solution 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.
Solution 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;
}
Solution 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