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?

You can create an extension for anything, even `object`(although that's [not considered best-practice][1]). 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
}
}


[1]:

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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.

See [MSDN][1].

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


[1]:

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According to this [site][1]:

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][2] at Microsft MSDN.


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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][1]

Thanks to [ShawnFeatherly for his answer][2]


[1]:

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