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Generic type restriction for enums

, 10 Jul 2011
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How to restrict a generic type identifier to only allow enums.

I was looking for a way to restrict a generic type identifier to only allow enums. Since you are not allowed to use where T : Enum, I checked the metadata for Enum and found that it implements ValueType, IComparable, IFormattable, IConvertible. You can't use ValueType as restriction, but struct works fine.

In other words, use “where T : struct, IComparable, IFormattable, IConvertible” to make a type restriction which should work well as an enum restriction.

The reason why I needed the specification is that I want to be able to fetch the DescriptionAttribute from enums. I use it to be able to add friendly names to enum values.

Example enum:

public enum MyEnum
{
    [Description("Some value"]
    SomeValue,

    [Description("Foo bar")]
    FooBar
}

Fetch value:

MyEnum value = MyEnum.FooBar;

// will display "Foo bar"
Console.WriteLine(value.GetDescription());

The extension method that does the magic:

public static string GetDescription<T>(this T instance)
    where T : struct, IConvertible, IFormattable, IComparable
{
    var type = typeof (T);
    var memberInfo = type.GetMember(instance.ToString())[0];

    var attribute = (DescriptionAttribute)memberInfo.GetCustomAttributes(
        typeof(DescriptionAttribute), false).FirstOrDefault();
    return attribute == null ? instance.ToString() : attribute.Description;
}

License

This article, along with any associated source code and files, is licensed under The GNU Lesser General Public License (LGPLv3)

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About the Author

jgauffin
Founder Gauffin Interactive AB
Sweden Sweden
Founder of OneTrueError, a .NET service which captures, analyzes and provide possible solutions for exceptions.
 
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Comments and Discussions

 
GeneralMy vote of 3 PinmemberRichard Deeming13-Jul-11 8:20 
Suggestionnicely done PinmemberIGood12-Jul-11 8:02 
GeneralRe: nicely done Pinmemberjgauffin12-Jul-11 19:34 
GeneralRe: nicely done PinmemberIGood13-Jul-11 16:33 
GeneralRe: nicely done Pinmemberjgauffin13-Jul-11 19:43 
GeneralRe: nicely done PinmemberIGood14-Jul-11 6:49 

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