The value of attributes

I often rediscover the value of Attributes in C#. Very often it is hard to find a situation where you have need of attributes, you may be thinking that it is only for meta coding like unit testing or dynamic code intepretation/serializing, but you're wrong. Attributes are useful every time you need to add meta data to a class, property or field. Let me give you the common example of adding label to an enum.

Problem: An enum defines genre in your music database. Now you want to display all available genres.

[Flags]
public enum Genre
{
    Other = 0,
    AlternativeRock = 1,
    HardRock = 2,
    WorldMusic = 4,
    Indie = 8,
    PostRock = 16
}

Start by defining the attribute Label.

public class LabelAttribute : Attribute
{
    public LabelAttribute(string text)
    {
        this.Text = text;
    }

public string Text { get; set; }

}

Now you can add this to your genre values.

[Flags]
public enum Genre
{ 
    [Label("Other")]
    Other = 0,

[Label("Alternative Rock")]
AlternativeRock = 1,

[Label("Hard rock")]
HardRock = 2,

[Label("World music")]
WorldMusic = 4,

[Label("Indie")]
Indie = 8,

[Label("Post-rock")]
PostRock = 16

}

With the simple use of an extension method you may accomplish alot.

public static class LabelAttributeHelper
{
    /// <summary>
    /// Get label of an enum field
    /// </summary>
    public static string Label<T>(this T val)
    {
        if (!typeof(T).IsEnum)
            throw new ArgumentException("T must be an Enum");

    FieldInfo field = typeof(T).GetField(val.ToString());
    LabelAttribute[] attributes =
        (LabelAttribute[])field.GetCustomAttributes(typeof(LabelAttribute), false);

    if (attributes.Length == 0)
        throw new ArgumentOutOfRangeException(&quot;Enum value &quot; + val.ToString() + &quot; is missing Label attribute&quot;);

    return attributes.First().Text;
}

}

Now you may easily type out all available genres with a simple procedure.

foreach (Genre genre in Enum.GetValues(typeof(Genre)))
    Console.WriteLine(genre.Label());

That's all for today. Have a nice weekend!