[AttributeUsage(AttributeTargets.All, 
   Inherited=true, 
   AllowMultiple=false)]
public class ItemsSourceAttribute : System.Attribute 

public class FontSizeItemsSource : IItemsSource
{
    public ItemCollection GetValues()
    {
        ItemCollection sizes = new ItemCollection();
        sizes.Add(5.0, "Five");
        sizes.Add(5.5);
        sizes.Add(6.0, "Six");
        sizes.Add(6.5);
        sizes.Add(7.0, "Seven");
        sizes.Add(7.5);
        sizes.Add(8.0, "Eight");
        sizes.Add(8.5);
        sizes.Add(9.0, "Nine");
        sizes.Add(9.5);
        sizes.Add(10.0);
        sizes.Add(12.0, "Twelve");
        sizes.Add(14.0);
        sizes.Add(16.0);
        sizes.Add(18.0);
        sizes.Add(20.0);
        return sizes;
    }
}

public class Person
{
    [Category("Writing")]
    [DisplayName("Writing Font Size")]
    [Description("This property uses the DoubleUpDown as the default editor.")]
    [ItemsSource(typeof(FontSizeItemsSource))]
    public double WritingFontSize { get; set; }
}