Order by enum description

Question

I am working on an ASP.NET MVC projet using EF code first, and I am facing a situation where I need to order by an enum description:

public partial class Ite         


        

Answer 1

I would go with dynamic expression. It's more flexible and can easily be changed w/o affecting the database tables and queries.

However, instead of sorting by description strings in the database, I would create ordered map in memory, associating int "order" value with each enum value like this:

public static class EnumHelper
{
    public static Expression> DescriptionOrder(this Expression> source)
        where TEnum : struct
    {
        var enumType = typeof(TEnum);
        if (!enumType.IsEnum) throw new InvalidOperationException();

        var body = ((TEnum[])Enum.GetValues(enumType))
            .OrderBy(value => value.GetDescription())
            .Select((value, ordinal) => new { value, ordinal })
            .Reverse()
            .Aggregate((Expression)null, (next, item) => next == null ? (Expression)
                Expression.Constant(item.ordinal) :
                Expression.Condition(
                    Expression.Equal(source.Body, Expression.Constant(item.value)),
                    Expression.Constant(item.ordinal),
                    next));

        return Expression.Lambda>(body, source.Parameters[0]);
    }

    public static string GetDescription(this TEnum value)
        where TEnum : struct
    {
        var enumType = typeof(TEnum);
        if (!enumType.IsEnum) throw new InvalidOperationException();

        var name = Enum.GetName(enumType, value);
        var field = typeof(TEnum).GetField(name, BindingFlags.Static | BindingFlags.Public);
        return field.GetCustomAttribute()?.Description ?? name;
    }
}

The usage would be like this:

case SortableTypeE.Type:
    var order = EnumHelper.DescriptionOrder((Item x) => x.MyEnum);
    result = sortOrder == SortOrder.TypeE.ASC
        ? items.OrderBy(order)
        : items.OrderByDescending(order);
    result = result.ThenBy(i => i.SomeOtherProperty);
    break;

which would generate expression like this:

x => x.MyEnum == Enum[0] ? 0 :
     x.MyEnum == Enum[1] ? 1 :
     ...
     x.MyEnum == Enum[N-2] ? N - 2 :
     N - 1;

where 0,1,..N-2 is the corresponding index in the value list sorted by description.