Centralize ternary logic in EF6/LINQ

Question

I seem to often (when dealing with enums) do things like this:

.Select(x => new {
    enumDesc = (
        x.status == GoodStatus ? "Good!"
        : x.status == BadStatus ? "Bad<angryface>"
        : x.status == Unknown ? "no clue"
    )
})

or

let enumDesc = (
    x.status == GoodStatus ? "Good!"
    : x.status == BadStatus ? "Bad<angryface>"
    : x.status == Unknown ? "no clue"
)

Some queries end up having dozens of these conditions, like a project I'm working on now. Doing it this way is nice because it can be translated into a SQL case statement so it's faster than iterating through after materialization. In my current project, however, I need to do this in more than one place/query. Is there any way to do this in a performant and reusable way (ie, translates to SQL) without refactoring the structures (eg moving the enum into a table)? I haven't been able to come up with one. If I could capture or alias just the ternary stuff, that would be good enough, or making an expression that "selects" from constants or attributes or something...

Answer 1

If you're open to avoiding if logic, consider the following:

There are often times where I see developers in C# lean towards using the DescriptionAttribute and combining that with an extension method to lookup the enum values' description value.

public enum Status
{
    [Description("no clue")]
    Unknown,
    [Description("Good!")]
    Good,
    [Description("Bad<angryface>")]
    Bad
}

public static class EnumExt
{
    public static string Description<T>(this T source)where T : struct, IConvertible
    {
        if (!typeof (T).IsEnum)
        {
            throw new ArgumentException("T must be an enumerated type");
        }

        var fi = source.GetType().GetField(source.ToString());
        var attributes = (DescriptionAttribute[])fi.GetCustomAttributes(typeof (DescriptionAttribute), false);
        return attributes.Length > 0 ? attributes[0].Description : source.ToString();
    }
}

Here's a working .NET Fiddle I threw together quick to demonstrate this.

Otherwise, if you want to map a string to an enum value without the usage of attributes -- I'd suggest a switch instead...

Answer 2

You can use a method that implements an Expression<Func<T, T2>> , see example below:

    public enum Status
    {
        GoodStatus,
        BadStatus,
        Unknown
    }

    public class foo
    {
        public Status status;
    }

    public void Execute()
    {
        List<foo> list = new List<foo>
        {
            new foo
            {
                status = Status.BadStatus
            },
            new foo
            {
                status = Status.GoodStatus
            },
            new foo
            {
                status = Status.GoodStatus
            },
            new foo
            {
                status = Status.Unknown
            }
        };


        var descStatus = list.Select(GetStatus).ToList();
    }

    public Expression<Func<foo, object>> GetStatus(foo _foo)
    {
        return x => new
        {
            enumDesc = _foo.status == Status.GoodStatus
                ? "Good!"
                : _foo.status == Status.BadStatus
                    ? "Bad<angryface>"
                    : _foo.status == Status.Unknown ? "no clue" : ""
        };
    }