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 (i.e., translates to SQL) without refactoring the structures (e.g. 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...

iirc you can create something like `Func Convert` and then use like `enumDesc = Convert(x)` — AD.Net, Apr 28 '16 at 14:36
You could have a look at the _CustomAttribute_ for your enum members. And then use it in combination with an ExtensionMethod. Something like this http://stackoverflow.com/questions/1799370/getting-attributes-of-enums-value — schlonzo, Apr 28 '16 at 14:38
I would not convert the value on the model, retrieve the value and leave it as-is, then when you represent it do the conversion using a general function. — Gusman, Apr 28 '16 at 14:39

score 0 · Answer 1 · answered Apr 28 '16 at 14:54

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...

I haven't tested it yet but from my understanding this wouldn't work before materializing the entities into objects, meaning it won't happen at the SQL layer, so I would need to iterate through the objects an additional time. — Josh, Apr 28 '16 at 15:22
No, wherever you need to use the value of the enum -- instead use the `.Description()`. Or if it's easier create a read-only property on the entity that evaluates as `public string StatusDesc { get { return Status.Description(); } }`. Then you can use the `.StatusDesc` for the value you want. — David Pine, Apr 28 '16 at 15:33
With this I get: Exception thrown: 'System.NotSupportedException' in EntityFramework.dll Additional information: LINQ to Entities does not recognize the method 'System.String Description[Status](Status)' method, and this method cannot be translated into a store expression. — Josh, Apr 28 '16 at 16:57

score 0 · Answer 2 · answered Apr 28 '16 at 14:55

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" : ""
        };
    }

Hmm I tried this based on another page I found, but it didn't work. I'll try again based on your example. — Josh, Apr 28 '16 at 15:23
With this I get: Exception thrown: 'System.NotSupportedException' in EntityFramework.dll Additional information: Unable to create a constant value of type 'OrderItemStatusClass'. Only primitive types or enumeration types are supported in this context. — Josh, Apr 28 '16 at 17:02
This does not run code with IQueryable coming from a context. if you need running on a Queryable, include .ToList() before the .Select(...) — Julio Borges, Apr 28 '16 at 17:48

Centralize ternary logic in EF6/LINQ

2 Answers2