Expression of type 'MyEnum' cannot be used for parameter of type

Question

I created Enum ToFrendlyString function for my enums, but i cant use in Linq.

 public enum MyEnum
    {

        Queued = 0,           
        [Description("In progress")]
        In_progress = 2,            
        [Description("No answer")]
        No_answer = 6,

    }



  public static class EnumToFrendlyString
    {

        public static string ToFrendlyString(this Enum value)
        {
            return value.GetEnumDescription();
        }


        public static string GetEnumDescription(this Enum value)
        {
            FieldInfo fi = value.GetType().GetField(value.ToString());

            var attributes =
                (DescriptionAttribute[])fi.GetCustomAttributes(
                    typeof(DescriptionAttribute),
                    false);

            if (attributes.Length > 0)
                return attributes[0].Description;

            return value.ToString();
        }
    }

When i try to use this function in Linq, im getting error

  var res = collection.AsQueryable().Where(p => p.UserID == UserID).OrderByDescending(p=> p.DateCreated).Select(p => new MyClass
                {                          
                     Date = p.DateCreated.ToString(),
                     Status = p.Status.ToFrendlyString(),                        
                }).Take(10).ToList();

If i make another function in same class, like

 private string MyStatusToString(MyEnum status)
       {
           return status.ToFrendlyString();
       }

and change my Linq to use this function, then everything works.

Error

Expression of type 'DAL.MyEnum' cannot be used for parameter of type 'System.Enum' of method 'System.String ToFrendlyString(System.Enum)'

Why are you querying for a string status when you have a perfectly good numeric Enum? — Artless, Feb 01 '13 at 17:26
Your error references `MyEnum` but your enum is `MyStatus`. Is `p.Status` actually an enum type? — mclark1129, Feb 01 '13 at 17:26
@Trickery, couze i return that result as ajax and i dont want to make JS logic to render as string. — Novkovski Stevo Bato, Feb 01 '13 at 17:28
@MikeC that is had writing error, i renamed my classes and so on, ill edit now — Novkovski Stevo Bato, Feb 01 '13 at 17:30
Your code works fine for me. Can you include short, but complete example code that shows the issue? What is the type of `collection`? — svick, Feb 01 '13 at 18:20
collection is type of MongoCollection<> . Did u tried with Linq-to-Sql? — Novkovski Stevo Bato, Feb 01 '13 at 18:29
@NovkovskiStevoBato I didn't, because your question didn't say anything about MongoDB or LINQ to SQL. — svick, Feb 01 '13 at 18:48

score 2 · Accepted Answer · edited May 23 '17 at 10:31

I'm not sure you can use Enum as the Type for an extension method like that - try this instead. I've taken the liberty of tidying the code up a bit, feel free to ignore those changes :)

public static class EnumToFrendlyString
{
    public static string ToFrendlyString<T>(this T value)
        where T : struct
    {
        return value.GetEnumDescription();
    }

    public static string GetEnumDescription<T>(this T value)
        where T : struct
    {
        return EnumDescriptionCache<T>.Descriptions[value];
    }

    private static class EnumDescriptionCache<T>
        where T : struct
    {
        public static Dictionary<T, string> Descriptions =
            Enum.GetValues(typeof(T))
                .Cast<T>()
                .ToDictionary(
                    value => value,
                    value => value.GetEnumDescriptionForCache());
    }

    private static string GetEnumDescriptionForCache<T>(this T value)
        where T : struct
    {
        if (!typeof(T).IsEnum)
        {
            throw new ArgumentException("Only use with enums", "value");
        }

        var descriptionAttribute = typeof(T)
            .GetField(value.ToString())
            .GetCustomAttributes(typeof(DescriptionAttribute), false)
            .Cast<DescriptionAttribute>()
            .FirstOrDefault();

        return (descriptionAttribute != null)
            ? descriptionAttribute.Description
            : value.ToString();
    }
}

I've added a private, generic class to cache the descriptions for your enum members to avoid lots of runtime use of reflection. It looks a bit odd popping in and out of the class to first cache then retrieve the values, but it should work fine :)

The warning I gave in this answer still applies - the enum value passed to the dictionary isn't validated, so you could crash it by calling ((MyEnum)5367372).ToFrendlyString().

Wow wow, looks like its working! If i don`t comment about any error then its 100% working. Can you give explanation why "where T : struct" do the magic here? About caching, since its generic function, do i must cache on first ToFrendlyString call since how i can init the cache dictonary? — Novkovski Stevo Bato, Feb 01 '13 at 18:28
`where T : struct` just limits the extension method's applicability to value types - this method should work fine without it. As @svick points out you actually *[can](http://stackoverflow.com/questions/276585/enumeration-extension-methods)* use `Enum` as the type for an extension method, so I'll be honest and say I'm not 100% sure why it wasn't working when you did. I'll update the answer with a caching example :) — Steve Wilkes, Feb 01 '13 at 18:38

score 0 · Answer 2 · answered Feb 01 '13 at 17:38

0

I am not sure but it can be that you didn t add the DAL project yet to your current project(Add reference -> projects in solutins -> Dal). Then it might work. (I had a similar issue once and this was my solution)

answered Feb 01 '13 at 17:38

Maximc

1,722
1
23
37

its already added, if it wasnt, Resharper or Build fill alert an error – Novkovski Stevo Bato Feb 01 '13 at 17:40
Not always. I had a constructor with a Enum but the project didn t had the project(with the neum) added in the reference, it only gave error on runtime with the same error. Adding the reference fixed it there for me. – Maximc Feb 01 '13 at 17:44

score 0 · Answer 3 · answered Feb 01 '13 at 19:07

It seems the problem is that your collection is IQueryable<T> and the query provider is trying to translate your Select() into a query string.

One way to avoid that is to perform Select() in memory using IEnumerable<T>:

var res = collection.AsQueryable()
            .Where(p => p.UserID == UserID)
            .OrderByDescending(p=> p.DateCreated)
            .Take(10)
            .AsEnumerable()
            .Select(p => new MyClass
            {                          
                 Date = p.DateCreated.ToString(),
                 Status = p.Status.ToFrendlyString(),                        
            })
            .ToList();

Expression of type 'MyEnum' cannot be used for parameter of type

3 Answers3

Linked