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

问题

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)'

回答1:

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().

回答2:

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)

回答3:

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();

来源：https://stackoverflow.com/questions/14651624/expression-of-type-myenum-cannot-be-used-for-parameter-of-type