Input type into method, Operator "<" cannot be used on type "Type"

Question

I was trying to make my code cleaner by moving something that I call a lot into a static method. The original code is:

List<ListItem> listItems = Enum
  .GetValues(typeof(TimelineinfoEnums.StateEnum))
  .Cast<TimelineinfoEnums.StateEnum>()
  .Select(x => new ListItem { 
     Text  = x.ToString(), 
     Value = (int)x })
  .ToList();

TimelineinfoEnums.StateEnum is just a place holder, the point of my method is to be able to call it, pass in a value, and make TimelineinfoEnums.StateEnum some other enum that I'm working with. After reading elsewhere on SO, that you have to use a string input for things like this because the "entity type is literally not known, or knowable, at compile time.", I tried this code:

 public static List<ListItem> GetDropdownList(string @enum)
 {     
     List<ListItem> listItems = Enum
       .GetValues(@enum.GetType())
       .Cast<@enum.GetType()>()
       .Select(
          x => new ListItem { Text = x.ToString(), Value = (int)x }).ToList();            
          return listItems;
        }

However, when I attempted to do so, on the second @enum.GetType()I got an error of:

Operator '<' cannot be applied to operands of type 'method group' and 'Type'

I've tried removing the .GetType() bit but then I get an error of

enum is a variable but is used like a type

I'm not too familar with Linq, so I'm likely missing something straightforward, thanks for any help!

Edit: ListItem Looks like this:

 public class ListItem
        {
            public int Value { get; set; }
            public string Text { get; set; }
        }

casting is some **compiletime**-operation. So there´s no way to do it, when you provide the actual type at **runtime**. — MakePeaceGreatAgain, Aug 24 '20 at 11:08
`.Cast<@enum.GetType()>()` this does not do what you think it does. Generics are a compile-time (meaning: JIT-compile-time) thing, not a runtime thing. The closest think you'll get to that is by using `Activator.CreateInstance`. — Dai, Aug 24 '20 at 11:14
Ok, just to be clear I am providing the value at compile-time. — Bubinga, Aug 24 '20 at 11:15
@Bubinga No you aren't, you're passing a `string` at runtime. — Dai, Aug 24 '20 at 11:15
Then I guess I don't really know what runtime is. How do you know that I am? — Bubinga, Aug 24 '20 at 11:16

Dai · Accepted Answer · 2020-08-24T11:33:57.147

You can still use generics, but use the struct, Enum constraint added in C# 7.3:

Change your code to this:

public static List<ListItem> GetDropdownList<TEnum>()
    where TEnum : struct, Enum
{     
     return Enum.GetValues( typeof(TEnum) )
        .Cast<TEnum>()
        .Select( e => new ListItem() { Text = e.ToString(), Value = Convert.ToInt32( e ) } );
        .ToList();

    public static IReadOnlyList<ListItem> Items => _items;
}

Note that the above code's performance can be improved by moving it to a static generic class and storing the result in a static field, as this will cache the result (as generic types exist once for each type, so there's no need to perform this computation more than once):

public static class EnumListItems<TEnum>
    where TEnum : struct, Enum
{
    private static readonly IReadOnlyList<ListItem> _items = Enum.GetValues( typeof(TEnum) )
        .Cast<TEnum>()
        .Select( e => new ListItem() { Text = e.ToString(), Value = Convert.ToInt32( e ) } );
        .ToList();

    public static IReadOnlyList<ListItem> Items => _items;
}

Used like so:

IReadOnlyList<ListItem> items = EnumListItems<MyEnum>.Items;

If you need a mutable list, use .ToList():

List<ListItem> items = EnumListItems<MyEnum>.Items.ToList();

Thanks for such a thorough answer, ngl I don't fully understand everything though, but that is just because I'm rather new to programming. +1 — Bubinga, Aug 24 '20 at 11:24
@Bubinga I accidentally pressed "Cut" on my code instead of "Copy" when I was expanding my answer. I've restored my original code now. Sorry for the confusion. — Dai, Aug 24 '20 at 11:34
How would you change this code to get the display name of the enum values? The ```[Display(Name = "Example")]``` thingo? — Bubinga, Aug 29 '20 at 06:24
@Bubinga Getting the value of a `DisplayAttribute` for enum members requires a lot of code, see here: https://stackoverflow.com/questions/13099834/how-to-get-the-display-name-attribute-of-an-enum-member-via-mvc-razor-code — Dai, Aug 29 '20 at 06:31

CodeCaster · Answer 2 · 2020-08-24T11:21:16.967

entity type is literally not known, or knowable, at compile time

You appear to have to use Cast<T>() to do anything meaningful with Linq, because Enum.GetValues()'s return type was never updated to support generic collections for Linq. Also, @enum.GetType() will always return System.String.

The only thing you want to extract, are the enum members' integer and textual value, and you can extract those from any enum. So:

private static List<ListItem> GetListItems<TEnum>()
{
    var result = new List<ListItem>();
    
    foreach (object e in Enum.GetValues(typeof(TEnum)))
    {
        result.Add(new ListItem 
        { 
            Text = e.ToString(),
            Value = (int)e
        });
    }
               
    return result;
}

When using C# 7.3 and up, you should constrain TEnum to System.Enum. And if you want Linq instead of a loop, you can use Enum.GetValues(typeof(TEnum)).Cast<System.Enum>().

Now if you don't know TEnum at compile time, look at How to call generic method with a given Type object? and Type.GetType(enumName).

Thanks for the explanation but it's giving me an error at ```(int)e``` saying that it, "Cannot convert type 'System.Enum' to 'int'" — Bubinga, Aug 24 '20 at 11:20

Input type into method, Operator "<" cannot be used on type "Type"

2 Answers2