Cannot convert type 'TEnum' to 'int'

Question

I'm trying to convert an enum to a List as mentioned in this example e.g.

Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>().Select(v => new SelectListItem {
    Text = v.ToString(),
    Value = ((int)v).ToString()
}).ToList();

This work but I want to amend it to work with a generic enum

public static List<SelectListItem> GetEnumList<TEnum>(TEnum value)
{

        return Enum.GetValues(typeof(TEnum)).Cast<TEnum>().Select(v => new SelectListItem
        {
            Text = v.ToString(),
            Value = ((int)v).ToString()
        }).ToList();
 }

However the above code doesn't compile and gives

Cannot convert type 'TEnum' to 'int'

for the line

  Value = ((int)v).ToString()

1. How do I fix this above code.

2. Why is it giving a compile error with generic enum and not with a normal enum

---

Edit: I have tried the suggestions in the thread but I get a further error:

Here is my full code:

public static IHtmlContent EnumDropDownListFor<TModel, TResult,TEnum>(
    this IHtmlHelper<TModel> htmlHelper,
    Expression<Func<TModel, TResult>> expression,
    TEnum enumValue,
    string optionLabel) 
{
    return htmlHelper.DropDownListFor(expression, (IEnumerable<SelectListItem>)GetEnumList(enumValue), optionLabel);
}

public static List<SelectListItem> GetEnumList<TEnum>(TEnum value) 
{
    return Enum.GetValues(typeof(TEnum)).Cast<TEnum>().Select(v => new SelectListItem
    {
        Text = v.ToString(),
        Value = Convert.ToInt32(v).ToString()
    }).ToList();

}

but I get a runtime error

ArgumentException: Type provided must be an Enum.

Parameter name: enumType

on the line

return Enum.GetValues(typeof(TEnum)).Cast<TEnum>().Select(v => new SelectListItem
    {
        Text = v.ToString(),
        Value = Convert.ToInt32(v).ToString()
    }).ToList();

What do I need to fix in the code to not get the runtime error.

Answer 1

You've told the compiler nothing about TEnum. As far as its concerned, it could be a string, a DateTime, a BankAccount, a Bullet or anything.

To get this to work, you can use Enum.Parse and Convert.ToInt32

UPD: Let me just format the code from comment and fix compilation errors for SO-copy-pasters :D

public static int GetEnumIntValue<T>(T value)
    where T : struct
{
    Type genericType = typeof(T);
    Debug.Assert(genericType.IsEnum);
    Enum enumValue = Enum.Parse(genericType, value.ToString()) as Enum;
    return Convert.ToInt32(enumValue);
}

Answer 2

You don't need a value, just the enum type. You can use something like this

Generic

public static List<SelectListItem> GetEnumList<TEnum>()
    where TEnum : struct, IConvertible, IFormattable
{
    return ((TEnum[])Enum.GetValues(typeof(TEnum))).Select(v => new SelectListItem
    {
        Text = v.ToString(),
        Value = v.ToString("d", null)
    }).ToList();
}

Non generic

public static List<SelectListItem> GetEnumList(Type enumType)
{
    return Enum.GetValues(enumType).Cast<IFormattable>().Select(v => new SelectListItem
    {
        Text = v.ToString(),
        Value = v.ToString("d", null)
    }).ToList();
}

Both methods are not compile type safe. Generic, because of the lack of an enum connstraint. Non generic - well, it's similar to the Enum static methods which are not compile type safe anyway. Both methods will throw runtime exception if called with non enum type.

Answer 3

public static List<SelectListItem> GetEnumList<TEnum>(TEnum value) where TEnum : IConvertible
{
    return Enum.GetValues(typeof(TEnum)).Cast<TEnum>().Select(v => new SelectListItem
    {
        Text = v.ToString(),
        Value = v.ToInt32(null).ToString()
    }).ToList();
}

1. Complier know that you use Enum type after .Cast<MyEnum>().
2. But after .Cast<TEnum>() compiler doesn't know about type which can be.

Answer 4

compiler complains because type parameter in generic method is not constrained

also method doesn't have to be generic

public static List<SelectListItem> GetEnumList(Enum value)
{
    return Enum.GetValues(value.GetType())
            .Cast<Enum>()
            .Select(v => new SelectListItem
                        {
                            Text = v.ToString(),
                            Value = Convert.ToInt32(v).ToString()
                        })
            .ToList();
}

here still can be a problem, if underlying type of enum was not int:

public enum LongEnum: ulong
{
    A = 0,
    B = 3000000000
}

// Run-time exception: Value was either too large or too small for an Int32.

Answer 5

This is what works for me, Extention method :

public static List<IdNameDto> ToIdNameList<TEnum>(this TEnum @enum) where TEnum : Enum
{
    return Enum.GetValues(typeof(TEnum)).Cast<TEnum>()
        .Select(x => new IdNameDto
        {
            Id = Convert.ToInt32(x),
            Name = x.ToString(),
        }).ToList();
}

dto :

public class IdNameDto
{
    public int Id { get; set; }
    public string Name { get; set; }
}