Unable to use Enum extension method with generic

Question

I know generics are done at compiled time, but I am getting confused with the way generics work (and I though I knew generics..).

I have created the following extension method:

public static class EnumExt
{
    /// <summary>
    /// Gets the description, if any, or the name of the enum as a string in a enum type
    /// </summary>
    public static string GetDescription<T>(this T enumType) where T : struct, IConvertible
    {
        FieldInfo fieldInfo = enumType.GetType().GetField(enumType.ToString());
        DescriptionAttribute[] descriptionAttributes = (DescriptionAttribute[])
            fieldInfo.GetCustomAttributes(typeof(DescriptionAttribute), false);
        if (descriptionAttributes.Length > 0)
        {
            return descriptionAttributes[0].Description;
        }
        return enumType.ToString();
    }
}

I have as an example the following enum

namespace MyProject.Model
{
    [Flags]
    public enum MyEnumType
    {
        [Description("None")]
        None = 0,
        [Description("Show Products (default)")]
        Products  = 1,
        [Description("Show Tariffs")]
        Tariffs = 2
    }
}

And now I'd like to use it from an HttpHelper in MVC that returns a string (html text) like this one. Please note My class has access to my EnumExt methods.

    public static IHtmlString CheckBoxesForEnumModel<TModel>(this HtmlHelper<TModel> htmlHelper)
    {
        if (!typeof(TModel).IsEnum)
        {
            throw new ArgumentException("this helper can only be used with enums");
        }
        TModel[] allEnumValues = (TModel[])Enum.GetValues(typeof(TModel));
        foreach (TModel item in allEnumValues)
        {
            var descErr = item.GetDescription(); //does not compile, but I know it's a MyEnumType.Tariffs..
            var descOk = MyEnumType.Tariffs.GetDescription(); //this line works
            //descOk = "Show Tariffs"
        }

        return new HtmlString("ideally this is some html checkboxes with each enum description");
    }

I know I can get all the Enum values and iterate through them using the TModel like this:

TModel[] allEnumValues = (TModel[])Enum.GetValues(typeof(TModel));

but if I know a TModel is an enum (it's a MyEnumType), why can't I use it to access the enum extension method like:

allValues[0].GetDescription<>(); //ERROR. this does not compile

I guess it's because somehow I must cast it to a specific type like MyEnumType, but how to do so and keep it generic?

Thanks in advance!

UPDATE : Thanks to the first answers I was able to compile by restricting TModel to struct, IConvertible

Answer 1

Because your extension method is defined for T where T : struct, IConvertible .

But TModel in CheckBoxesForEnumModel doesn't conform to these generic type constraints.

You should change the signature from

public static IHtmlString CheckBoxesForEnumModel<TModel>(this HtmlHelper<TModel> htmlHelper)

to

public static IHtmlString CheckBoxesForEnumModel<TModel>(this HtmlHelper<TModel> htmlHelper)
    where TModel : struct, IConvertible

or more restrictive.

Answer 2

You need a constraint for your method. Do,

public static IHtmlString CheckBoxesForEnumModel<TModel>(this HtmlHelper<TModel> htmlHelper) where TModel : struct, IConvertible
{
   if (!typeof(TModel).IsEnum)
   {
      throw new ArgumentException("this helper can only be used with enums");
   }

   //Here some code to get all the values and the names for this Enum        
   //But HOW?? 

   return new HtmlString("ideally this is some html checkboxes with each enum description");
}

I hope the reason is obvious.

Answer 3

You created an extension method for your enum based on T where T is struct an implements IConvertible .

But in your HtmlHelper extension method your TModel does not have the same constraints, so there is not way that compiler can associate your enum extension method based on a type that is a struct and IConvertible with your TModel which is just a type.

Adding the same constraints to your HtmlHelper method will do the trick:

public static IHtmlString CheckBoxesForEnumModel<TModel>(this HtmlHelper<TModel> htmlHelper) where TModel : struct, IConvertible
{
    string description = htmlHelper.ViewData.Model.GetDescription();
}