当x不是变量而是枚举类型时，如何将typeof（x）中的x作为方法参数传递

Question

I have the following enum

public enum StatusEnum
{
    Open= 1,
    SemiOpen = 2,
    Closed= 3
}

I pass it in my ASP.NET 5 view to my custom HTML Helper

@Html.EnumDropDownListFor(m => m.SwitchStatus, typeof (StatusEnum), "- Please select Item -")

which is a method that uses Generic enum as parameter

  public static IHtmlContent EnumDropDownListFor<TModel, TResult, TEnum>(
        this IHtmlHelper<TModel> htmlHelper,
        Expression<Func<TModel, TResult>> expression,
        TEnum enumValue,
        string optionLabel) 
    {
        // bunch of logic omitted as not relevant to error
        //calling another method passing the TEnum
        return null;
    }

which works.

However I need to pass the enum type between a number of methods as I need to treat it as an TEnum and not a Type which typeof will pass.

From the above I add a where clause to tell the method the TEnum is an enum.

  public static IHtmlContent EnumDropDownListFor<TModel, TResult, TEnum>(
        this IHtmlHelper<TModel> htmlHelper,
        Expression<Func<TModel, TResult>> expression,
        TEnum enumValue,
        string optionLabel) where TEnum : struct, IConvertible, IFormattable
    {
        // bunch of logic omitted as not relevant to error
        //calling another method passing the TEnum
        return null;
    }

However then I get a red line in my view under

@Html.EnumDropDownListFor 

in Visual Studio

showing the error message

Error CS0453 The type 'Type' must be a non-nullable value type in order to use it as parameter 'TEnum' in the generic type or method EnumDropDownListFor

Basically as I understand it I need to pass it a TEnum and not a Type

However if I remove the typeof from my line

 @Html.EnumDropDownListFor(m => m.SwitchStatus, StatusEnum, "- Please select Item -")

I understandably get

StatusEnum is a type which is not valid in the given context

What I basically want to do is do is call typeof inside the method like this

public static IHtmlContent EnumDropDownListFor<TModel, TResult, TEnum>(
        this IHtmlHelper<TModel> htmlHelper,
        Expression<Func<TModel, TResult>> expression,
        TEnum enumValue,
        string optionLabel) where TEnum : struct, IConvertible, IFormattable
    {
       Type enumValue2 = typeof(StatusEnum);
        // bunch of logic omitted as not relevant to error
        //calling another method passing the TEnum
        return null;
    }

and for that I need the MVC View to look something like

 @Html.EnumDropDownListFor(m => m.SwitchStatus, Before_typeof(StatusEnum), "- Please select Item -")

so I am allowed to pass it and do the typeof in the method.

Is this possible?

---

Take note: I can get it working with the non generic version

ie

    public static IHtmlContent EnumDropDownListFor<TModel, TResult>(
         this IHtmlHelper<TModel> htmlHelper,
         Expression<Func<TModel, TResult>> expression,
         Type enumValue,
         string optionLabel)
    {
        //logic
        return null;

    }

which works perfectly

I just wanted to know if the generic version is possible somehow

Answer 1

When you pass in typeof (StatusEnum) as second parameter, the TEnum becomes a Type , which isn't what you want. It seems to me you want to put in the value of an enum in there, since the field is named enumValue . So you should pass in StatusEnum.Okay , or something like that, not a type.

You already have the type from the type parameter, so if it is to do about the type, you don't need to pass in that parameter at all.