How to pass the x in typeof(x) as a method parameter when x is not a variable but an enum type

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

i.e.

    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

possible duplicate http://stackoverflow.com/questions/4656758/mvc3-razor-dropdownlistfor-enums — lordkain, Nov 18 '15 at 08:17

score 0 · Answer 1 · answered Nov 18 '15 at 08:17

0

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.

answered Nov 18 '15 at 08:17

Patrick Hofman

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I have updated my post with an example of passing the "type" working. I just wanted to know if it is possible using TEnum generics instead – zoaz Nov 18 '15 at 08:36
The non generic way yes. Using TEnum in combination with the Where clause No. That is all in my original question – zoaz Nov 18 '15 at 08:42
But what is the purpose of the `where`? The `TEnum` must be a `struct`, etc.? While you pass in a `Type`? – Patrick Hofman Nov 18 '15 at 08:43
See this thread. http://stackoverflow.com/questions/33755630/cannot-convert-type-tenum-to-int – zoaz Nov 18 '15 at 08:45

How to pass the x in typeof(x) as a method parameter when x is not a variable but an enum type

1 Answers1