Access Enum custom attributes whilst looping over values

Question

I have added a custom attribute to some enum values (to give them a screen friendly string value). I am trying to build a list of SelectListItems for use on an MVC page, but I am running into trouble accessing the custom attribute.

My enum looks like this.

public enum MyEnum
{
   [StringValue("None")] None = 0,
   [StringValue("First Value")] FirstValue = 1,
   [StringValue("Second Value")] SecondValue = 2
}

The attribute looks like this.

public class StringValueAttribute : Attribute 
{
    public StringValueAttribute(string value)
    {
        this.StringValue = value;
    }

    public string StringValue { get; protected set; }        
}

I created a helper class so that I can easily access the StringValue attribute from an instance of the Enum.

public static string GetStringValue(this Enum value)
{
    Type type = value.GetType();
    FieldInfo fieldInfo = type.GetField(value.ToString());
    StringValueAttribute[] attribs = fieldInfo.GetCustomAttributes(typeof(StringValueAttribute), false) as StringValueAttribute[];
    return attribs != null && attribs.Length > 0 ? attribs[0].StringValue : null;
}

I could call it like this.

MyEnum test = MyEnum.FirstValue;
string stringValue = test.GetStringValue();

Finally, onto the code I am stuck with. I can loop over the Enum Values easily, but the values are not instances of MyEnum so I cannot call my helper function. And when I try to access the FieldInfo it always returns null. Here is what I have so far.

public static List<SelectListItem> GetFlagsSelectList<T>(int? selectedValue)
{
    List<SelectListItem> items = new List<SelectListItem>();
    foreach (int value in Enum.GetValues(typeof(T)))
    {
        items.Add(new SelectListItem
        {
            Text = Enum.GetName(typeof(T), value), 
            Value = value.ToString(), 
            Selected = selectedValue.HasValue && selectedValue.Value == value
        });
    }
    return items;
}

Is it possible to access the custom attribute within the foreach loop?

EDIT:

I think I asked this unclearly. I would like to access the custom attribute inside the foreach loop. Calling Enum.GetName(typeof(T), value) simply returns the name of the property (e.g. FirstValue) which I do NOT want.

I would like to do something like:

foreach (int value in Enum.GetValues(typeof(T)))
{
    string name = Enum.ToObject(typeof (T), value).GetStringValue();
}

But T could be any type so I can't call my GetStringValue() method there.

I have tried doing this:

foreach (int value in Enum.GetValues(typeof(T)))
{
    FieldInfo fieldInfo = typeof(T).GetField(value.ToString());
    StringValueAttribute[] attribs = fieldInfo.GetCustomAttributes(typeof(StringValueAttribute), false) as StringValueAttribute[];
    string name = attribs != null && attribs.Length > 0 ? attribs[0].StringValue : Enum.GetName(typeof(T), value),;

    items.Add(new SelectListItem
    {
        Text = name,
        Value = value.ToString(),
        Selected = selectedValue.HasValue && selectedValue.Value == value
    });
}

But I always get an exception because the FieldInfo object always returns null.

Answer 1

Try

static string GetStringValue2(Enum value) {
 ....
}

public static List<SelectListItem> GetFlagsSelectList<T>(int? selectedValue) where T : struct {
  var items = new List<SelectListItem>();
  foreach (T value in Enum.GetValues(typeof(T))) {
    var stringValue = GetStringValue2((Enum)(object)value);
    items.Add(new SelectListItem {
      Text = Enum.GetName(typeof(T), value),
      Value = Convert.ToInt32(value).ToString(),
      Selected = selectedValue.HasValue && selectedValue.Value == Convert.ToInt32(value)
    });
  }
  return items;
}

Answer 2

I wrote a blog post about this a while back (for the XmlEnumAttribute, but the same applies here).

public static string ConvertToString(Enum e)
{
  // Get the Type of the enum
  Type t = e.GetType();

  // Get the FieldInfo for the member field with the enums name
  FieldInfo info = t.GetField(e.ToString("G"));

  // Check to see if the XmlEnumAttribute is defined on this field
  if (!info.IsDefined(typeof(XmlEnumAttribute), false))
  {
    // If no XmlEnumAttribute then return the string version of the enum.
    return e.ToString("G");
  }

  // Get the XmlEnumAttribute
  object[] o = info.GetCustomAttributes(typeof(XmlEnumAttribute), false);
  XmlEnumAttribute att = (XmlEnumAttribute)o[0];
  return att.Name;
}

Hope that helps.