Question

Here is the full implementation I am considering:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web.Mvc;

namespace Utils {
    public static class IDictionaryExt {
        public static IEnumerable<SelectListItem> ToSelectListItems<T, R>(this IDictionary<T, R> dic, T selectedKey) {
            return dic.Select(x => new SelectListItem() { Text = x.Value.ToString(), Value = x.Key.ToString(), Selected=(dynamic)x.Key == (dynamic)selectedKey });
        }
    }
}

Notice the equality check using the dynamic casts: (dynamic)x.Key == (dynamic)selectedKey. Is this the best way to check equality between selectedKey and x.Key here? Based on @Gabe’s comment in Operator '==' can't be applied to type T?, I believe it is: overload resolution is deferred to to runtime, but we do get “normal” overload resolution (i.e. considering ValueTypes and other Objects with == overloads versus Objects with default reference equality).

Answer 1

The best way to handle this situation is to use EqualityComparer<T>.Default

return dic.Select(x => new SelectListItem() { Text = x.Value.ToString(), Value = x.Key.ToString(), Selected= EqualityComparer<T>.Default.Equals(x.Key, selectedKey) });