Know if TResult is empty collection

Question

I have this code

public static class FallbackUtils<TResult, TRequest>
      where TRequest : IFallbackRequest
   {
      public static IEnumerable<TResult> GetAllValues(TRequest request,
         Func<TRequest, TResult> callback)          
      {
         yield return callback(request);

         while (request.CanFallback())
         {
            request.Fallback();
            yield return callback(request);
         }
      }  
   }

Sometimes TResult is going to be an IEnumerable. In those cases I want to be able to keep not only null but empty collections from being returned. The problem is that I don't know how to ask TResult if it's empty I know only how to ask if it's null. Something like this

public static IEnumerable<TResult> GetAllValues(TRequest request,
         Func<TRequest, TResult> callback, bool ignoreNullOrEmpty = false)          
      {
         var item = callback(request);

         if (ignoreNullOrEmpty)
         {
            if (item != null && item.Any())
               yield return item;
         }
         else
            yield return item;

         while (request.CanFallback())
         {
            request.Fallback();

            item = callback(request);
            if (ignoreNullOrEmpty)
            {
               if (item != null && item.Any())
                  yield return item;
            }
            else
               yield return item;
         }
      }

I want to do the filtering inside this method to clean up a lot of duplicate code that is doing this same thing.

Any ideas of what should I do or what I'm doing wrong? Thanks in advance.

Edin · Answer 1 · 2014-05-26T23:31:28.670

If you need to know if your TResult is IEnumerable, then you just need try to cast to it. Afterwards you can handle the empty collection and null values as follows:

// is it an IEnumerable?
var item = callback(request);
var enumerable = item as IEnumerable;
if(enumerable != null) 
{
  var enumerator = enumerable.GetEnumerator();
  if(!enumerator.MoveNext())
  {
     // handle an empty collection here
  }
  else
  {
     do 
     {
        if(enumerator.Current == null)
          // handle the null value here
     } while(enumerator.MoveNext());
  }
}

Since you probably do not know the type of the items being inside the IEnumerable, you cannot use IEnumerable<T> and Linq extensions.

This is the best answer here. – Xcelled May 26 '14 at 23:26 — Xcelled, May 26 '14 at 23:26

Selman Genç · Accepted Answer · 2014-05-26T23:36:37.033

2

Well there is no Any method on IEnumerable, you could write your own method:

public static bool Any(this IEnumerable source)
{
    var enumerator = source.GetEnumerator();

    return enumerator.MoveNext();
}

Then you can call it:

if (item != null && 
    item is IEnumerable && 
    ((IEnumerable)item).Any())

edited May 26 '14 at 23:36

answered May 26 '14 at 23:17

Selman Genç

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@Chorinator be warned, this will NOT work for an IEnumerable since they're two different interfaces. – Xcelled May 27 '14 at 01:25
@Xcelled194 you are **wrong**. it will work for IEnumerable because it inherits from IEnumerable. – Selman Genç May 27 '14 at 01:29
Whoops, forgot about that. – Xcelled May 27 '14 at 03:54

Know if TResult is empty collection

2 Answers2