Why can't iterator methods take either 'ref' or 'out' parameters?

Question

I tried this earlier today:

public interface IFoo
{
    IEnumerable GetItems_A( ref int somethingElse );
    IEnumerable GetItems_B( ref in         


        

Answer 1

I've gotten around this problem using functions, when the value that I need to return is derived from the iterated items:

// One of the problems with Enumerable.Count() is
// that it is a 'terminator', meaning that it will
// execute the expression it is given, and discard
// the resulting sequence. To count the number of
// items in a sequence without discarding it, we 
// can use this variant that takes an Action
// (or Action), invokes it and passes it the
// number of items that were yielded.
//
// Example: This example allows us to find out
//          how many items were in the original
//          source sequence 'items', as well as
//          the number of items consumed by the
//          call to Sum(), without causing any 
//          LINQ expressions involved to execute
//          multiple times.
// 
//   int start = 0;    // the number of items from the original source
//   int finished = 0; // the number of items in the resulting sequence
//
//   IEnumerable> items = // assumed to be an iterator
//
//   var result = items.Count( i => start = i )
//                   .Where( p => p.Key = "Banana" )
//                      .Select( p => p.Value )
//                         .Count( i => finished = i )
//                            .Sum();
//
//   // by getting the count of items operated 
//   // on by Sum(), we can calculate an average:
// 
//   double average = result / (double) finished; 
//
//   Console.WriteLine( "started with {0} items", start );
//   Console.WriteLine( "finished with {0} items", finished );
//

public static IEnumerable Count( 
    this IEnumerable source, 
    Action receiver )
{
  int i = 0;
  foreach( T item in source )
  {
    yield return item;
    ++i ;
  }
  receiver( i );
}

public static IEnumerable Count( 
    this IEnumerable source, 
    Action receiver )
{
  long i = 0;
  foreach( T item in source )
  {
    yield return item;
    ++i ;
  }
  receiver( i );
}