yield keyword

using System;
using System.Collections;
using System.Collections.Generic;


namespace YieldDemo
{
   public static class YieldDemo
   {
       ///
       /// Filtrarea unui sir de stringuri cu yield
       ///
       ///
       ///
       public static IEnumerable NamesWitchStartsWithALetter(string[] names)
       {
           foreach ( string name in names )
               if ( name.StartsWith ( "A" ) )
                   yield return name;
       }

       ///
       /// crearea unui iterator pentru un numar limitat de patrate perfect
       ///
       ///
       ///
       public static IEnumerable<int> Squares(int squaresCount)
       {
           int sqCounter = 1;
           while (sqCounter <= squaresCount)        
           {                  
              yield return sqCounter * sqCounter;
              sqCounter++;
           }
       }
        /// <summary>
        /// iterator pentru primele n numere din sirul Fibonacci
        /// </summary>
        /// <param name="fibLength"></param>
        /// <returns></returns>
        public static IEnumerable<int> Fibonacci ( int fibLength )
        {
            if ( fibLength < 0 )
                yield break;

            if ( fibLength == 0 )
            {
                yield return 1;
                yield break;
            }
            if ( fibLength == 1 )
            {
                yield return 1;
                yield return 1;
                yield break;
            }

            int first = 1;
            int second = 1;
            int counter = 1;
            yield return 1;
            yield return 1;
            while ( counter++ < fibLength )
            {
                int retValue = first + second;
                first = second;
                second = retValue;
                yield return retValue;
            }
        }
    }
}