1         public static IEnumerable<T> Sequence<T>(Func<T, T> getNext, T startVal, Func<T, bool> endReached)
 2         {
 3             if (getNext == null)
 4                 yield break;
 5             yield return startVal;
 6             T val = startVal;
 7             while (endReached == null || !endReached(val))
 8             {
 9                 val = getNext(val);
10                 yield return val;
11             }
12         }

        static void Main(string[] args){var oddNumbersForm1 = Sequence<int>(x => x + 2, 1, x => x >= 20);foreach (int x in oddNumbersForm1){Console.WriteLine(x);}Console.ReadKey();}

 1         public class Range<T> : IEnumerable<T>
 2         {
 3             private T start;
 4             private T end;
 5             private Comparison<T> compare;
 6             private IEnumerable<T> sequence;
 7
 8             private static int Compare<U>(U one,U other)
 9             {
10                 return Comparer<U>.Default.Compare(one,other);
11             }
12
13             public Range(T start, T end, Func<T, T> getNext, Comparison<T> compare)
14             {
15                 this.start = start;
16                 this.end = end;
17                 this.compare = compare;
18                 this.sequence = Sequence<T>(getNext, start, v => compare(getNext(v), end) > 0);
19             }
20
21             public Range(T start, T end, Func<T, T> getNext)
22                 : this(start, end, getNext, Compare) { }
23
24             public IEnumerator<T> GetEnumerator()
25             {
26                 return sequence.GetEnumerator();
27             }
28
29             System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
30             {
31                 return ((IEnumerable<T>)this).GetEnumerator();
32             }
33
34             public bool Contains(T value)
35             {
36                 return compare(value, start) >= 0 && compare(end, value) >= 0;
37             }
38         }

1 foreach(int x in CFunc(10,TFunc)){
2     Console.WriteLine(x);
3 }