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

public class Test
{
    private static IEnumerable<T> Swap<T>(IEnumerable<T> source, int smallerIndex, int greaterIndex) {
using (IEnumerator<T> e = source.GetEnumerator()) {
    IList<T> saved = new List<T>(greaterIndex-smallerIndex+1);
    int index = 0;
    while (e.MoveNext()) {
        // If we're outside the swapped indexes, yield return the current element
        if (index < smallerIndex || index > greaterIndex) {
            index++;
            yield return e.Current;
        } else if (index == smallerIndex) {
            var atSmaller = e.Current;
            // Save all elements starting with the current one into a list;
            // Continue until you find the last index, or exhaust the sequence.
            while (index != greaterIndex && e.MoveNext()) {
                saved.Add(e.Current);
                index++;
            }
            // Make sure we're here because we got to the greaterIndex,
            // not because we've exhausted the sequence
            if (index == greaterIndex) {
                // If we are OK, return the element at greaterIndex
                yield return e.Current;
            }
            // Enumerate the saved items
            for (int i = 0 ; i < saved.Count-1 ; i++) {
                yield return saved[i];
            }
            // Finally, return the item at the smallerIndex
            yield return atSmaller;
            index++;
        }
    }
}
    }
	public static void Main()
	{
        var data = new[] {0,1,2,3,4,5,6};
        foreach (var x in Swap(data, 2, 5)) {
            Console.WriteLine(x);
        }
		
	}
}