appel1

using System.Collections.Generic;
using System.Linq;
using System.IO;
using NUnit.Framework;

public class TestFixture
{
  public static IEnumerable<string> FormatFactory()
  {
    yield return "Some string {0}";

appel1

public static class ExcelReaderSheetNames
{
  public static IEnumerable<string> ReadSheetNames(Strema stream)
  {
    using (var r = ExcelReaderFactory.CreateReader(stream)
    {
      do 
      {
        yield return r.Name;
      }