ToTestFile.cs

public static class TestUtilities
{
  private static ConcurrentDictionary<string, TestFile> cache = new  ConcurrentDictionary<string, TestFile>();
  
  public static TestFile ToTestFile(this string fileName)
  {
    TestFile result;
    if (!cache.TryGetValue(fileName, out result))
    {
      result = new TestFile(cache);
      cache[testFile] = result;
    }
    return result;
  }
}