A System.String extension method Substring

Almost every time I use String.Substring() I have to do a few IndexOf calls to find the "startIndex" en "length" to use. This includes declaring local variables and verifying that the index found is not smaller than zero. This extension method solves these issues. (Long live C# 3.0!)

public static string Substring(this String str, string startText, string endText, StringComparison comparisonType)
{
  string lsResult = "";
  Int32 liStartPos = str.IndexOf(startText, comparisonType);
  if (liStartPos >= 0) {
    liStartPos = liStartPos + startText.Length;
    Int32 liEndPos = str.IndexOf(endText, liStartPos, comparisonType);
    if (liEndPos >= 0) {
      lsResult = str.Substring(liStartPos, liEndPos - liStartPos);
    }
  }
  return lsResult;
}

public static string Substring(this String str, string startText, string endText)
{
  string lsResult = "";
  Int32 liStartPos = str.IndexOf(startText, StringComparison.CurrentCulture);
  if (liStartPos >= 0) {
    liStartPos = liStartPos + startText.Length;
    Int32 liEndPos = str.IndexOf(endText, liStartPos, StringComparison.CurrentCulture);
    if (liEndPos >= 0) {
      lsResult = str.Substring(liStartPos, liEndPos - liStartPos);
    }
  }
  return lsResult;
}

I wasn't sure if overloading existing methods was alowed, but it works.