随机总是给出相同的值

Question

我正在创建一个简单的类来生成假的lorem ipsum文本，所以我有：

public static class LoremIpsumGenerator {

  private static Random _random = new Random();

  private static String _lorem = Constants.LoremIpsumText;
  private static IEnumerable<String> _words = _lorem.ToWordList();

  public static String GetText(Int32 totalLength = 200, Int32? paragraphLength = null) {

  IList<String> words = _words.OrderBy(x => Guid.NewGuid()).ToList();

  String text = String.Empty;
  String paragraph = _lorem.Substring(0, 12);

  Int32 index = 0;

  while (text.Length < totalLength) {

    //Int32 a = (paragraphLength == null ? _random.Next(20, 80) : paragraphLength.Value);

    Int32 limit = _random.Next(20, 80);

    while (paragraph.Length < limit) {

      paragraph += (String.IsNullOrEmpty(paragraph) ? words[index].ToFirstLetterUppercase() : words[index].ToLower()) + " ";        

      index++;

      if (index > words.Count)
        index = 0;

    }

    paragraph = paragraph.TrimEnd();
    text += paragraph + ". ";
    paragraph = String.Empty;

  }

  return text;

  } // GetText

} // LoremIpsumGenerator

问题在于：

Int32 l = (paragraphLength == null ? _random.Next(20, 80) : paragraphLength.Value);

它给了我几乎总是相同的价值......例如我得到了：

21, 21, 47, 47, 47, 47, 47, 47, 47, 47, 47
or 
50, 50, 50, 50, 50, 50, 50, 50, 50, 50, 50

有谁知道如何防止这种情况？

更新

public static class LoremIpsumGenerator {

  private static Random _random = new Random();

  private static String _lorem = Constants.LoremIpsumText;
  private static IEnumerable<String> _words = _lorem.ToWordList();

  public static String GetText(Int32 totalLength = 200, Int32? paragraphLength = null) {

  IList<String> words = _words.OrderBy(x => Guid.NewGuid()).ToList();

  String text = String.Empty;
  String paragraph = _lorem.Substring(0, 12);

  Int32 index = 0;

  while (text.Length < totalLength) {

    Int32 limit = _random.Next(20, 80);

    while (paragraph.Length < limit) {

      paragraph += (String.IsNullOrEmpty(paragraph) ? words[index].ToFirstLetterUppercase() : words[index].ToLower()) + " ";        

      index++;

      if (index > words.Count)
        index = 0;

    }

    paragraph = paragraph.TrimEnd();
    text += paragraph + ". ";
    paragraph = String.Empty;

  }

  } // GetText

} // LoremIpsumGenerator