/// <summary>
  /// 将字符串转换成32位整数,转换失败返回null
  /// </summary>
  /// <param name="str">转换的字符串</param>
  /// <returns>转换之后的整数,或null</returns>
  public static int? TryParseToInt32(this string str)
  {
    if (string.IsNullOrWhiteSpace(str))
      return null;
    var result = 0;
    if (int.TryParse(str, out result))
      return result;
    else
      return null;
  }

  /// <summary>
  /// 去除子字符串
  /// </summary>
  /// <param name="str">原字符串</param>
  /// <param name="substring">要去除的字符串</param>
  /// <returns>去除子字符串之后的结果</returns>
  public static string DeSubstring(this string str, string substring)
  {
    if (string.IsNullOrEmpty(str) || string.IsNullOrEmpty(substring) || !str.Contains(substring))
    {
      return str;
    }

    return Regex.Replace(str, Regex.Escape(substring), string.Empty);
  }

  /// <summary>
  /// 去除子字符串
  /// </summary>
  /// <param name="str">原字符串</param>
  /// <param name="substrings">要去除的子字符串</param>
  /// <returns>去除子字符串之后的结果</returns>
  public static string DeSubstring(this string str, params string[] substrings)
  {
    if (string.IsNullOrEmpty(str))
      return str;
    if (substrings == null)
      return str;
    var newStr = str;
    foreach (var item in substrings)
    {
      newStr = newStr.DeSubstring(item);
    }
    return newStr;
  }

  /// <summary>
  /// 获取子序列
  /// </summary>
  /// <typeparam name="T">序列中元素类型</typeparam>
  /// <param name="source">源数据</param>
  /// <param name="startIndex">开始索引(返回时包括)</param>
  /// <param name="endIndex">结束索引(返回时包括)</param>
  /// <returns>子序列</returns>
  public static IEnumerable<T> SubEnumerable<T>(this IEnumerable<T> source, int startIndex, int endIndex)
  {
    if (source == null)
      yield return default(T);
    var length = source.Count();
    if (startIndex < 0 || endIndex < startIndex || startIndex >= length || endIndex >= length)
      throw new ArgumentOutOfRangeException();

    var index = -1;
    foreach (var item in source)
    {
      index++;
      if (index < startIndex)
        continue;
      if (index > endIndex)
        yield break;
      yield return item;
    }
  }

  /// <summary>
  /// 通过对指定的值进行比较返回序列中的非重复元素。
  /// </summary>
  /// <typeparam name="T">序列中元素类型</typeparam>
  /// <typeparam name="TResult">指定的比较属性类型</typeparam>
  /// <param name="source">源数据</param>
  /// <param name="selector">应用于每个元素的转换函数</param>
  /// <returns>一个包含源序列中的按指定属性非重复元素</returns>
  public static IEnumerable<T> Distinct<T, TResult>(this IEnumerable<T> source, Func<T, TResult> selector)
  {
    if (source == null)
      throw new ArgumentNullException(nameof(source));
    if (selector == null)
      throw new ArgumentNullException(nameof(selector));
    var set = new HashSet<TResult>();
    foreach (var item in source)
    {
      var result = selector(item);
      if (set.Add(result))
      {
        yield return item;
      }
    }
  }

  /// <summary>
  /// 通过对指定的值进行比较返回序列中重复的元素
  /// </summary>
  /// <typeparam name="T">序列中的数据类型</typeparam>
  /// <typeparam name="TResult">指定的比较属性类型</typeparam>
  /// <param name="source">源数据</param>
  /// <param name="selector">应用于每个元素的转换函数</param>
  /// <returns>一个包含源序列中的按指定元素的重复元素</returns>
  public static IEnumerable<T> Identical<T>(this IEnumerable<T> source)
  {
    if (source == null)
      throw new ArgumentNullException(nameof(source));
    var setT = new HashSet<T>();
    foreach (var item in source)
    {
      if (!setT.Add(item))
      {
        yield return item;
      }
    }
  }

  /// <summary>
  /// 通过对指定的值进行比较返回序列中重复的元素
  /// </summary>
  /// <typeparam name="T">序列中的数据类型</typeparam>
  /// <typeparam name="TResult">指定的比较属性类型</typeparam>
  /// <param name="source">源数据</param>
  /// <param name="selector">应用于每个元素的转换函数</param>
  /// <returns>一个包含源序列中的按指定元素的重复元素</returns>
  public static IEnumerable<T> Identical<T, TResult>(this IEnumerable<T> source, Func<T, TResult> selector)
  {
    if (source == null)
      throw new ArgumentNullException(nameof(source));
    if (selector == null)
      throw new ArgumentNullException(nameof(selector));
    var setTResult = new HashSet<TResult>();
    foreach (var item in source)
    {
      var result = selector(item);
      if (!setTResult.Add(result))
      {
        yield return item;
      }
    }
  }