持續更新的.NET實用擴展方法，具體內容如下

1. 字符串轉換為可空數值類型(int, long, float...類似)

  /// <summary>  /// 將字符串轉換成32位整數,轉換失敗返回null  /// </summary>  /// <param name="str">轉換的字符串</param>  /// <returns>轉換之后的整數,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;  }

2. 去除子字符串

  /// <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結果</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結果</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;  }

3. 獲取子序列

  /// <summary>  /// 獲取子序列  /// </summary>  /// <typeparam name="T">序列中元素類型</typeparam>  /// <param name="source">源數據</param>  /// <param name="startIndex">開始索引(返回時包括)</param>  /// <param name="endIndex">結束索引(返回時包括)</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;    }  }

4. 通過指定鍵對序列去重, 不必實現IEqualityComparer接口

  /// <summary>  /// 通過對指定的值進行比較返回序列中的非重復元素。  /// </summary>  /// <typeparam name="T">序列中元素類型</typeparam>  /// <typeparam name="TResult">指定的比較屬性類型</typeparam>  /// <param name="source">源數據</param>  /// <param name="selector">應用于每個元素的轉換函數</param>  /// <returns>一個包含源序列中的按指定屬性非重復元素</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;      }    }  }