public static  class ConvertExt
    {

      /// <summary>
        /// NULL 處理、替換
      /// </summary>
      public static string ReplaceExt(this string obj,string oldStr,object newObj)
      { 
          //檢查要替換的新內容是否為空
          string newStr = newObj == null ? string.Empty : (newObj != DBNull.Value ? newObj.ToString() : string.Empty);

          return obj.Replace(oldStr,newStr);
      }