對於列表<T>。「添加

Question

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「對象引用未設置爲對象的實例」在我的方法中，我有多個過濾數據和返回對象的語句。因爲我想用視圖模型服務視圖，我實例化一個列表，然後將其傳遞到視圖中...

簡化示例。

List<viewModel> returnedViewModel = new List<viewModel>(); 

    foreach (var item in filteredData) 
    { 
     returnedViewModel.Add(// this line is throwing error 
     new viewModel 
     { 
      data = item, 
     }); 
    } 

問題是我得到「對象引用未設置爲對象的實例」。爲其中一個特定的filteredData對象返回了viewModel.Add，而其他則不會導致該問題。

我是在解決了這個問題的可能來源之後。

我使用EF並在我的connectionString中有一個MARS語句。這可能是一個問題嗎？

全statment：

public ActionResult RenderPartialSearchableEventStacks(string viewType, 
                 string orderColumn, 
                 string sortOrder, 
                 int? stackNumber, 
                 string adminName, 
                 string clientName, 
                 string stackType, 
                 string stackStatus, 
                 int? adminId = null) 
{ 
    IEnumerable<blsEventStack> eventStacks = unitOfWork.EventStackRepository.Filter(n => (stackNumber == null) || n.EventStackId == stackNumber, 
                        n => (String.IsNullOrEmpty(adminName)) || n.ManagingAdminName.Contains(adminName), 
                        n => (String.IsNullOrEmpty(clientName)) || n.RelatedClientName.Contains(clientName), 
                        n => (String.IsNullOrEmpty(stackType)) || n.EventStackType.Contains(stackType), 
                        n => (String.IsNullOrEmpty(stackStatus)) || n.EventStackLastEventStatus.Contains(stackStatus)); 

    IEnumerable<blsEventStack> viewTypeRelatedEventStacks = eventStacks; 

    if (viewType == "allEventStacks") 
    { 
     viewTypeRelatedEventStacks = unitOfWork.EventStackRepository.GetAll(); 
    } 

    if (viewType == "assignedEventStacks") 
    { 
     // Needed to filter out results with empty (unassigned) admin name 
     viewTypeRelatedEventStacks = from x in eventStacks 
            where x.ManagingAdminName != null 
            select x; 
    } 

    if (viewType == "singleAdminAssignedEventStacks") 
    { 
     string singleAdminName; 

     if (adminId != null) 
     { 
      // Gets admin name using adminId 
      singleAdminName = unitOfWork.AdminRepository.GetById(adminId).AdminName; 
     } 
     else 
     { 
      MembershipUser admin = Membership.GetUser(); 

      blsAdmin loggedInAdmin = (from x in unitOfWork.AdminRepository.GetAll() 
             where x.AdminEmail == admin.Email 
             select x).FirstOrDefault(); 

      singleAdminName = loggedInAdmin.AdminName; 

      ViewBag.adminView = true; 
     } 

     // Gets all eventStacks that belong to a give adminName 
     viewTypeRelatedEventStacks = from x in eventStacks 
            where x.ManagingAdminName == singleAdminName 
            select x; 

     ViewBag.adminId = adminId; 
    } 

    IEnumerable<blsEventStack> returnedEventStacks = viewTypeRelatedEventStacks; 

    if (orderColumn == "stack") 
    { 
     if (sortOrder == "descendingStack") 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.EventStackId descending 
            select x; 
      sortOrder = null; 
     } 
     else 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            select x; 
      sortOrder = "descendingStack"; 
     } 
    } 

    if (orderColumn == "managingAdmin") 
    { 
     if (sortOrder == "descendingManagingAdmin") 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.ManagingAdminName descending 
            select x; 
      sortOrder = null; 
     } 
     else 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.ManagingAdminName 
            select x; 
      sortOrder = "descendingManagingAdmin"; 
     } 
    } 

    if (orderColumn == "relatedClient") 
    { 
     if (sortOrder == "descendingRelatedClient") 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.RelatedClientName descending 
            select x; 
      sortOrder = null; 
     } 
     else 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.RelatedClientName 
            select x; 
      sortOrder = "descendingRelatedClient"; 
     } 
    } 

    if (orderColumn == "stackType") 
    { 
     if (sortOrder == "descendingStackType") 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.EventStackType descending 
            select x; 
      sortOrder = null; 
     } 
     else 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.EventStackType 
            select x; 
      sortOrder = "descendingStackType"; 
     } 
    } 

    if (orderColumn == "latestEventTime") 
    { 
     if (sortOrder == "descendingLatestEventTime") 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.EventStackLastEventTime descending 
            select x; 
      sortOrder = null; 
     } 
     else 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.EventStackLastEventTime 
            select x; 
      sortOrder = "descendingLatestEventTime"; 
     } 
    } 

    if (orderColumn == "latestEventStatus") 
    { 
     if (sortOrder == "descendingLatestEventStatus") 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.EventStackLastEventStatus descending 
            select x; 
      sortOrder = null; 
     } 
     else 
     { 
      returnedEventStacks = from x in viewTypeRelatedEventStacks 
            orderby x.EventStackLastEventStatus 
            select x; 
      sortOrder = "descendingLatestEventStatus"; 
     } 
    } 

    List<ViewModelAllEventStacks> returnedViewModel = new List<ViewModelAllEventStacks>(); 

    foreach (var item in returnedEventStacks) 
    { 
     returnedViewModel.Add( 
     new ViewModelAllEventStacks 
     { 
      EventStack = item, 
      AdminId = item.AdminEventLogs.FirstOrDefault().AdminId, 
      ClientId = item.ClientEventLogs.FirstOrDefault().ClientId 
     }); 
    } 

    ViewBag.stackNumber = stackNumber; 
    ViewBag.adminName = adminName; 
    ViewBag.clientName = clientName; 
    ViewBag.stackType = stackType; 
    ViewBag.stackStatus = stackStatus; 

    // ViewBag passing state of existing order 
    ViewBag.sortOrder = sortOrder; 

    // ViewBag passing view data (ex. assignedEventStacks, allEventStacks, etc.) 
    ViewBag.viewType = viewType; 

    return PartialView("BLS_AllEventStacks", returnedViewModel); 
} 

Answer 1

有4個可能的原因：

1. 你在調試代碼是不是出了什麼實際運行和已過時。
2. 構造函數viewModel中存在空例外。
3. data屬性中發生空例外情況。
4. 正如評論中指出的那樣，有一些代碼被過濾出來，實際上是將列表設置爲null。

基於編輯的問題

returnedViewModel.Add( 
    new ViewModelAllEventStacks 
    { 
     EventStack = item, 
     AdminId = item.AdminEventLogs.FirstOrDefault().AdminId, 
     ClientId = item.ClientEventLogs.FirstOrDefault().ClientId 
    }); 

如果沒有AdminEventLogs或ClientEventLogs這將引發空異常錯誤。也有可能，但不太可能，item爲空。

Answer 2

我相信這兩條線是問題所在。當您使用item.AdminEventLogs.FirstOrDefault()時，如果item.AdminEventLogs沒有任何元素，則它將爲空，因此訪問.AdminId將引發錯誤。同樣的道理也適用於item.ClientEventLogs.FirstOrDefault()

AdminId = item.AdminEventLogs.FirstOrDefault().AdminId, 
ClientId = item.ClientEventLogs.FirstOrDefault().ClientId 

您可以使用.Any()檢查item.AdminEventLogs和item.ClientEventLogs是否有任何元素。如果是這樣，使用的第一個元件的AdminId和ClientId，否則規定AdminId和ClientId爲0（假設AdminId和ClientId是整數）

AdminId = item.AdminEventLogs.Any() ? item.AdminEventLogs.First().AdminId : 0, 
ClientId = item.ClientEventLogs.Any() ? item.ClientEventLogs.First().ClientId : 0