LINQ實體框架值不能為null。 參數名稱：字符串

Question

我試圖從我的實體數據模型返回視圖模型對象的集合到我的視圖，並得到以下錯誤：

值不能為空。 參數名稱：字符串 說明：在執行當前Web請求期間發生未處理的異常。 請查看堆棧跟蹤，以獲取有關錯誤及其在代碼中起源的更多信息。

Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: String

    Source Error:


Line 22:         {
Line 23:             IEnumerable<CollegeContactViewModel> contacts = db.CollegeContacts.Where(cc => cc.CollegeContactID >0).AsEnumerable().ToList()
Line 24:                 .Select(c => new CollegeContactViewModel()
Line 25:                 {
Line 26:                     Id          = c.CollegeContactID,

這是我的索引動作

public ActionResult Index()
        {
            IEnumerable<CollegeContactViewModel> contacts = db.CollegeContacts.AsEnumerable().ToList()
                .Select(c => new CollegeContactViewModel()
                {
                    Id          = c.CollegeContactID,
                    Status      = c.CollegeContStatus,
                    Center      = c.CollegeContCenter,
                    Salutation  = c.CollegeContSalutation,
                    FirstName   = c.CollegeContFname,
                    LastName    = c.CollegeContLname,
                    Position    = c.CollegeContPosition,
                    Department  = c.CollegeContDept,
                    Institution = c.CollegeContInstitution,
                    Address = new Address()
                    {
                        AddressLine1 = c.CollegeContAdd1,
                        AddressLine2 = c.CollegeContAdd2,
                        City         = c.CollegeContCity,
                        State        = c.CollegeContSt,
                        PostalCode   = c.CollegeContZip
                    },
                    Email                = c.CollegeContEmail,
                    Phone                = c.CollegeContPhone,
                    Opt                  = c.CollegeContOpt,
                    ContactDate          = c.CollegeContDate,
                    OnMailingList        = c.CollegeContMailListSingle,
                    NumberOfCatalogsSent = int.Parse(c.NumCatalogsSent),
                    DateSent             = c.Datesent.ToString(),
                    DateEntered          = c.DateEntered.ToString(),
                    Reception            = c.Reception,
                    Partner              = c.Partner,
                    Website              = c.SAwebsite,
                    Poster               = c.CollegeContPoster
                });
            return View(contacts.ToList());
        }

CollegeContactViewModel

public class CollegeContactViewModel
    {
        public int Id { get; set; }
        public string Status { get; set; }
        public string Center { get; set; }
        public string Salutation { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Position { get; set; }
        public string Department { get; set; }
        public string Institution { get; set; }
        public Address Address { get; set; }
        public string Email { get; set; }
        public string Phone { get; set; }
        public string Opt { get; set; }
        public string ContactDate { get; set; }
        public bool? OnMailingList { get; set; }
        public int NumberOfCatalogsSent { get; set; }
        public string DateSent { get; set; }
        public string DateEntered { get; set; }
        public string Reception { get; set; }
        public string Partner { get; set; }
        public string Website { get; set; }
        public string Poster { get; set; }
    }

對於數據庫中的另一個實體，我具有完全相同的代碼類型，並且工作正常。 此外，除主鍵外，所有字段都允許為null，因此不能為空。 我也逐步瀏覽了代碼，然后填充了ID。

有人可以告訴我為什么我不斷收到此異常嗎？

Answer 1

掃描您的代碼中的字符串參數，這看起來像是最有可能（僅？）的候選者：

NumberOfCatalogsSent = int.Parse(c.NumCatalogsSent),

如果c.NumCatalogsSent為null，則將得到錯誤。

---

我會改用這種方法（EF v4 可以轉換null-coalescing運算符）：

NumberOfCatalogsSent = int.Parse(c.NumCatalogsSent ?? "0"),