LINQ实体框架值不能为null。 参数名称：字符串

Question

我试图从我的实体数据模型返回视图模型对象的集合到我的视图，并得到以下错误：

值不能为空。 参数名称：字符串 说明：在执行当前Web请求期间发生未处理的异常。 请查看堆栈跟踪，以获取有关错误及其在代码中起源的更多信息。

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,

这是我的索引动作

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; }
    }

对于数据库中的另一个实体，我具有完全相同的代码类型，并且工作正常。 此外，除主键外，所有字段都允许为null，因此不能为空。 我也逐步浏览了代码，然后填充了ID。

有人可以告诉我为什么我不断收到此异常吗？

Answer 1

扫描您的代码中的字符串参数，这看起来像是最有可能（仅？）的候选者：

NumberOfCatalogsSent = int.Parse(c.NumCatalogsSent),

如果c.NumCatalogsSent为null，则将得到错误。

---

我会改用这种方法（EF v4 可以转换null-coalescing运算符）：

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