Question

我有点奇怪的错误让我有点困惑，所以让我描绘一下：

我有一个控制器，用房屋评论更新数据库。如果房子不存在，它会创建一个，然后更新它。

所以这是房子不存在的代码：

if (checkHouse == null)
{
        var department = db.Universitys.Single(d => d.universityName == "NotSet");//make dynamic
        var hws = new House();
        hws.address1 = reviewmodelview.Address1;
        hws.address2 = reviewmodelview.Address2;
        hws.postCode = reviewmodelview.postCode;
        hws.noOfDisputes = 0;
        department.Houses.Add(hws);
        db.SaveChanges(); //created the house and saved the changes, now apply the review to it.

        var theHouse = db.Houses.Single(d => d.address1 == reviewmodelview.Address1);//findhouse
        var rvw = new Review(); //create review
        rvw.condition = reviewmodelview.condition;
        rvw.furniture = reviewmodelview.furniture;
        rvw.overall = reviewmodelview.overall;
        rvw.isApproved = false;
        rvw.review = reviewmodelview.review;
        rvw.user = User.Identity.Name;
        rvw.reviewDate = DateTime.Now;
        theHouse.Reviews.Add(rvw);
        db.SaveChanges();
}

以下是房子确实存在的代码：

else
{
    var theHouse = db.Houses.Single(d => d.address1 == reviewmodelview.Address1);//findhouse
    var rvw = new Review();//create review
    rvw.condition = reviewmodelview.condition;
    rvw.furniture = reviewmodelview.furniture;
    rvw.overall = reviewmodelview.overall;
    rvw.isApproved = false;
    rvw.review = reviewmodelview.review;
    rvw.user = User.Identity.Name;
    rvw.reviewDate = DateTime.Now;
    theHouse.Reviews.Add(rvw);
    db.SaveChanges();
}

现在的问题是，如果房子存在，则添加评论。如果房子不存在，房子将被添加到数据库，但代码中断：

theHouse.Reviews.Add(rvw);

如果我将鼠标悬停在评论上，则说它为空。错误是“对象引用未设置为对象的实例。”

但是，如果我再试一次，但这次添加数据库中已存在的房屋地址并在

处添加断点

theHouse.Reviews.Add(rvw);

它说：{System.Data.Objects.DataClasses.EntityCollection<Data.Manager.Review>}

我无法弄清楚为什么两个相同的代码片段的评论集合为空！

房屋模型是：

namespace Data.Manager
{
    public class House
    {
        public virtual int HouseID { get; set; }
        public virtual string postCode { get; set; }
        public virtual string address1 { get; set; }
        public virtual string address2 { get; set; }
        public virtual int noOfDisputes { get; set; } //number of disputes added by tennants
        public virtual int averageRating { get; set; }
        public virtual ICollection<Review> Reviews { get; set; }        
    }
}

如果我能解释为什么会这样，我会永远爱你。

Answer 1

如果您使用ICollection<Review> Reviews创建新房，则必须先将其初始化，然后才能使用Reviews.Add。它适用于已经存在的房屋的原因是，当您加载现有房屋时，集合将被初始化并填充现有值。

在致电theHouse.Reviews = new List<Review>();之前，您必须按照theHouse.Reviews.Add的规定行事。您也可以在House的构造函数中提前初始化它，以确保在任何访问尝试之前对其进行初始化。

Answer 2

尝试在House构造函数中初始化您的评论集合

namespace Data.Manager
{
    public class House
    {
        public House()
        {
          Reviews = new List<Review>();
        }

        public virtual int HouseID { get; set; }
        public virtual string postCode { get; set; }
        public virtual string address1 { get; set; }
        public virtual string address2 { get; set; }
        public virtual int noOfDisputes { get; set; } //number of disputes added by tennants
        public virtual int averageRating { get; set; }
        public virtual ICollection<Review> Reviews { get; set; }        
    }
}

Answer 3

如上一个答案所述，您需要在向其添加项目之前初始化Review对象。

只需保存一次更改而不是两次调用数据库，就可以使代码略微提高。

        if (checkHouse == null)
        {

            var department = db.Universitys.Single(d => d.universityName == "NotSet");//make dynamic
            var hws = new House();
            hws.address1 = reviewmodelview.Address1;
            hws.address2 = reviewmodelview.Address2;
            hws.postCode = reviewmodelview.postCode;
            hws.noOfDisputes = 0;
            hws.Reviews = new List<Review>();

            var rvw = new Review(); //create review
            rvw.condition = reviewmodelview.condition;
            rvw.furniture = reviewmodelview.furniture;
            rvw.overall = reviewmodelview.overall;
            rvw.isApproved = false;
            rvw.review = reviewmodelview.review;
            rvw.user = User.Identity.Name;
            rvw.reviewDate = DateTime.Now;
            hws.Reviews.Add(rvw);
            department.Houses.Add(hws);

            db.SaveChanges();

        }

MVC C＃奇怪的NullReferenceException

3 个答案: