尝试添加一对多关系时出现NullReferenceException

Question

Item可以包含多个Sizes。当我尝试向项目添加新大小时，会抛出NullReference错误。当我尝试将图像添加到我的项目时，会发生同样的事情。

  

对象引用未设置为对象的实例。

代码

var size = new Size(){
    BasePrice = currentBasePrice, // not null, checked in debugger
    DiscountPrice = currentDiscountPrice // not null, checked in debugger
};

// item is not null, checked in debugger
item.Sizes.Add(size); // nothing here is null, however it throws null reference error here

项目模型

public class Item
{
    public int ID { get; set; }
    public int CategoryID { get; set; }
    virtual public Category Category { get; set; }
    virtual public ICollection<Size> Sizes { get; set; }
    virtual public ICollection<Image> Images { get; set; }
}

尺寸模型

public class Size
{
    public int ID { get; set; }
    public int ItemID { get; set; }
    virtual public Item Item { get; set; } // tried to delete this, did not help
    public decimal BasePrice { get; set; }
    public decimal? DiscountPrice { get; set; }
}

Answer 1

您需要向Item初始化Sizes集合添加构造函数。自动属性简化了后备变量，但不对其进行初始化。

public Item() 
{
    this.Sizes = new List<Size>();
}

Answer 2

我假设Item.Sizes为空。您尚未初始化集合，因此item.Sizes.Add会抛出NullReferenceException。

public class Item
{
    public int ID { get; set; }
    public int CategoryID { get; set; }
    virtual public Category Category { get; set; }
    virtual public ICollection<Size> Sizes { get; set; }
    virtual public ICollection<Image> Images { get; set; }

    public Item()
    {
        Sizes = new List<Size>();
    }
}