Question

我首先使用EF4.1代码创建一个带有SQL CE 4后端的简单数据库应用程序。我有一个Product类和一个CallItem类定义如下：

    class CallItem
    {
        public int id { get; set; }
        public float discount { get; set; } 
        public virtual Product Product { get; set; }
    }

    class Product
    {
        public int id { get; set; }

        public decimal BaseCost { get; set; }
        public int UnitSize { get; set; }
        public bool isWasteOil { get; set; }

        public string Code { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }
        public string Ingredients { get; set; }
    }

edit - 当我使用LINQ查询创建CallItems集合时，我无法访问附加到每个CallItem的Product的属性，例如

var callItems = from ci in context.CallItems select ci;

foreach(CallItem callItem in callItems)
{
    RunSheet nrs = new RunSheet();
    nrs.prodCode = callitem.Product.Code;
}

询问数据库显示正在填充CallItems中的Productid。但是，以下行在运行时生成NullReferenceException：

nrs.prodCode = callitem.Product.Code;

因为callitem.Product正在评估为null。这是否与延迟加载有关，如果是，我该如何解决这个问题呢？

RunSheet是另一个类，nrs是一个实例，其属性'prodCode'我想用CallItem的Product代码填充。

谢谢！

Answer 1

从那段代码中你所展示的它应该可行。你尝试过显式加载吗？

var callItems = from ci in context.CallItems.Include(c => c.Product) select ci;

foreach(CallItem callItem in callItems)
{
    RunSheet nrs = new RunSheet();
    nrs.prodCode = callitem.Product.Code;
}

Answer 2

public class CallItem
    {
        public int Id { get; set; }
        public float Discount { get; set; }
        public virtual Product Product { get; set; }
    }
 public class Product
    {
        public int Id { get; set; }

        public decimal BaseCost { get; set; }
        public int UnitSize { get; set; }
        public bool IsWasteOil { get; set; }

        public string Code { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }
        public string Ingredients { get; set; }
    }
using (var context = new StackOverFlowContext())
            {
                var p = new Product
                                {
                                    Id = 1,
                                    BaseCost = 200,
                                    Code = "Hola",
                                    Description = "Soe description",
                                    Ingredients = "Some  ingredients",
                                    IsWasteOil = true,
                                     Name = "My Product",
                                     UnitSize = 10

                                };

                var item = new CallItem
                                    {
                                        Id = 101,
                                        Discount = 10,
                                         Product = p
                                    };
                context.CallItems.Add(item);
                context.SaveChanges();
                var result = from temp in context.CallItems
                             select temp;
                Console.WriteLine("CallItem Id"+result.First().Id);
                Console.WriteLine("ProductId"+result.First().Product.Id);
            }

我用以下输出

编写了上面的代码

CallItemId 1
ProductId 1

sql Profiler显示了这个

SELECT TOP (1) 
[c].[Id] AS [Id], 
[c].[Discount] AS [Discount], 
[c].[Product_Id] AS [Product_Id]
FROM [dbo].[CallItems] AS [c]

评论太长了，所以我把它放在这里。

EF4.1 - 属性在运行时评估为null

2 个答案: