UWP - 实体框架核心（dbcontext）访问列表子成员 [英] UWP - entity framework core (dbcontext) access to list submember

问题描述

大家好，



我遇到很多麻烦并且一直在调查很多小时，所以我完全陷入困境。



我按照页面中的教程：

UWP入门 - 新数据库 - EF Core | Microsoft Docs

这是创建一个SQLite数据库，并能够在UWP中使用EntityFrameworkCore。



直到这里一切都按预期工作。

但是......我无法使用

Hi All,

I am having a lot of trouble with this and been investigating many many hours, so I am completely stuck.

I followed the tutorial in the page:
Getting Started on UWP - New Database - EF Core | Microsoft Docs
This is to create a SQLite database and be able to use it in UWP with EntityFrameworkCore.

Until here everything works as intended.
But... I am not able to use the

public List<Post> Posts { get; set; }

当我尝试使用它时，我收到错误：

When I try to use it, I get the error:

'Object reference not set to an instance of an object.'

但是我尝试用几种方法初始化对象，并且无法使其工作。



我会说明显的方法是：

But I tried to initialize the object in several ways, and cannot make it work.

I would say that the obvious approach is this:

var blog = new Blog { Url = "theurl", Posts = new List<Posts>() };
db.Blogs.Add(blog);
var thebest = new Post{ Title = "MyTest"};
db.Blogs.SingleOrDefault(x => x.Url == "theurl").Posts.Add(thebest);

但是我处于以下循环中：

But I am in a loop of:

'Object reference not set to an instance of an object.'

我认为问题是当我使用SingleOrDefault或FirstOrDefault等等时......它会返回一个引用？而不是原来的？

在DbContext中使用这个列表的正确方法是什么？



我尝试过：



查看和查看互联网。



以多种不同方式初始化变量。

I think that the problem is when I am using SingleOrDefault or FirstOrDefault etc etc... it returns a reference? and not the original?
What is the correct way to use this list inside the DbContext?

What I have tried:

Looking and looking in the internet.

Initialize the variables in several different ways.

var blog = new Blog { Url = "theurl", Posts = new List<Posts>() };
db.Blogs.Add(blog);
var thebest = new Post{ Title = "MyTest"};
db.Blogs.SingleOrDefault(x => x.Url == "theurl").Posts.Add(thebest);

var blog = new Blog { Url = NewBlogUrl.Text };
db.Blogs.Add(blog);
db.Blogs.SingleOrDefault(x => x.Url == NewBlogUrl.Text).Posts = new List<Post>();

始终相同的异常。

Always same exception.

推荐答案

实体框架中的集合公开为 DbSet 集合。

Collections in Entity Framework are exposed as DbSet collections.

根据这个：

According to this:

加载相关数据 - EF Core | Microsoft Docs [ ^ ]：

EF Core尚不支持延迟加载。您可以在我们的待办事项上查看延迟加载项目 [ ^ ]跟踪此功能。

Lazy loading is not yet supported by EF Core. You can view the lazy loading item on our backlog[^] to track this feature.

您将需要使用其中一个而是在该页面上显示的热切/显式加载选项。

You will need to use one of the eager / explicit loading options shown on that page instead.

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