我们在使用Entity Framework框架进行CRUD时，经常会出现各种各样的错误，下面请看我的实验结果。
以下是只用一个上下文对象进行操作：
第一次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.Single(t => t.Id == 2); post.AuthorId = 1; blog.SaveChanges();
结果：成功
第二次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.Single(t => t.Id == 2); post.Author = blog.Authors.Single(t => t.Id == 3); blog.SaveChanges();
结果：成功
第三次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.Single(t => t.Id == 2); post.AuthorId = 1; post.Author = blog.Authors.Single(t => t.Id == 4); blog.SaveChanges();
结果：失败，报错如下：
Conflicting changes to the role 'Post_Author_Target' of the relationship 'ConsoleApplication1.DDD.Infrastructure.Post_Author' have been detected.
第四次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.Single(t => t.Id == 2); post.Author = new Author() { Id = 4 }; blog.Entry(post.Author).State = EntityState.Unchanged; blog.SaveChanges(); 结果：成功
第五次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.Single(t => t.Id == 2); //author = new Author() { Id = 1, Name = "zwj" };//直接实例化或下面查询得出 author = blog.Authors.AsNoTracking().Single(t => t.Id == 3); post.Author = author; blog.SaveChanges(); 结果：成功，但是Authors表中会新增一笔记录，并将新的ID赋值给Posts表；原因：Author与Post不在同一个上下文中，Post所在的上下文中追踪不到Author的信息，故当成新增。
以下是在两个不同的上下文中进行操作：
第六次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.AsNoTracking().Single(t => t.Id == 2); //blog.Entry(post).State = EntityState.Detached;上面的AsNoTracking可以由这句实现 BlogDbContext blog2 = new BlogDbContext(); post.AuthorId = 5; blog2.Entry(post).State = EntityState.Modified; blog2.SaveChanges();
结果：成功
第七次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.AsNoTracking().Single(t => t.Id == 2); BlogDbContext blog2 = new BlogDbContext(); post.Author = blog2.Authors.Single(t => t.Id == 5); blog2.Entry(post).State = EntityState.Modified; blog2.SaveChanges();
结果：失败，报错如下：
A referential integrity constraint violation occurred: The property value(s) of 'Author.Id' on one end of a relationship do not match the property value(s) of 'Post.AuthorId' on the other end.
第八次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.AsNoTracking().Single(t => t.Id == 2); BlogDbContext blog2 = new BlogDbContext(); author = blog2.Authors.AsNoTracking().Single(t => t.Id == 1); post.Author = author; blog2.Entry(post).State = EntityState.Modified; blog2.SaveChanges();
结果：失败，报错同第七次
第九次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.AsNoTracking().Single(t => t.Id == 2); author = blog.Authors.AsNoTracking().Single(t => t.Id == 3); BlogDbContext blog2 = new BlogDbContext(); post.Author = author; blog2.Entry(post).State = EntityState.Modified; blog2.SaveChanges();
结果：失败，报错同第七次
第十次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.AsNoTracking().Single(t => t.Id == 2); BlogDbContext blog2 = new BlogDbContext(); author = new Author() { Id = 1, Name = "zwj" }; post.Author = author; blog2.Entry(post).State = EntityState.Modified; blog2.SaveChanges(); 结果：失败，报错同第七次
第十一次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.AsNoTracking().Single(t => t.Id == 2); BlogDbContext blog2 = new BlogDbContext(); author = blog2.Authors.Single(t => t.Id == 1); post.Author = author; post.AuthorId = author.Id; blog2.Entry(post).State = EntityState.Modified; blog2.SaveChanges();
结果：成功，但我认为主要是通过赋值AuthorId来完成的，与第六次相同，去掉赋值AuthorId，则与第七次相同的报错
第十二次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.AsNoTracking().Single(t => t.Id == 2); BlogDbContext blog2 = new BlogDbContext(); author = blog2.Authors.Single(t => t.Id == 3); blog2.Entry(post).Reference(t => t.Author).CurrentValue = author; blog2.Entry(post).State = EntityState.Modified; blog2.SaveChanges();
结果：失败，报错同第七次
第十三次：
BlogDbContext blog = new BlogDbContext(); post = blog.Posts.AsNoTracking().Single(t => t.Id == 2); BlogDbContext blog2 = new BlogDbContext(); blog2.Posts.Attach(post); post.Author = blog2.Authors.Single(t => t.Id == 1); blog2.SaveChanges();
结果：成功
最后得出结论：
1.在同一个上下文中，不论是采用直接赋值导航属性或是直接赋值外键属性，除不能同时对导航属性及外键属性赋不相关联的值外，都可以成功；
2.不在同一个上下文中，若想在完成CUD时，则必需先确保要进行操作的实体处于Detached状态，然后再进行相应的更新，涉及导航属性时，只能采用赋值外键属性，不能通过导航属性直接赋值，否则会报错（这个我觉得是个BUG，不知大家有什么好的解决办法没有），若采用先在新的上下文中Attached，再进行更新操作则与第1条结论相同。