using(var myDb = new MyDbEntities())
{

    user user = new user();
    user.username = "me";
    user.email = "me@me.com";

    myDb.Users.Add(user);
    myDb.users.Attach(user);
    myDb.Entry(user).State = EntityState.Modified;//this is for modiying/update existing entry
    myDb.SaveChanges();
}

最简单的方法是这样的。

var book = new Model.Book
{
    BookNumber =  _book.BookNumber,
    BookName = _book.BookName,
    BookTitle = _book.BookTitle,
};
using (var db = new MyContextDB())
{
    var result = db.Books.SingleOrDefault(b => b.BookNumber == bookNumber);
    if (result != null)
    {
        try
        {
            // you can't attach book since it doesn't exist in the database yet
            // attach result instead
            db.Books.Attach(result);
            result = book; // this will update all the fields at once
            db.SaveChanges();
        }
        catch (Exception ex)
        {
            throw;
        }
    }
}

这里是这个问题的最佳解决方案:在视图中添加所有的ID(键)。考虑将多个表命名为(First, Second和Third)

@Html.HiddenFor(model=>model.FirstID)
@Html.HiddenFor(model=>model.SecondID)
@Html.HiddenFor(model=>model.Second.SecondID)
@Html.HiddenFor(model=>model.Second.ThirdID)
@Html.HiddenFor(model=>model.Second.Third.ThirdID)

在c#代码中，

[HttpPost]
public ActionResult Edit(First first)
{
  if (ModelState.Isvalid)
  {
    if (first.FirstID > 0)
    {
      datacontext.Entry(first).State = EntityState.Modified;
      datacontext.Entry(first.Second).State = EntityState.Modified;
      datacontext.Entry(first.Second.Third).State = EntityState.Modified;
    }
    else
    {
      datacontext.First.Add(first);
    }
    datacontext.SaveChanges();
    Return RedirectToAction("Index");
  }

 return View(first);
}

如Renat所说，删除:db.Books.Attach(book);

另外，将结果查询改为使用“AsNoTracking”，因为该查询会抛出实体框架的模型状态。它认为“结果”是现在要追踪的书，而你不想要那个。

var result = db.Books.AsNoTracking().SingleOrDefault(b => b.BookNumber == bookNumber);

所以你有一个更新的实体，你想用最少的代码在数据库中更新它……

并发性总是很棘手，但我假设您只是希望您的更新能够胜出。下面是我对相同情况的处理方法，并修改名称以模仿您的类。换句话说，只需将attach更改为add，这对我来说很管用:

public static void SaveBook(Model.Book myBook)
{
    using (var ctx = new BookDBContext())
    {
        ctx.Books.Add(myBook);
        ctx.Entry(myBook).State = System.Data.Entity.EntityState.Modified;
        ctx.SaveChanges();
    }
}

Not related to this specific example, but I came across a challenge when trying to use EF and a DateTime field as the concurrency check field. It appears the EF concurrency code doesn't honor the precision setting from the metadata (edmx) i.e. Type="DateTime" Precision="3". The database datetime field will store a millisecond component within the field (i.e. 2020-10-18 15:49:02.123). Even if you set the original value of the Entity to a DateTime that includes the millisecond component, the SQL EF generates is this:

UPDATE [dbo].[People]
SET [dateUpdated] = @0
WHERE (([PeopleID] = @1) AND ([dateUpdated] = @2))
-- @0: '10/19/2020 1:07:00 AM' (Type = DateTime2)
-- @1: '3182' (Type = Int32)
-- @2: '10/19/2020 1:06:10 AM' (Type = DateTime2)

正如您所看到的，@2是一个没有毫秒组件的STRING表示。这将导致更新失败。

因此，如果您打算使用DateTime字段作为并发键，那么在检索记录时必须从数据库字段中剥离毫秒/ tick，并且仅使用类似剥离的DateTime传递/更新字段。

    //strip milliseconds due to EF concurrency handling
    PeopleModel p = db.people.Where(x => x.PeopleID = id);
    if (p.dateUpdated.Millisecond > 0)
    {
        DateTime d = new DateTime(p.dateUpdated.Ticks / 10000000 * 10000000);
        object[] b = {p.PeopleID, d};
        int upd = db.Database.ExecuteSqlCommand("Update People set dateUpdated=@p1 where peopleId=@p0", b);
        if (upd == 1)
            p.dateUpdated = d;
        else
            return InternalServerError(new Exception("Unable to update dateUpdated"));
    }
return Ok(p);

当用新值更新字段时，也去掉毫秒

(param)int id, PeopleModel person;
People tbl = db.People.Where(x => x.PeopleID == id).FirstOrDefault();
db.Entry(tbl).OriginalValues["dateUpdated"] = person.dateUpdated;
//strip milliseconds from dateUpdated since EF doesn't preserve them
tbl.dateUpdated = new DateTime(DateTime.Now.Ticks / 10000000 * 10000000);