如何使用TryUpdateModel [英] How to use TryUpdateModel

问题描述

我正在准备本教程。我从本教程看到，为了更新，作者正在使用以下代码：

I am readying this tutorial. I see from this tutorial that for update the author is using the following code:

....
var studentToUpdate = db.Students.Find(id);
if (TryUpdateModel(studentToUpdate, "",
   new string[] { "LastName", "FirstMidName", "EnrollmentDate" }))
{
    try
    {
        db.Entry(studentToUpdate).State = EntityState.Modified;
        db.SaveChanges();

        return RedirectToAction("Index");
    }
   ...
}

了解为什么需要以下行：

But I don't understand why the following line is needed:

db.Entry(studentToUpdate).State = EntityState.Modified;

当我删除此行时，代码仍然运行正常，更新完成。
有人可以帮助我天气吗？

When I remove this line, the code still works well and update is done perfectly. Can someone help me weather that line is needed? If so, why when I remove it, the update works well.

推荐答案

它工作得很好，因为你发现 studentToUpdate 从您的上下文中，这是实体的附加方式，并且当您调用 TryUpdateModel 方法所做的更改时， code> SaveChanges 方法。

It works well because you find the studentToUpdate from your context, that's way the entity is attached and the changes that are made by the TryUpdateModel method are saved when you call the SaveChanges method.

如果您正在使用独立的实体，例如：

If you were working with a detached entity, for example doing this:

var studentToUpdate=new Student(){Id=id};
if (TryUpdateModel(studentToUpdate, "",
   new string[] { "LastName", "FirstMidName", "EnrollmentDate" }))
{
    try
    {
        db.Entry(studentToUpdate).State = EntityState.Modified;
        db.SaveChanges();

        return RedirectToAction("Index");
    }
...
}

在这种情况下必须调用 Entry 方法将实体附加到您的上下文并更改其状态。

In this case you have to call the Entry method to attach the entity to your context and change its state.

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