如何更新实体框架.NET 4的实体

问题描述

我的code是这样的：

my code is something like this:

public class Program
{
 [STAThread]
 static void main()
 {
  DataAccessClass dal = new DataAccessClass();
  List<Person> list = dal.GetPersons();
  Person p = list[0];
  p.LastName = "Changed!";
  dal.Update(p);
 }
}

public class DataAccessClass
{
 public static List<Person> GetPersons()
 {
  MyDBEntities context = new MyDBEntities();
  return context.Persons.ToList();
 }

 public void Update(Person p)
 {
  // what sould be written here?
 }
}

现在请告诉我，我应该怎么写的更新（）方法？一切我写的过程中遇到各种异常。（请注意装入的数据进行跟踪，连接或类似的东西）

now please tell me what should i write in the Update() method?everything i write , encounters various exceptions.(please pay attention that the data loaded is tracked , connected or something like that)

推荐答案

现在的问题是，你的个人的实体仍然附着在GetPersons创建上下文。如果要使用附加单位工作，你不得不在两个选择和更新操作都使用相同的上下文实例。你有两个选择来解决你的问题。

The problem is that your Person entities are still attached to context created in GetPersons. If you want to work with attached entities you have to use same context instance in both select and update operations. You have two choices to solve your problem.

1）正确处理连接实体

1) Correctly handled attached entities

public class Program
{
  [STAThread]
  static void main()
  {
    using (DataAccessClass dal = new DataAccessClass())
    {
      List<Person> list = dal.GetPersons();
      Person p = list[0];
      p.LastName = "Changed!";
      dal.Save();
    }
  }
}

public class DataAccessClass : IDisposable
{
  private MyDBEntities _context = new MyDBEntities();

  public List<Person> GetPersons()
  {
    return _context.Persons.ToList();
  }

  public void Save()
  {
    // Context tracks changes on your entities. You don't have to do anything. Simply call
    // SaveChanges and all changes in all loaded entities will be done in DB.
    _context.SaveChanges();
  }

  public void Dispose()
  {
    if (_context != null)
    {
      _context.Dispose();
      _context = null;
    }
  }
}

2）不要使用连接实体

2) Don't use attached entities

public class Program
{
  [STAThread]
  static void main()
  {
    DataAccessClass dal = new DataAccessClass())
    List<Person> list = DataAccessClass.GetPersons();
    Person p = list[0];
    p.LastName = "Changed!";
    dal.Update(p);
  }
}

public class DataAccessClass
{
  public static List<Person> GetPersons()
  {
    // Closing context will detach entities
    using (MyDBEntities context = new MyDBEntities())
    {
      return context.Persons.ToList();
    }
  }

  public void Update(Person p)
  {
    using (MyDBEntities context = new MyDBEntities())
    {
      context.Persons.Attach(p);
      // Detached entities don't track changes so after attaching you have to say
      // what changes have been done
      context.ObjectStateManager.ChangeObjectState(p, System.Data.EntityState.Modified);
      context.SaveChanges();
    }
  }
}

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