Entity Framework DbContext Change Tracker API

Dec 17, 2014

5 mins read

DbSet有一個Local，是用來存放從資料庫中取得的資料

namespace ConsoleApplication1
{
    using ConsoleApplication1.Models;
    using System;
    using System.Data.Entity;
class Program
{
    static void Main(string[] args)
    {
        // 初始化資料庫
        Database.SetInitializer(new MyInit());
        using (DemoContext db = new DemoContext())
        {
            db.Database.Initialize(false);
            Console.WriteLine(db.Topic.Local.Count);
            db.Topic.Load();
            Console.WriteLine(db.Topic.Local.Count);
        }
    }
}

}

[![](http://2.bp.blogspot.com/-nFIf_sip55A/VJFaB_gYo0I/AAAAAAAABzs/FxfhAybwr8Q/s1600/01.png)](http://2.bp.blogspot.com/-nFIf_sip55A/VJFaB_gYo0I/AAAAAAAABzs/FxfhAybwr8Q/s1600/01.png)

Local屬性是一個ObservableCollection，可以透過CollectionChanged事件來得到Local集合中發生變化的通知

namespace ConsoleApplication1
{
    using ConsoleApplication1.Models;
    using System;
    using System.Data.Entity;
class Program
{
    static void Main(string[] args)
    {
        // 初始化資料庫
        Database.SetInitializer(new MyInit());
        using (DemoContext db = new DemoContext())
        {
            db.Database.Initialize(false);

            db.Topic.Local.CollectionChanged += (s, a) =&gt;
            {
                if (a.NewItems != null)
                {
                    foreach (Topic item in a.NewItems)
                    {
                        Console.WriteLine("Added:" + item.TopicTitle);
                    }
                }

                if (a.OldItems != null)
                {
                    foreach (Topic item in a.OldItems)
                    {
                        Console.WriteLine("Removed:" + item.TopicTitle);
                    }
                }
            };

            Topic topic = new Topic { TopicTitle = "abc" };
            db.Topic.Add(topic);
            db.Topic.Remove(topic);
        }
    }
}

}

[![](http://1.bp.blogspot.com/-SKyRpTru4p4/VJFaB-AddFI/AAAAAAAABzQ/2Yak7QteYlg/s1600/02.png)](http://1.bp.blogspot.com/-SKyRpTru4p4/VJFaB-AddFI/AAAAAAAABzQ/2Yak7QteYlg/s1600/02.png)

當資料取出到DbContext之後，接下來的資料異動會透過EntityState來識別，當呼叫SaveChanges時，執行相對應的SQL處理 Detached: 該實體尚未存在於DbContext中 Unchanged: 該實體剛從資料庫取出或是呼叫過SaveChanges之後，還沒有任何異動 Added: 該實體已經加入到DbContext中，但尚未呼叫SaveChanges Deleted: 該實體已從DbContext中刪除，但尚未呼叫SaveChanges Modifyied:該實體已在DbContext中更新過，但尚未調用SaveChanges

namespace ConsoleApplication1
{
    using ConsoleApplication1.Models;
    using System;
    using System.Data.Entity;
class Program
{
    static void Main(string[] args)
    {
        // 初始化資料庫
        Database.SetInitializer(new MyInit());
        using (DemoContext db = new DemoContext())
        {
            db.Database.Initialize(false);

            // 建立一個新實體，初始狀態會是Detached
            Topic topic = new Topic() { TopicTitle = "abc" };
            Console.WriteLine("{0}, {1}", db.Topic.Local.Count, db.Entry(topic).State);

            // 呼叫Attach可以把資料附加進Local集合中，狀態會是UnChanged
            db.Topic.Attach(topic);
            Console.WriteLine("{0}, {1}", db.Topic.Local.Count, db.Entry(topic).State);

            // 把實體加入DbSet後，狀態會是Added
            db.Topic.Add(topic);
            Console.WriteLine("{0}, {1}", db.Topic.Local.Count, db.Entry(topic).State);

            // 呼叫SaveChanges後，狀態會是UnChanged
            db.SaveChanges();
            Console.WriteLine("{0}, {1}", db.Topic.Local.Count, db.Entry(topic).State);

            // 異動資料後，狀態會是Modified
            topic.TopicTitle = "123";
            Console.WriteLine("{0}, {1}", db.Topic.Local.Count, db.Entry(topic).State);

            // 把實體從DbSet移除後，狀態會是Deleted
            db.Topic.Remove(topic);
            Console.WriteLine("{0}, {1}", db.Topic.Local.Count, db.Entry(topic).State);
        }
    }
}

}

[![](http://1.bp.blogspot.com/-9aIonFcLz1Y/VJFaB9ZSzbI/AAAAAAAABzM/MehYP1SWpVw/s1600/03.png)](http://1.bp.blogspot.com/-9aIonFcLz1Y/VJFaB9ZSzbI/AAAAAAAABzM/MehYP1SWpVw/s1600/03.png)

除了透過Entry取得實體的狀態外，還可以取得實體不同的值目前的值(Current Value) 原始的值(Original Value) 資料庫裡面的值(Database Value)

namespace ConsoleApplication1
{
    using ConsoleApplication1.Models;
    using System;
    using System.Data.Entity;
class Program
{
    static void Main(string[] args)
    {
        // 初始化資料庫
        Database.SetInitializer(new MyInit());
        using (DemoContext db = new DemoContext())
        {
            db.Database.Initialize(false);

            Topic topic = db.Topic.Find(1);
            topic.TopicTitle = "123";
            db.Database.ExecuteSqlCommand("UPDATE Topic SET TopicTitle='456' WHERE TopicID=1");
            Console.WriteLine("CurrentValue:{0}", db.Entry(topic).CurrentValues.GetValue&lt;string&gt;("TopicTitle"));
            Console.WriteLine("OriginalValue:{0}", db.Entry(topic).OriginalValues.GetValue&lt;string&gt;("TopicTitle"));
            Console.WriteLine("DatabaseValue:{0}", db.Entry(topic).GetDatabaseValues().GetValue&lt;string&gt;("TopicTitle"));
        }
    }
}

}

[![](http://4.bp.blogspot.com/-kRdKg81HKhk/VJFaDLFWQ0I/AAAAAAAABzU/Z9Jsu8pjr-Q/s1600/04.png)](http://4.bp.blogspot.com/-kRdKg81HKhk/VJFaDLFWQ0I/AAAAAAAABzU/Z9Jsu8pjr-Q/s1600/04.png)

如果不想加入Tracker的話，可以呼叫AsNoTracking()即可

namespace ConsoleApplication1
{
    using ConsoleApplication1.Models;
    using System;
    using System.Data.Entity;
class Program
{
    static void Main(string[] args)
    {
        // 初始化資料庫
        Database.SetInitializer(new MyInit());
        using (DemoContext db = new DemoContext())
        {
            db.Database.Initialize(false);

            Console.WriteLine(db.Topic.Local.Count);
            db.Topic.AsNoTracking().Load();
            Console.WriteLine(db.Topic.Local.Count);
        }
    }
}

}