要專業系統地學習EF推薦《你必須掌握的Entity Framework 6.x與Core 2.0》。這本書作者（汪鵬，Jeffcky）的博客：https://www.cnblogs.com/CreateMyself/

上一篇我們知道了快照和代理怎麽回事，所以來測試看看

測試添加操作，插入一千條數據看看

快照五次，時間分別為： 1335ms 635ms 1025ms 1242ms 717ms

//快照插入一千條數據
var watch = new Stopwatch();
watch.Start();

List<Book> books = new List<Book>();
 
for (int i = 0; i < 1000; i++)
{
    books.Add(new Book { Name = $"book{i + 1}", PageSize = 1000 + i, AddTime = DateTime.Now });
}
ctx.Books.AddRange(books);
ctx.SaveChanges();

watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds);  

代理添加，時間分別為： 1287ms 701ms 1460ms 1289ms 968ms

//  代理插入一千條數據
 

ctx.Configuration.AutoDetectChangesEnabled = false;
var watch = new Stopwatch();
watch.Start();
ctx.Database.Log = Console.WriteLine;
List<Book> books = new List<Book>();
for (int i = 0; i < 3000; i++)
{
    books.Add(new Book { Name = $"book{i + 1}", PageSize = 1000 + i, AddTime = DateTime.Now });
}
ctx.Books.AddRange(books);
ctx.SaveChanges();

watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds);
 

有一個問題，添加一千條數據，EF是逐條添加的，但是順序為什麽不對呢?

再來看看修改數據，我們對三千條數據進行修改

快照追蹤方式，我記錄了兩組數據

1、1692ms 518ms 474ms 561ms 519ms 為什麽這後面的幾個同時這麽短？那是因為我沒有改代碼，繼續執行修改，那麽其實雖然說給屬性賦值了，但並沒有修改，EF也不會去執行

2、3303ms 2724ms 2250ms 2796ms 現在我就將每一次修改的數據進行改變

var watch = new Stopwatch();
watch.Start();

var books = ctx.Books.ToList();
books.ForEach(book =>
{
    book.Name = "book";
    book.PageSize = 999;
    book.AddTime = new DateTime(1999, 9, 9);
});
ctx.SaveChanges();

watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds);

代理方式，現在就有差距了，會比快照耗時一些

1、3010ms 3144ms 1851ms 2989ms

2、1788ms 3006ms 2738ms 1681ms

ctx.Configuration.AutoDetectChangesEnabled = false;
var watch = new Stopwatch();
watch.Start();

var books = ctx.Books.ToList();
books.ForEach(book =>
{
    book.Name = "book2";
    book.PageSize = 888;
    book.AddTime = new DateTime(1999, 9, 9);
});
ctx.SaveChanges();

watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds);

如果說我們只是修改三千條數據中的其中三條呢？

快照追蹤

643ms 424ms 609ms 575ms

var watch = new Stopwatch();
watch.Start();

var books = ctx.Books.ToList();
books.ForEach(book =>
{
    if (book.PageSize < 1003)
    {
        book.Name = "book2";
        book.PageSize = 777;
        book.AddTime = new DateTime(1999, 9, 9);
    }
});
ctx.SaveChanges();
watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds);

現在我們把三條之外的兩千多條實體的屬性也賦值，但是還是原來的值

1838ms 1291ms 2992ms 3175ms

var watch = new Stopwatch();
watch.Start();

var books = ctx.Books.ToList();
books.ForEach(book =>
{
    if (book.PageSize < 1003)
    {
        book.Name = "book2";
        book.PageSize = 777;
        book.AddTime = new DateTime(1999, 9, 9);
    }
    else
    {
        book.Name = book.Name;
        book.PageSize = book.PageSize;
        book.AddTime = book.AddTime;
    }
});
ctx.SaveChanges();
watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds);

代理追蹤

584ms 513ms 492ms

var watch = new Stopwatch();
watch.Start();
ctx.Configuration.AutoDetectChangesEnabled = fal
var books = ctx.Books.ToList();
books.ForEach(book =>
{
    if (book.PageSize < 1003)
    {
        book.Name = "book";
        book.PageSize = 888;
        book.AddTime = new DateTime(1999, 9, 9);
    }
});
ctx.SaveChanges();
watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds);

3247ms 2481ms 3051ms

var watch = new Stopwatch();
watch.Start();
ctx.Configuration.AutoDetectChangesEnabled = f
var books = ctx.Books.ToList();
books.ForEach(book =>
{
    if (book.PageSize < 1003)
    {
        book.Name = "book";
        book.PageSize = 888;
        book.AddTime = new DateTime(1999, 9, 9);
    }
    else
    {
        book.Name = book.Name;
        book.PageSize = book.PageSize;
        book.AddTime = book.AddTime;
    }
});
ctx.SaveChanges();
watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds);

感覺沒什麽變化，可能是我的數據太少了

最後引用作者的一段話：

“對於代理變更追蹤，筆者認為似乎沒有什麽可用之處，幾乎沒看見過將屬性標記為virtual作為代理類來用，再者EF團隊在代理式變更追蹤方面並沒有過多投入，二者的性能比較就當做了解。

virtual只是用於導航屬性的延遲加載，如果我們想用代理式變更追蹤，除非真正的理解它的問題並有一個合理的理由去使用它，當然這也就意味著，當用快照式變更追蹤遇到性能瓶頸時，可以考慮使用代理式變更追蹤（大部分情況下還是不會）”

EF6學習筆記十七：快照追蹤與代理追蹤性能測試