c# tolist() 淺析

在介紹tolist()的時候我先個大家介紹一下linq的延遲加載：

var products = new List<Product>
     {
         new Product { Name = "CD Player", Id = 1, Category = "Electronics" },
         new Product { Name = "DVD Player", Id = 2, Category = "Electronics" },
         new Product { Name = "Blu-Ray Player", Id = 3, Category = "Electronics" },
         new Product { Name = "LCD TV", Id = 4, Category = "Electronics" },
         new Product { Name = "Wiper Fluid", Id = 5, Category = "Automotive" },
         new Product { Name = "LED TV", Id = 6, Category = "Electronics" },
         new Product { Name = "VHS Player", Id = 7, Category = "Electronics" },
         new Product { Name = "Mud Flaps", Id = 8, Category = "Automotive" },
         new Product { Name = "Plasma TV", Id = 9, Category = "Electronics" },
         new Product { Name = "Washer", Id = 10, Category = "Appliances" },
         new Product { Name = "Stove", Id = 11, Category = "Electronics" },
         new Product { Name = "Dryer", Id = 12, Category = "Electronics" },
         new Product { Name = "Cup Holder", Id = 13, Category = "Automotive" },
     };

// select all electronics, there are 7 of them
IEnumerable<Product> electronicProducts = products.Where(p => p.Category == "Electronics");
  
 // now clear the original list we queried
 products.Clear();
  
 // now iterate over those electronics we selected first
 Console.WriteLine(electronicProducts.Count());

許多擴展方法(包括Where() )的查詢結果是創建一個迭代器通過移動列表來執行查詢。 因此,此時的electronicProducts不是List<Product>,只是IEnumerable<Product>,它會在您使用這個列表時動態求值. 這就是LINQ中強大的延遲執行,在你需要結果前,都不會對表達式求值。 此時我們可以去查詢electronicProducts,這樣我們就可以得到結果列表!

但是如果我們換一種寫法這個值就會是7：

var electronicProducts = products.Where(p =>  p.Category == "Electronics").ToList();

現在,  List<T>代替 electronicProducts 作為IEnumerable<T> 動態執行的原始集合,這將是另一個新的集合,修改不會影響原來的集合。

當然,這有優點也有缺點。 通常,如果你只是要遍歷的結果和過程,你不需要(也不想)將它存儲在一個單獨的列表,這只會浪費內存,后來還需要垃圾收集。 然而,如果你想保存子集,並將它分配給另一個類,ToList()是非常方便的,你不需要擔心改變原來的集合。

ToList()內存消耗

tolist和lazy load是對立的 ，tolist是直接將linq語句的執行結果放於list集合中，直接將結果存放在內存中，所以數據量大的時候，使用tolist要謹慎。