c# tolist() 浅析

在介绍tolist()的时候我先个大家介绍一下linq的延迟加载：

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());

许多扩展方法(包括Where() )的查询结果是创建一个迭代器通过移动列表来执行查询。 因此,此时的electronicProducts不是List<Product>,只是IEnumerable<Product>,它会在您使用这个列表时动态求值. 这就是LINQ中强大的延迟执行,在你需要结果前,都不会对表达式求值。 此时我们可以去查询electronicProducts,这样我们就可以得到结果列表!

但是如果我们换一种写法这个值就会是7：

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

现在,  List<T>代替 electronicProducts 作为IEnumerable<T> 动态执行的原始集合,这将是另一个新的集合,修改不会影响原来的集合。

当然,这有优点也有缺点。 通常,如果你只是要遍历的结果和过程,你不需要(也不想)将它存储在一个单独的列表,这只会浪费内存,后来还需要垃圾收集。 然而,如果你想保存子集,并将它分配给另一个类,ToList()是非常方便的,你不需要担心改变原来的集合。

ToList()内存消耗

tolist和lazy load是对立的 ，tolist是直接将linq语句的执行结果放于list集合中，直接将结果存放在内存中，所以数据量大的时候，使用tolist要谨慎。