LINQ相关问题

Question

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Text; 

namespace Comparer.csd 
{ 
    class Program 
    { 
     /* Confusion is regarding how ToList() method works. Does it do the deep copy or shallow copy?? 

     /**********OUTPUT 
     a a b 
     a b c 
     a c a 
     ----------------- 
     a a b 
     a c a 
     ----------------- 
     1 
     2 
     3 
     OUTPUT ENDS************/ 

     static void Main(string[] args) 
     { 
      List<test> list = new List<test>(); 
      list.Add(new test("a", "b", "c")); 
      list.Add(new test("a", "c", "a")); 
      list.Add(new test("a", "a", "b")); 

      /// sort the list based on first name and last name 

      IEnumerable<test> soretedCollection = from t in list 
            orderby t._fname ascending, t._mname ascending 
            select t; 
      Print(soretedCollection); 
      /// remove the first object from the list 
      list.RemoveAt(0); 
      /// print the list . 
      /// Removal of the item from the list is reflected here so I guess sorted collection and list both 
      /// are refering the same data structure 
      /// what if I will do 
      /// list = soretedCollection.ToList<test>(); /// is it going to change the reference of the list if some other object 
      /// is holding the reference?? 
      Print(soretedCollection); 

      Dictionary<int, int> dic = new Dictionary<int, int>();    
      dic.Add(1, 1); 
      dic.Add(2, 1); 
      dic.Add(3, 1); 
      List<int> keys = dic.Keys.ToList<int>(); 
      /// remove the dictionary entry with key=2 
      dic.Remove(2); 
      /// how come this time it did not remove the second item becuase it is removed from the dictionary. 
      for (int i = 0; i < keys.Count; ++i) 
      { 
       Console.WriteLine(keys[i].ToString()); 
      } 

      Console.Read(); 
     } 

     static void Print(IEnumerable<test> list) 
     { 
      foreach (test t in list) 
      { 
       t.Print(); 
      } 
      Console.WriteLine("---------------------"); 
     } 
    } 

} 

Answer 1

调用.ToList()力量渴望执行完整枚举的 - 这样做的结果是从原来的枚举一个单独的列表，所以.ToList()后的任何更改将不在该列表中得到反映。这个列表中的实际项目与@Johannes Rudolph指出的原始枚举中的相同（相同的对象引用） - 所以是的，这是一个浅拷贝。

的IEnumerable<test>虽然将懒洋洋地在源集合执行 - 只有当你积极地列举项目将枚举创建一个枚举取源集合，因为它是在这一点上（通过使用foreach或.ToList() IE）及时 - 这意味着如果在创建枚举器之前底层集合中存在更改，这些将会反映在枚举中。

Answer 2

ToList将始终创建列表的浅表副本，这是返回的列表将引用相同对象作为源IEnumerable做，但返回的列表是源代码的副本。

参见MSDN。