在 IEnumerable 上使用 ToList() 方法<t>这已经是一个列表<t></t></t>

Question

Using the ToList() method on IEnumerable which is already a List :

1. Will it create a new List?
2. In performance manner: its complexity will still be O(n) or less?

Answer 1

1. It will create a new list.
2. It will copy over the elements internally using Array.CopyTo which is (from the docs ) O(n)

you can see the code of the constructor of list (which is what is being called) here

Answer 2

It is simple yet some clarification required.

1. Yes. It will create new list instance.

2. Now when it comes to create new list. It will copy all items in it. If it is valuetype then actual value is being copy and if it is reference type then that ref is being copied. It is O(n).

    class Program { static async Task Main(string[] args) { //Value Type List<int> valueTypeitems = new List<int>(); valueTypeitems.Add(1); valueTypeitems.Add(2); var newlist = valueTypeitems.ToList(); valueTypeitems[0] = 3; // This will only reflect in first list. valueTypeitems.ForEach(cc => Console.WriteLine(cc)); newlist.ForEach(cc => Console.WriteLine(cc)); //Reference type List<Test> refItems = new List<Test>(); refItems.Add(new Test() { Value = 1 }); refItems.Add(new Test() { Value = 2 }); var newrefList = refItems.ToList(); refItems.ForEach(cc => Console.WriteLine(cc.Value)); newrefList.ForEach(cc => Console.WriteLine(cc.Value)); refItems[0].Value = 10; //This reflect in both list. As object is same but you change value. refItems.ForEach(cc => Console.WriteLine(cc.Value)); newrefList.ForEach(cc => Console.WriteLine(cc.Value)); //if you actually change ref in one list then it will not reflect refItems[0] = new Test() { Value = 30 }; refItems.ForEach(cc => Console.WriteLine(cc.Value)); newrefList.ForEach(cc => Console.WriteLine(cc.Value)); Console.ReadLine(); } public class Test { public int Value { get; set; } } }