C# 将列表复制到新列表-更高效&;最佳做法
将列表复制到新列表的适当方式是什么? 将列表复制到新列表的最有效方法是什么 通过高效,而不是代码效率,更多的是在幕后框架的意义上C# 将列表复制到新列表-更高效&;最佳做法,c#,C#,将列表复制到新列表的适当方式是什么? 将列表复制到新列表的最有效方法是什么 通过高效,而不是代码效率,更多的是在幕后框架的意义上 List<String>List2 = List.ToList(); list2=List.ToList(); 或: List<String>List2 = new List<String>(); foreach (string item in List) { List2.Add(item); } list2=新列表();
List<String>List2 = List.ToList();
list2=List.ToList();
或:
List<String>List2 = new List<String>();
foreach (string item in List)
{
List2.Add(item);
}
list2=新列表();
foreach(列表中的字符串项)
{
清单2.添加(项目);
}
更新:
List<String>List2 = new List<String>();
foreach (string item in List)
{
List2.Add(item);
}
那么更高效的IL代码呢?我相信这两个示例是相同的,而.ToList()实现的可能是后者 最好的性能如下所示:
List<String> list2 = new List<String>(list.Count);
foreach(String item in list)
list2.Add(item);
List list2=新列表(List.Count);
foreach(列表中的字符串项)
清单2.添加(项目);
重要的部分是创建具有足够容量的list2来保存其内容
如果以后不需要修改任何一个列表,则只需要一个参考副本:
List<String> list2 = list;
List list2=列表;
鉴于列表
有一个IEnumerable
构造函数,我更喜欢这种形式:
List<string> newList = new List<string>(otherList);
List newList=新列表(otherList);
编辑
正如Ondrej在下面的反编译代码中指出的那样,List
的构造函数预先分配数组的大小,并将内容复制到上面。这将比创建一个新列表,然后在其他列表上逐个添加项目要快得多,特别是在第二个示例中,您没有指定要预分配的项目数量。您可以使用它来获取IEnumerable
List list1=新列表();
//填写清单1
列表2=新列表(列表1);
就效率而言,第一种会更快List
的底层实现是一个ArrayList,因此在调用.Add
时,可能需要调整底层数组的大小
另一方面,.ToList
可以确定新的列表的正确初始大小
,并避免foreach
技术遭受的重新分配操作
考虑到这一点,我推荐.ToList
。代码越少,速度就越快
下面是一个简单的程序,您可以运行它来验证ToList
是否确实更快:
void Main()
{
List<int> items = new List<int>();
items = Enumerable.Range(0, 1000000).ToList();
CopyWithToList(items);
CopyWithForeach(items);
}
public void CopyWithToList<T>(List<T> list)
{
var sw = Stopwatch.StartNew();
List<T> copy = list.ToList();
sw.Stop();
Console.WriteLine("CopyWithToList: {0}", sw.Elapsed);
}
public void CopyWithForeach<T>(List<T> list)
{
var sw = Stopwatch.StartNew();
List<T> copy = new List<T>();
foreach (T item in list) {
copy.Add(item);
}
sw.Stop();
Console.WriteLine("CopyWithForeach: {0}", sw.Elapsed);
}
void Main()
{
列表项=新列表();
items=可枚举的.Range(0,1000000).ToList();
复印件清单(项目);
CopyWithForeach(项目);
}
公共无效副本列表(列表)
{
var sw=Stopwatch.StartNew();
List copy=List.ToList();
sw.Stop();
WriteLine(“CopyWithToList:{0}”,sw.appeased);
}
公共作废CopyWithForeach(列表)
{
var sw=Stopwatch.StartNew();
列表副本=新列表();
foreach(列表中的T项){
副本。添加(项目);
}
sw.Stop();
WriteLine(“CopyWithForeach:{0}”,sw.appeased);
}
ToList的功能(简称):
公共静态列表列表(此IEnumerable源代码)
{
返回新列表(源);
}
ctor做什么(缩短):
公共列表(IEnumerable集合)
{
ICollection collection2=作为ICollection的集合;
int count=collection2.count;
此._items=新的T[计数];
collection2.CopyTo(此._项,0);
这个。_size=count;
}
因此ToList()的效率要高得多——它确实是先分配空间,然后一步复制所有项目。测试表明,最好的性能是
.ToList()
方法(对于包含21474836个元素的列表,它在笔记本核心i5 CPU上运行约48毫秒)
其他方法的速度都比较慢,而使用.Add()
的方法的性能最差
下面是一些测试代码:
class Program
{
static void Main()
{
var list = new List<int>();
for (int i = 0; i < int.MaxValue / 100; i++)
{
list.Add(i);
}
TimeItAccurate(ListCopy_1, list, 10);
TimeItAccurate(ListCopy_2, list, 10);
TimeItAccurate(ListCopy_3, list, 10);
TimeItAccurate(ListCopy_4, list, 10);
TimeItAccurate(ListCopy_5, list, 10);
}
private static List<int> ListCopy_1(List<int> list)
{
var newList = list.ToList();
return newList;
}
private static List<int> ListCopy_2(List<int> list)
{
var newList = new List<int>(list.Count);
foreach (var i in list)
{
newList.Add(i);
}
return newList;
}
private static List<int> ListCopy_3(List<int> list)
{
var newList = new List<int>(list.ToArray());
return newList;
}
private static List<int> ListCopy_4(List<int> list)
{
var newList = new List<int>(list.Count);
newList.AddRange(list);
return newList;
}
private static List<int> ListCopy_5(List<int> list)
{
var newList = new List<int>(list);
return newList;
}
public static void TimeItAccurate<TIn, TResult>(Func<TIn, TResult> func, TIn argument, int iterationsCount)
{
#region Pre-heat
for (int i = 0; i < 10; i++)
{
var t = func.Invoke(argument);
}
#endregion
var stopwatch = new Stopwatch();
var result = default(TResult);
stopwatch.Start();
for (int i = 0; i < iterationsCount; i++)
{
result = func.Invoke(argument);
}
stopwatch.Stop();
Console.WriteLine("Result:\n{4}(...) == {0}\n\n{1} iterations done in {2} ms.\nAverage time: {3:f5} ms.",
result,
iterationsCount,
stopwatch.ElapsedMilliseconds,
stopwatch.ElapsedMilliseconds / (double)iterationsCount,
func.Method.Name);
}
}
类程序
{
静态void Main()
{
var list=新列表();
对于(int i=0;i
结果是:
Result (.ToList()):
ListCopy_1(...) == System.Collections.Generic.List`1[System.Int32]
10 iterations done in 474 ms.
Average time: 47.40000 ms.
Result (for-cycle with .Add()):
ListCopy_2(...) == System.Collections.Generic.List`1[System.Int32]
10 iterations done in 1896 ms.
Average time: 189.60000 ms.
Result (ctor with .ToArray()):
ListCopy_3(...) == System.Collections.Generic.List`1[System.Int32]
10 iterations done in 981 ms.
Average time: 98.10000 ms.
Result (.AddRange()):
ListCopy_4(...) == System.Collections.Generic.List`1[System.Int32]
10 iterations done in 959 ms.
Average time: 95.90000 ms.
Result (new List<int>(list)):
ListCopy_5(...) == System.Collections.Generic.List`1[System.Int32]
10 iterations done in 480 ms.
Average time: 48.00000 ms.
Result(.ToList()):
ListCopy_1(…)==System.Collections.Generic.List`1[System.Int32]
在474毫秒内完成10次迭代。
平均时间:47.40000毫秒。
结果(对于带有.Add()的循环):
ListCopy_2(…)==System.Collections.Generic.List`1[System.Int32]
10次迭代
class Program
{
static void Main()
{
var list = new List<int>();
for (int i = 0; i < int.MaxValue / 100; i++)
{
list.Add(i);
}
TimeItAccurate(ListCopy_1, list, 10);
TimeItAccurate(ListCopy_2, list, 10);
TimeItAccurate(ListCopy_3, list, 10);
TimeItAccurate(ListCopy_4, list, 10);
TimeItAccurate(ListCopy_5, list, 10);
}
private static List<int> ListCopy_1(List<int> list)
{
var newList = list.ToList();
return newList;
}
private static List<int> ListCopy_2(List<int> list)
{
var newList = new List<int>(list.Count);
foreach (var i in list)
{
newList.Add(i);
}
return newList;
}
private static List<int> ListCopy_3(List<int> list)
{
var newList = new List<int>(list.ToArray());
return newList;
}
private static List<int> ListCopy_4(List<int> list)
{
var newList = new List<int>(list.Count);
newList.AddRange(list);
return newList;
}
private static List<int> ListCopy_5(List<int> list)
{
var newList = new List<int>(list);
return newList;
}
public static void TimeItAccurate<TIn, TResult>(Func<TIn, TResult> func, TIn argument, int iterationsCount)
{
#region Pre-heat
for (int i = 0; i < 10; i++)
{
var t = func.Invoke(argument);
}
#endregion
var stopwatch = new Stopwatch();
var result = default(TResult);
stopwatch.Start();
for (int i = 0; i < iterationsCount; i++)
{
result = func.Invoke(argument);
}
stopwatch.Stop();
Console.WriteLine("Result:\n{4}(...) == {0}\n\n{1} iterations done in {2} ms.\nAverage time: {3:f5} ms.",
result,
iterationsCount,
stopwatch.ElapsedMilliseconds,
stopwatch.ElapsedMilliseconds / (double)iterationsCount,
func.Method.Name);
}
}
Result (.ToList()):
ListCopy_1(...) == System.Collections.Generic.List`1[System.Int32]
10 iterations done in 474 ms.
Average time: 47.40000 ms.
Result (for-cycle with .Add()):
ListCopy_2(...) == System.Collections.Generic.List`1[System.Int32]
10 iterations done in 1896 ms.
Average time: 189.60000 ms.
Result (ctor with .ToArray()):
ListCopy_3(...) == System.Collections.Generic.List`1[System.Int32]
10 iterations done in 981 ms.
Average time: 98.10000 ms.
Result (.AddRange()):
ListCopy_4(...) == System.Collections.Generic.List`1[System.Int32]
10 iterations done in 959 ms.
Average time: 95.90000 ms.
Result (new List<int>(list)):
ListCopy_5(...) == System.Collections.Generic.List`1[System.Int32]
10 iterations done in 480 ms.
Average time: 48.00000 ms.