C# 迭代字典c中vs foreach差异的循环#
我有一个followC# 迭代字典c中vs foreach差异的循环#,c#,.net,performance,dictionary,foreach,C#,.net,Performance,Dictionary,Foreach,我有一个followforeach循环来完成这项工作。我很想知道下面的例子-对于性能问题,使用for循环而不是foreach循环是否更好 因为我读到for循环比foreach循环快,所以我也有点困惑 foreach (KeyValuePair<string, StringValues> v in values) { string key = v.Key; StringValues val = v.Value; if (val.Count &
foreach
循环来完成这项工作。我很想知道下面的例子-对于性能问题,使用for
循环而不是foreach
循环是否更好
因为我读到for
循环比foreach
循环快,所以我也有点困惑
foreach (KeyValuePair<string, StringValues> v in values)
{
string key = v.Key;
StringValues val = v.Value;
if (val.Count > 0)
{
if (!string.IsNullOrWhiteSpace(val[0]))
{
switch (key)
{
case ABC:
One = val[0];
break;
case PQR:
Two = val[0];
break;
//.. bunch of other case block here with similar stuff
}
}
}
}
foreach(值中的KeyValuePair v)
{
字符串键=v.键;
StringValues val=v.值;
如果(值计数>0)
{
如果(!string.IsNullOrWhiteSpace(val[0]))
{
开关(钥匙)
{
案例ABC:
1=val[0];
打破
案例PQR:
二=val[0];
打破
//…这里还有一堆类似的箱子
}
}
}
}
只有在非常极端的情况下才有意义。在foreach
中,性能下降是您必须写入另一个变量,而在for
中,您不需要这样做。
foreach基本上是这样的:
for(inti=0,i
由于词典没有定义的顺序,在IDictionary
界面中缺少任何类型的索引器,这可能会使不使用foreach
/GetEnumerator()
的情况下很难进行迭代。鉴于
Dictionary Dictionary=Enumerable.Range(0,10).ToDictionary(i=>i,i=>-i);
…因为您知道键包含一个连续的整数范围,所以可以使用for
循环遍历所有可能的键值
//这利用了我们知道字典中存在0..9中的键这一事实
for(int key=0;key
然而,如果你不能做出这样的假设,它就会变得更加棘手。您可以迭代以获取每个元素的键…但该集合也不允许索引,因此您回到了从foreach
与for
困境开始的位置。但是,如果您坚持使用for
,一种方法是对数组执行键,然后迭代该数组
//将Keys属性复制到数组以允许索引
int[]keys=newint[dictionary.Count];
dictionary.key.CopyTo(key,0);
//这对字典中键的分布没有任何假设
for(int index=0;index
当然,CopyTo()
将在您自己有机会枚举键之前枚举一次完整的时间,因此这只会影响性能
如果您使用的是一组预先已知的固定键,或者您不介意每次字典的键更改时都必须维护一个单独的键集合,那么更好的方法是将键缓存在一个可以索引的结构中
int[]keyCache=Enumerable.Range(0,10).ToArray();
// ...
//这将检索与字典分开存储的已知键
for(int index=0;index
可能会很有诱惑力地使用;毕竟,它很容易使用
for(int index=0;index
但是,这对性能非常不利ElementAt()
仅当输入集合实现了IList
时才能执行,而Dictionary
既不继承,也不继承。否则,对于尝试检索的每个索引,都必须从头开始。考虑枚举上面定义的整个10元字典…
| Index requested | Elements enumerated | Total elements enumerated |
|:---------------:|:----------------------------:|:-------------------------:|
| 0 | 0 | 1 |
| 1 | 0, 1 | 3 |
| 2 | 0, 1, 2 | 6 |
| 3 | 0, 1, 2, 3 | 10 |
| 4 | 0, 1, 2, 3, 4 | 15 |
| 5 | 0, 1, 2, 3, 4, 5 | 21 |
| 6 | 0, 1, 2, 3, 4, 5, 6 | 28 |
| 7 | 0, 1, 2, 3, 4, 5, 6, 7 | 36 |
| 8 | 0, 1, 2, 3, 4, 5, 6, 7, 8 | 45 |
| 9 | 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 | 55 |
…从这个我用
使用系统;
使用System.Collections.Generic;
使用System.Linq;
运用系统反思;
使用BenchmarkDotNet.Attributes;
使用BenchmarkDotNet.Jobs;
名称空间SO61507883
{
[SimpleJob(RuntimeMoniker.Net48)]
[SimpleJob(RuntimeMoniker.NetCoreApp31)]
公共课基准
{
公共静态IReadOnlyList字典
{
得到;
}=Array.AsReadOnly(新的int[]{10,1_000});
[ParamsSource(名称(字典化))]
公共整数大小
{
获得;设置;
}
公共词典源
{
获得;设置;
}
//仅由*_CachedKeys()基准方法使用
公共int[]密钥缓存
{
获得;设置;
}
[全局设置()]
公共作废设置()
{
Source=可枚举范围(0,大小)
.ToDictionary(i=>i,i=>-i);
KeyCache=newint[Size];
Source.Keys.CopyTo(KeyCache,0);
}
[基准测试()]
public(int-keySum,int-valueSum)GetEnumerator()
{
int-keySum=0;
int valueSum=0;
使用(Dictionary.Enumerator Enumerator=Source.GetEnumerator())
while(枚举数.MoveNext())
{
KeyValuePair=枚举数。当前值;
keySum+=pair.Key;
valueSum+=成对值;
}
返回值(keySum、valueSum);
}
[基准(基线=真实)]
public(int-keySum,int-valueSum)ForEach()
{
int-keySum=0;
int valueSum=0;
foreach(源中的KeyValuePair对)
{
keySum+=pair.Key;
valueSum+=成对值;
}
返回(键
// * Summary *
BenchmarkDotNet=v0.12.1, OS=Windows 10.0.18363.657 (1909/November2018Update/19H2)
Intel Core i7 CPU 860 2.80GHz (Nehalem), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=3.1.201
[Host] : .NET Core 3.1.3 (CoreCLR 4.700.20.11803, CoreFX 4.700.20.12001), X64 RyuJIT
.NET 4.8 : .NET Framework 4.8 (4.8.4121.0), X64 RyuJIT
.NET Core 3.1 : .NET Core 3.1.3 (CoreCLR 4.700.20.11803, CoreFX 4.700.20.12001), X64 RyuJIT
| Method | Job | Runtime | Size | Mean | Error | StdDev | Ratio | RatioSD |
|---------------------------------- |-------------- |-------------- |------- |--------------------:|------------------:|------------------:|----------:|--------:|
| GetEnumerator | .NET 4.8 | .NET 4.8 | 10 | 118.4 ns | 1.71 ns | 1.76 ns | 1.02 | 0.02 |
| ForEach | .NET 4.8 | .NET 4.8 | 10 | 116.4 ns | 1.44 ns | 1.28 ns | 1.00 | 0.00 |
| For_Indexer_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 10 | 147.6 ns | 2.96 ns | 3.17 ns | 1.26 | 0.02 |
| While_Indexer_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 10 | 149.2 ns | 1.72 ns | 1.61 ns | 1.28 | 0.02 |
| For_TryGetValue_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 10 | 154.5 ns | 1.16 ns | 0.97 ns | 1.33 | 0.01 |
| While_TryGetValue_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 10 | 160.8 ns | 1.93 ns | 1.71 ns | 1.38 | 0.01 |
| For_Indexer_CopyToKeys | .NET 4.8 | .NET 4.8 | 10 | 177.5 ns | 1.37 ns | 1.14 ns | 1.53 | 0.02 |
| While_Indexer_CopyToKeys | .NET 4.8 | .NET 4.8 | 10 | 185.6 ns | 3.69 ns | 4.80 ns | 1.59 | 0.05 |
| For_Indexer_CachedKeys | .NET 4.8 | .NET 4.8 | 10 | 154.5 ns | 2.83 ns | 2.64 ns | 1.33 | 0.03 |
| While_Indexer_CachedKeys | .NET 4.8 | .NET 4.8 | 10 | 155.3 ns | 2.35 ns | 2.08 ns | 1.33 | 0.02 |
| For_ElementAt | .NET 4.8 | .NET 4.8 | 10 | 1,009.2 ns | 8.61 ns | 7.19 ns | 8.67 | 0.12 |
| While_ElementAt | .NET 4.8 | .NET 4.8 | 10 | 1,140.9 ns | 14.36 ns | 13.43 ns | 9.81 | 0.16 |
| | | | | | | | | |
| GetEnumerator | .NET Core 3.1 | .NET Core 3.1 | 10 | 118.6 ns | 2.39 ns | 3.19 ns | 0.98 | 0.03 |
| ForEach | .NET Core 3.1 | .NET Core 3.1 | 10 | 120.3 ns | 1.28 ns | 1.14 ns | 1.00 | 0.00 |
| For_Indexer_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 10 | 126.1 ns | 0.67 ns | 0.56 ns | 1.05 | 0.01 |
| While_Indexer_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 10 | 135.5 ns | 2.28 ns | 2.02 ns | 1.13 | 0.02 |
| For_TryGetValue_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 10 | 131.0 ns | 2.41 ns | 2.25 ns | 1.09 | 0.02 |
| While_TryGetValue_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 10 | 133.9 ns | 1.42 ns | 1.19 ns | 1.11 | 0.01 |
| For_Indexer_CopyToKeys | .NET Core 3.1 | .NET Core 3.1 | 10 | 162.4 ns | 2.32 ns | 2.06 ns | 1.35 | 0.02 |
| While_Indexer_CopyToKeys | .NET Core 3.1 | .NET Core 3.1 | 10 | 166.3 ns | 1.29 ns | 1.21 ns | 1.38 | 0.02 |
| For_Indexer_CachedKeys | .NET Core 3.1 | .NET Core 3.1 | 10 | 136.0 ns | 1.27 ns | 1.19 ns | 1.13 | 0.02 |
| While_Indexer_CachedKeys | .NET Core 3.1 | .NET Core 3.1 | 10 | 142.3 ns | 2.84 ns | 4.59 ns | 1.14 | 0.02 |
| For_ElementAt | .NET Core 3.1 | .NET Core 3.1 | 10 | 952.4 ns | 10.08 ns | 8.94 ns | 7.92 | 0.13 |
| While_ElementAt | .NET Core 3.1 | .NET Core 3.1 | 10 | 953.8 ns | 8.86 ns | 7.40 ns | 7.93 | 0.12 |
| | | | | | | | | |
| GetEnumerator | .NET 4.8 | .NET 4.8 | 1000 | 9,344.9 ns | 80.50 ns | 75.30 ns | 1.00 | 0.01 |
| ForEach | .NET 4.8 | .NET 4.8 | 1000 | 9,360.2 ns | 82.04 ns | 64.05 ns | 1.00 | 0.00 |
| For_Indexer_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 1000 | 15,122.4 ns | 81.71 ns | 68.23 ns | 1.62 | 0.01 |
| While_Indexer_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 1000 | 15,106.4 ns | 85.68 ns | 75.96 ns | 1.61 | 0.02 |
| For_TryGetValue_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 1000 | 16,160.3 ns | 270.09 ns | 252.64 ns | 1.73 | 0.03 |
| While_TryGetValue_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 1000 | 16,452.4 ns | 146.51 ns | 129.88 ns | 1.76 | 0.02 |
| For_Indexer_CopyToKeys | .NET 4.8 | .NET 4.8 | 1000 | 17,407.1 ns | 251.38 ns | 222.84 ns | 1.86 | 0.03 |
| While_Indexer_CopyToKeys | .NET 4.8 | .NET 4.8 | 1000 | 17,034.0 ns | 295.71 ns | 404.77 ns | 1.85 | 0.05 |
| For_Indexer_CachedKeys | .NET 4.8 | .NET 4.8 | 1000 | 16,277.5 ns | 69.91 ns | 58.38 ns | 1.74 | 0.02 |
| While_Indexer_CachedKeys | .NET 4.8 | .NET 4.8 | 1000 | 15,131.9 ns | 55.97 ns | 46.74 ns | 1.62 | 0.01 |
| For_ElementAt | .NET 4.8 | .NET 4.8 | 1000 | 4,859,257.3 ns | 18,862.72 ns | 15,751.22 ns | 519.24 | 4.36 |
| While_ElementAt | .NET 4.8 | .NET 4.8 | 1000 | 3,837,001.5 ns | 7,396.43 ns | 6,556.74 ns | 409.85 | 3.11 |
| | | | | | | | | |
| GetEnumerator | .NET Core 3.1 | .NET Core 3.1 | 1000 | 9,029.9 ns | 21.69 ns | 18.12 ns | 1.00 | 0.00 |
| ForEach | .NET Core 3.1 | .NET Core 3.1 | 1000 | 9,022.4 ns | 13.08 ns | 10.92 ns | 1.00 | 0.00 |
| For_Indexer_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 1000 | 11,396.9 ns | 18.42 ns | 15.38 ns | 1.26 | 0.00 |
| While_Indexer_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 1000 | 12,504.6 ns | 13.82 ns | 10.79 ns | 1.39 | 0.00 |
| For_TryGetValue_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 1000 | 12,244.1 ns | 73.90 ns | 69.13 ns | 1.36 | 0.01 |
| While_TryGetValue_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 1000 | 12,437.4 ns | 22.48 ns | 18.77 ns | 1.38 | 0.00 |
| For_Indexer_CopyToKeys | .NET Core 3.1 | .NET Core 3.1 | 1000 | 13,717.9 ns | 36.98 ns | 30.88 ns | 1.52 | 0.00 |
| While_Indexer_CopyToKeys | .NET Core 3.1 | .NET Core 3.1 | 1000 | 14,099.6 ns | 20.44 ns | 18.12 ns | 1.56 | 0.00 |
| For_Indexer_CachedKeys | .NET Core 3.1 | .NET Core 3.1 | 1000 | 12,640.4 ns | 23.31 ns | 19.47 ns | 1.40 | 0.00 |
| While_Indexer_CachedKeys | .NET Core 3.1 | .NET Core 3.1 | 1000 | 12,610.5 ns | 20.97 ns | 17.51 ns | 1.40 | 0.00 |
| For_ElementAt | .NET Core 3.1 | .NET Core 3.1 | 1000 | 3,402,799.3 ns | 15,880.59 ns | 14,077.73 ns | 377.13 | 1.73 |
| While_ElementAt | .NET Core 3.1 | .NET Core 3.1 | 1000 | 3,399,305.2 ns | 5,822.01 ns | 5,161.06 ns | 376.76 | 0.74 |
| | | | | | | | | |
| GetEnumerator | .NET 4.8 | .NET 4.8 | 100000 | 885,621.4 ns | 1,617.29 ns | 1,350.51 ns | 1.00 | 0.00 |
| ForEach | .NET 4.8 | .NET 4.8 | 100000 | 884,591.8 ns | 1,781.29 ns | 1,390.72 ns | 1.00 | 0.00 |
| For_Indexer_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 100000 | 1,424,062.0 ns | 2,791.28 ns | 2,474.39 ns | 1.61 | 0.00 |
| While_Indexer_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 100000 | 1,435,667.1 ns | 3,696.89 ns | 3,277.19 ns | 1.62 | 0.00 |
| For_TryGetValue_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 100000 | 1,502,486.1 ns | 3,750.98 ns | 3,325.15 ns | 1.70 | 0.00 |
| While_TryGetValue_ConsecutiveKeys | .NET 4.8 | .NET 4.8 | 100000 | 1,558,335.7 ns | 4,619.63 ns | 3,857.60 ns | 1.76 | 0.00 |
| For_Indexer_CopyToKeys | .NET 4.8 | .NET 4.8 | 100000 | 1,685,000.7 ns | 4,676.88 ns | 3,651.40 ns | 1.90 | 0.01 |
| While_Indexer_CopyToKeys | .NET 4.8 | .NET 4.8 | 100000 | 1,722,418.0 ns | 3,431.67 ns | 3,042.08 ns | 1.95 | 0.01 |
| For_Indexer_CachedKeys | .NET 4.8 | .NET 4.8 | 100000 | 1,499,782.0 ns | 2,951.84 ns | 2,616.73 ns | 1.70 | 0.00 |
| While_Indexer_CachedKeys | .NET 4.8 | .NET 4.8 | 100000 | 1,583,570.2 ns | 3,880.57 ns | 3,440.03 ns | 1.79 | 0.00 |
| For_ElementAt | .NET 4.8 | .NET 4.8 | 100000 | 37,917,621,633.3 ns | 47,744,618.60 ns | 44,660,345.86 ns | 42,868.63 | 93.80 |
| While_ElementAt | .NET 4.8 | .NET 4.8 | 100000 | 38,343,003,642.9 ns | 262,502,616.47 ns | 232,701,732.10 ns | 43,315.66 | 229.53 |
| | | | | | | | | |
| GetEnumerator | .NET Core 3.1 | .NET Core 3.1 | 100000 | 900,980.9 ns | 2,477.29 ns | 2,068.65 ns | 1.00 | 0.00 |
| ForEach | .NET Core 3.1 | .NET Core 3.1 | 100000 | 899,775.7 ns | 1,040.30 ns | 868.70 ns | 1.00 | 0.00 |
| For_Indexer_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 100000 | 1,177,153.8 ns | 1,689.80 ns | 1,411.06 ns | 1.31 | 0.00 |
| While_Indexer_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 100000 | 1,255,795.4 ns | 2,562.23 ns | 2,139.58 ns | 1.40 | 0.00 |
| For_TryGetValue_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 100000 | 1,226,163.3 ns | 2,317.36 ns | 1,809.25 ns | 1.36 | 0.00 |
| While_TryGetValue_ConsecutiveKeys | .NET Core 3.1 | .NET Core 3.1 | 100000 | 1,245,130.0 ns | 4,146.38 ns | 3,237.22 ns | 1.38 | 0.00 |
| For_Indexer_CopyToKeys | .NET Core 3.1 | .NET Core 3.1 | 100000 | 1,430,340.4 ns | 7,834.82 ns | 6,945.37 ns | 1.59 | 0.01 |
| While_Indexer_CopyToKeys | .NET Core 3.1 | .NET Core 3.1 | 100000 | 1,472,807.7 ns | 5,363.80 ns | 4,754.87 ns | 1.64 | 0.01 |
| For_Indexer_CachedKeys | .NET Core 3.1 | .NET Core 3.1 | 100000 | 1,289,902.4 ns | 2,739.78 ns | 2,139.04 ns | 1.43 | 0.00 |
| While_Indexer_CachedKeys | .NET Core 3.1 | .NET Core 3.1 | 100000 | 1,276,484.8 ns | 4,652.23 ns | 3,884.82 ns | 1.42 | 0.00 |
| For_ElementAt | .NET Core 3.1 | .NET Core 3.1 | 100000 | 33,717,209,257.1 ns | 200,565,125.50 ns | 177,795,759.65 ns | 37,460.45 | 216.07 |
| While_ElementAt | .NET Core 3.1 | .NET Core 3.1 | 100000 | 34,064,932,086.7 ns | 225,399,893.36 ns | 210,839,200.10 ns | 37,841.10 | 204.02 |