C# for、while和foreach遍历产生GC
发表于2018-09-02
“foreach会造成额外的gc开销”这个坑，在Unity社区里已经是个常识。
避免这个坑的方式，是尽量改写为 等价于foreach的for/while代码 来避免额外的gc开销。
最近在网上看到了有关于Foreach遍历会产生GC的文章今天测试了下，在这里记录下怕忘记。
先上测试代码
using UnityEngine;
using System.Collections;
using System.Collections.Generic;
public class People
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public string Name;
public int Age;
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public class FontTextureSet : MonoBehaviour
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int[] tempsInts = new[] ｛ 1, 3, 4, 5, 6 ｝;
private People[] peoples;
private List<People> peoplesList;
private List<int> intList;
private Dictionary<int, People> peopleDictionary;
private Dictionary<int, int> intDictionary;
private int tem;
// Use this for initialization
void Awake()
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peoples = new People[5];
peoplesList = new List<People>();
intList = new List<int>();
peopleDictionary = new Dictionary<int, People>();
intDictionary = new Dictionary<int, int>();
for (int i = 0; i < 5; i++)
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People people = new People();
people.Age = i;
people.Name = ((char) 'A' + i).ToString();
peoples[i] = people;
peoplesList.Add(people);
peopleDictionary.Add(i, people);
intDictionary.Add(i,i);
intList.Add(i);
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void Update()
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Profiler.BeginSample("Foreach Int Array");
foreach (int v in tempsInts)
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tem = v;
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Profiler.EndSample();
Profiler.BeginSample("Foreach People Array");
foreach (People p in peoples)
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Profiler.EndSample();
Profiler.BeginSample("Foeach Int List");
foreach (int i in intList)
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Profiler.EndSample();
Profiler.BeginSample("Foreach People List");
foreach (People people in peoplesList)
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Profiler.EndSample();
Profiler.BeginSample("Foreach People Dictionary");
foreach (KeyValuePair<int, People> keyValuePair in peopleDictionary)
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Profiler.EndSample();
Profiler.BeginSample("Foeach peopleDictionary.Values");
foreach (People people in peopleDictionary.Values)
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Profiler.EndSample();
Profiler.BeginSample("Foreach Int Dictionary");
foreach (KeyValuePair<int, int> keyValuePair in intDictionary)
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Profiler.EndSample();
Profiler.BeginSample("Foreach intDictionary.Values");
foreach (int value in intDictionary.Values)
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Profiler.EndSample();
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ok，在来看看GC情况
哦 my god 这是真的吗，除了数组的遍历其他都产生了GC，这要是真的在Update里用了foreach遍历Directionary和List那不是要疯了，这代码要是让主程看到他肯定要哭，好吧，废话不多说，先来分析一下结果：
1、就是foreach变量数组不会产生GC，遍历Directionary和List都会产生GC
2、foreach遍历List产生GC是24B，Directionary是28B，注意Directionary遍历有多种这里我只测试了两种用foreach的遍历方法，有Directionary.Values遍历会产生40B的GC
3、GC的产生与数据类型无关，与数据结构有关
接下来能是for和while其实他们两差不多在遍历Directionary时都会产生GC，有一种情况除外，先来看测试代码
using UnityEngine;
using System.Collections;
using System.Collections.Generic;
public class People
｛
public string Name;
public int Age;
｝
public class FontTextureSet : MonoBehaviour
｛
int[] tempsInts = new[] ｛ 1, 3, 4, 5, 6 ｝;
private People[] peoples;
private List<People> peoplesList;
private List<int> intList;
private Dictionary<int, People> peopleDictionary;
private Dictionary<int, int> intDictionary;
private int tem;
// Use this for initialization
void Awake()
｛
peoples = new People[5];
peoplesList = new List<People>();
intList = new List<int>();
peopleDictionary = new Dictionary<int, People>();
intDictionary = new Dictionary<int, int>();
for (int i = 0; i < 5; i++)
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People people = new People();
people.Age = i;
people.Name = ((char) 'A' + i).ToString();
peoples[i] = people;
peoplesList.Add(people);
peopleDictionary.Add(i, people);
intDictionary.Add(i,i);
intList.Add(i);
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void Update()
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Profiler.BeginSample("For Int Array");
for (int i = 0; i < tempsInts.Length; i++)
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//tempsInts[i];
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Profiler.EndSample();
Profiler.BeginSample("For People Array");
for (int i=0; i<peoples.Length; i++)
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Profiler.EndSample();
Profiler.BeginSample("For Int List");
for (int i=0; i< intList.Count; i++)
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Profiler.EndSample();
Profiler.BeginSample("For People List");
for (int i=0; i<peoplesList.Count; i++)
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Profiler.EndSample();
Profiler.BeginSample("For People Dictionary.Values");
for (int i =0;i<peopleDictionary.Values.Count; i++)
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Profiler.EndSample();
Profiler.BeginSample("For peopleDictionary");
IEnumerator e = peopleDictionary.GetEnumerator();
e.MoveNext();
for (int i=0; i<peopleDictionary.Count; e.MoveNext(), i++)
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Profiler.EndSample();
Profiler.BeginSample("While Dictionary");
IEnumerator enumerator = peopleDictionary.GetEnumerator();
//e.MoveNext();
while (e.MoveNext())
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Profiler.EndSample();
Profiler.BeginSample("While List");
int num = 0;
while (num < peoplesList.Count)
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num++;
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Profiler.EndSample();
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结果图如下：
结论：
1、Dictionary不管是for、while还是foreach都会产生GC都是28B（注意这里没有用var这个关键字，下面我会对用var的遍历做测试）
2、For遍历Dictionary.Values产生的GC是72B，比foreach多32B
3、For和while遍历数组和List都不会产生GC
上面我提到了var这个关键字，ok，下面我们就对这个做个测试
using UnityEngine;
using System.Collections;
using System.Collections.Generic;
public class People
｛
public string Name;
public int Age;
｝
public class FontTextureSet : MonoBehaviour
｛
int[] tempsInts = new[] ｛ 1, 3, 4, 5, 6 ｝;
private People[] peoples;
private List<People> peoplesList;
private List<int> intList;
private Dictionary<int, People> peopleDictionary;
private Dictionary<int, int> intDictionary;
private int tem;
// Use this for initialization
void Awake()
｛
peoples = new People[5];
peoplesList = new List<People>();
intList = new List<int>();
peopleDictionary = new Dictionary<int, People>();
intDictionary = new Dictionary<int, int>();
for (int i = 0; i < 5; i++)
｛
People people = new People();
people.Age = i;
people.Name = ((char) 'A' + i).ToString();
peoples[i] = people;
peoplesList.Add(people);
peopleDictionary.Add(i, people);
intDictionary.Add(i,i);
intList.Add(i);
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void Update()
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Profiler.BeginSample("Foreach Var Dictionary");
foreach (var VARIABLE in peopleDictionary)
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Profiler.EndSample();
Profiler.BeginSample("Foreach Var Dictionary.Values");
foreach (var value in peopleDictionary.Values)
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Profiler.EndSample();
Profiler.BeginSample("Foreach Var List");
foreach (var people in peoplesList)
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Profiler.EndSample();
Profiler.BeginSample("For Var Dictionary");
var e = peopleDictionary.GetEnumerator();
e.MoveNext();
for (int i=0; i<peopleDictionary.Count; e.MoveNext(), i++)
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Profiler.EndSample();
Profiler.BeginSample("While Var Dictionary");
var enumerator = peopleDictionary.GetEnumerator();
//e.MoveNext();
while (e.MoveNext())
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Profiler.EndSample();
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结果如下图所示：
可以看出对于foreach来说没什么变化，但对for和while来说意义非凡啊，Directionary的遍历没有GC了。
OK，最后来做个总结：
1、遍历不要用foreach，全用for和while
2、遍历Dictionary是有一下两种方式的一种：
var e = peopleDictionary.GetEnumerator();
e.MoveNext();
for (int i=0; i<peopleDictionary.Count; e.MoveNext(), i++)
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var enumerator = peopleDictionary.GetEnumerator();
//e.MoveNext();
while (e.MoveNext())
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以上的测试只是简单测试不全，但已经达到目的了：就是以后遍历怎么遍历不会产生GC。
来自：https://blog.csdn.net/hyf2713/article/details/52335113