C# For、Foreach性能比较

一直听说foreach会有gc产生，有说.net3.5之前有，.net4则修复，一直没有测试，今天测试记录一下

测试在Update中进行：

1. foreach遍历字典有GC，不过只有第一次有。例如：在update中，两句代码都用foreach遍历同一个字典，则只有第一个foreach有gc产生。

//第一种方法
foreach (var item in peopleDictionary)
{
}
//第二种方法
foreach (KeyValuePair<int, People> keyValuePair in peopleDictionary)
{

}

2. for循环遍历字典，同样的，如果两句代码遍历同一个字典。也只有第一次有GC产生。可以使用下面代码。

IEnumerator ee = peopleDictionary.GetEnumerator();
ee.MoveNext();
for (int i = 0; i < peopleDictionary.Count; ee.MoveNext(), i++)
{
}

3. 使用foreach循环遍历其中的key或values，则会分别产生额外GC，同样只只产生一次。如下面代码

foreach (var item in peopleDictionary.Values)
{
}
foreach (var item in peopleDictionary.Keys)
{
}

4. 再来一个，每帧都有GC产生的方法，哈哈。

//每帧都有GC产生
IEnumerator ee = peopleDictionary.GetEnumerator();
ee.MoveNext();
for (int i = 0; i < peopleDictionary.Count; ee.MoveNext(), i++)
{
}
//只有一次有GC产生，关键字var
var ee = peopleDictionary.GetEnumerator();
ee.MoveNext();
for (int i = 0; i < peopleDictionary.Count; ee.MoveNext(), i++)
{
}

5. GC产生的大小，与字典存的数据结构有关，存int的GC 就小于存Class GC。

总结：

1. 遍历List、Array。for循环、 foreach循环、while循环都没有GC。

2. 遍历Dictionary。for循环、 foreach循环，第一次都会产生GC。只有一次也可以放心使用。

最后来一段测试代码，大家可以自行测试

using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using UnityEngine.Profiling;
//using System.Collections.Specialized;

public class People
{
    public string Name;
    public int Age;
}

public class Test : 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);
        }
    }
    void Update()
    {
        Profiler.BeginSample("Foreach Int Dictionary  PeoplePeoplePeople");
        foreach (KeyValuePair<int, People> keyValuePair in peopleDictionary)
        {
        }
        Profiler.EndSample();

        Profiler.BeginSample("Foreach Int Dictionary-2-2-2");
        foreach (KeyValuePair<int, People> keyValuePair in peopleDictionary)
        {

        }
        Profiler.EndSample();

        Profiler.BeginSample("Foreach Int Dictionary-5-5-5");
        foreach (var item in peopleDictionary)
        {

        }
        Profiler.EndSample();


        Profiler.BeginSample("For Int Dictionary1111----");
        var e = peopleDictionary.GetEnumerator();
        e.MoveNext();
        for (int i = 0; i < peopleDictionary.Count; e.MoveNext(), i++)
        {

        }
        Profiler.EndSample();

        Profiler.BeginSample("For Int Dictionary2222----");
        IEnumerator ee = peopleDictionary.GetEnumerator();
        ee.MoveNext();
        for (int i = 0; i < peopleDictionary.Count; ee.MoveNext(), i++)
        {

        }
        Profiler.EndSample();


        Profiler.BeginSample("Foreach Int Dictionary-1-1-1");
        foreach (var item in peopleDictionary.Keys)
        {

        }
        Profiler.EndSample();

        Profiler.BeginSample("Foreach Int Dictionary000----");
        foreach (var item in peopleDictionary.Values)
        {

        }
        Profiler.EndSample();

        Profiler.BeginSample("Foreach Int Dictionary999----");
        foreach (People item in peopleDictionary.Values)
        {

        }
        Profiler.EndSample();
    }
}