Linq 性能和延迟执行

Question

我已经为 .Net CF 运行了一些测试。基本上，我想比较 for、foreach、扩展方法 ForEach 和 LINQ 查询。这是整个代码（你可以跳过它，以达到困扰我的地步）

namespace ForEachForLINQPerTest
{
 class IntBox
 {
    public int fieldX;
    public int PropertyX { get; set; }
 }

 public partial class MainPage : PhoneApplicationPage
 {
    /// <summary>
    /// size of tested List
    /// </summary>
    public const int TEST_SIZE = 1000000;

    //
    private List<int> m_intList = new List<int>(TEST_SIZE);

    //
    private List<IntBox> m_intBoxList = new List<IntBox>(TEST_SIZE);

    //
    private Stopwatch m_stopwatch = null;
    // Constructor
    public MainPage()
    {
        InitializeComponent();

        for (int i = 0; i < TEST_SIZE; ++i)
        {
            m_intBoxList.Add( new IntBox());
            m_intList.Add(0);
        }
    }

    private void startButton_Click(object sender, RoutedEventArgs e)
    {
        var forTest = ForTest(); // Jitter preheat
        forTest = ForTest();
        forResultTextBlock.Text = forTest;

        var foreachTest = ForEachTest();
        foreachTest = ForEachTest();
        foreachResultTextBlock.Text = foreachTest;

        var exTest = Extenstion();
        exTest = Extenstion();
        ExtensionResultTextBlock.Text = exTest;

        var linqTest = LINQTest();
        linqTest = LINQTest();
        LINQResultTextBlock.Text = linqTest;
    }

    private string LINQTest()
    {
        m_stopwatch = new Stopwatch();
        m_stopwatch.Start();
        long temp = 0;
        var result = from x in m_intList
                     select temp += x;            
        m_stopwatch.Stop();
        var intListTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();
        result.ToList();


        m_stopwatch.Start();
        var result2 = from x in m_intBoxList
                     select temp += x.fieldX;    
        m_stopwatch.Stop();
        var intBoxListFieldTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();
        result2.ToList();

        m_stopwatch.Start();
        var result3 = from x in m_intBoxList
                      select temp += x.PropertyX;    
        m_stopwatch.Stop();
        var intBoxListPropertyTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();
        result3.ToList();
        return String.Format("LINQ test List<int> = {0} \n List<IntBox> field = {1} \n List<IntBos> property = {2}", intListTime, intBoxListFieldTime, intBoxListPropertyTime); 
    }

    private string Extenstion()
    {
        m_stopwatch = new Stopwatch();
        m_stopwatch.Start();
        long temp = 0;
        m_intList.ForEach(i => temp += i);
        m_stopwatch.Stop();
        var intListTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();


        m_stopwatch.Start();
        m_intBoxList.ForEach(i => temp += i.fieldX);
        m_stopwatch.Stop();
        var intBoxListFieldTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();

        m_stopwatch.Start();
        m_intBoxList.ForEach(i => temp += i.PropertyX);
        m_stopwatch.Stop();
        var intBoxListPropertyTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();
        return String.Format("Extenstion test List<int> = {0} \n List<IntBox> field = {1} \n List<IntBos> property = {2}", intListTime, intBoxListFieldTime, intBoxListPropertyTime); 
    }

    private string ForEachTest()
    {
        m_stopwatch = new Stopwatch();
        long temp = 0;
        m_stopwatch.Start();
        foreach(int item in m_intList)
        {
           temp += item;
        }
        m_stopwatch.Stop();
        var intListTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();


        m_stopwatch.Start();
        foreach (IntBox item in m_intBoxList)
        {
            temp += item.fieldX;
        }
        m_stopwatch.Stop();
        var intBoxListFieldTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();

        m_stopwatch.Start();
        foreach (IntBox item in m_intBoxList)
        {
            temp += item.PropertyX;
        }
        m_stopwatch.Stop();
        var intBoxListPropertyTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();

        return String.Format("ForEach test List<int> = {0} \n List<IntBox> field = {1} \n List<IntBos> property = {2}", intListTime, intBoxListFieldTime, intBoxListPropertyTime); 
    }

    private string ForTest()
    {
        m_stopwatch = new Stopwatch();
        m_stopwatch.Start();
        long temp = 0;
        for (int i = 0; i < TEST_SIZE; ++i)
        {
            temp += m_intList[i];
        }
        m_stopwatch.Stop();
        var intListTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();


        m_stopwatch.Start();
        for (int i = 0; i < m_intList.Count; ++i)
        {
            temp += m_intBoxList[i].fieldX;
        }
        m_stopwatch.Stop();
        var intBoxListFieldTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();

        m_stopwatch.Start();
        for (int i = 0; i < m_intList.Count; ++i)
        {
           temp +=  m_intBoxList[i].PropertyX;
        }
        m_stopwatch.Stop();
        var intBoxListPropertyTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();

        return String.Format("For loop test List<int> = {0} \n List<IntBox> field = {1} \n List<IntBos> property = {2}", intListTime, intBoxListFieldTime, intBoxListPropertyTime); 
    }
 }
}

我很困惑

        m_stopwatch = new Stopwatch();
        m_stopwatch.Start();
        long temp = 0;
        var result = from x in m_intList
                     select temp += x;            
        m_stopwatch.Stop();
        var intListTime = m_stopwatch.ElapsedMilliseconds;
        m_stopwatch.Reset();
        result.ToList();

输出是：

For 循环测试 List = 93
List field = 119 // ref -> field
List property = 136 // ref -> property -> 字段属性只是 CF 的函数

ForEach 测试列表 = 88
列表字段 = 140
列表属性 = 152

Extensions test List = 176
// 另一个函数被调用。 列表字段 = 220
列表属性 = 239

LINQ 测试列表 = 0 为什么？
列表字段 = 163
列表属性 = 165

为什么intListTime == 0？我究竟做错了什么？此外，字段和属性的最后两个值几乎相同（运行几次）。这是否意味着 LINQ 查询中的 PropertyX 是内联评估的？

Answer 1

第一次为零，因为表达式树是在编译时构建的，它会在您未包含在计时中的 ToList 调用上进行评估。

对于字段和属性访问时间，我不会太担心 - 实际上，在发布版本中，简单的属性访问器将被内联，提供与字段访问相同的性能。对于 linq 情况，您可能会看到相同的性能，因为 linq 内部可能会将属性/字段访问转换为方法调用，并且会导致相同的时间（因为我相信与字段/属性相比，方法调用开销可能会很大访问。

Answer 2

这称为“延迟执行”。直到需要时才评估 linq 语句。将 ToList 移动到停止时钟之前，时间会增加