达网络 - 如果是名单< T> .ForEach者优先通过一个标准的foreach循环？ [英] .Net - When is List<T>.ForEach prefered over a standard foreach loop?

问题描述

的泛型列表类有一个 .ForEach（动作< T>动作）方法。现在，我已经做了一些的简单的他们俩是如何执行的时序，它似乎是通用的ForEach是较差的表演。该（摘录编译器友好的）代码如下 -

The generic list class has a .ForEach(Action<T> action) method. Now i've done some simple timings of how they both perform and it seems that the generic ForEach is the poorer performer. The (Snippet Compiler Friendly) code is below -

public static class timer{
    public static long foreachloop = 0;
    public static long Gforeachloop = 0;}

public class something{
    public List<string> myStrings = new List<string>();

    public something()
    {
    	for(int i = 1; i<=5000000;i++)
    	{
    		myStrings.Add(i.ToString());
    	}
    }}

public class cls1{
    private static List<string> Strings = new List<string>();
    private static List<string> OtherStrings = new List<string>();

    public static void RunSnippet()
    {
    	something s = new something();

    	Stopwatch watch = new Stopwatch();
    	watch.Start();
    	foreach(string x in s.myStrings)
    	{
    		Strings.Add(x);
    	}
    	watch.Stop();
    	timer.foreachloop = watch.ElapsedMilliseconds;

    	watch.Reset();
    	watch.Start();

    	s.myStrings.ForEach(delegate(string n){OtherStrings.Add(n);});

    	s.myStrings.Clear();

    	watch.Stop();
    	timer.Gforeachloop = watch.ElapsedMilliseconds;

    	WL("FOREACH-"+timer.foreachloop + ",Count = " + Strings.Count);
    	WL("GFOREACH-"+timer.Gforeachloop + ",Count = " + OtherStrings.Count);
    }

    #region Helper methods

    public static void Main()
    {
    	try
    	{
    		RunSnippet();
    	}
    	catch (Exception e)
    	{
    		string error = string.Format("---\nThe following error occurred while executing the snippet:\n{0}\n---", e.ToString());
    		Console.WriteLine(error);
    	}
    	finally
    	{
    		Console.Write("Press any key to continue...");
    		Console.ReadKey();
    	}
    }

    private static void WL(object text, params object[] args)
    {
    	Console.WriteLine(text.ToString(), args);	
    }

    private static void RL()
    {
    	Console.ReadLine();	
    }

    private static void Break() 
    {
    	System.Diagnostics.Debugger.Break();
    }

    #endregion
}

FOREACH出来在177ms和GFOREACH在707ms。

FOREACH comes out at 177ms and GFOREACH at 707ms.

现在我猜有一个很好的理由使用它，但我只是想不出之一。很明显的表现是不是这个原因所以，问题是什么时候会是最好的选择？

Now I'm guessing there's a good reason for using it but i just can't think of one. Clearly performance isn't the reason so the question is when would it be the best option?

感谢。

推荐答案

这从埃里克利珀博客文章介绍了相关背景：

This blog post from Eric Lippert gives the background:

的 http://blogs.msdn.com/ericlippert/archive/2009/05/18/foreach-vs -foreach.aspx

他在谈论一个扩展方法的共同建议，做同样的事情的IEnumerable< T> ，但哲学异议适用于列表< T> .ForEach 以及

He's talking about the common suggestion of an extension method to do the same thing for IEnumerable<T>, but the philosophical objection applies to List<T>.ForEach as well.

这表明，也许这方法是从来没有这样一个好主意，虽然看起来酷。它更清晰只使用的foreach 。

This suggests that maybe that method was never such a good idea, although it looks "cool". It's clearer to just use foreach.

我认为，这样的方法可以被认为是的一个的经典修复闭合过循环变量错误。

I've suggested that such methods can be thought of as a fix for the classic closure-over-loop-variable bug.

但在实践中我刚刚拿到了地察觉这种虫子更好。

But in practice I've just got better at spotting such bugs.

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