两者之间不太可能有巨大的性能差异。与往常一样，当面对“哪个更快?”的问题时，您应该始终认为“我可以测量这个”。

在循环体中编写两个做相同事情的循环，执行并计时，并查看速度的差异。使用一个几乎为空的主体和一个与您实际要做的类似的循环主体来执行此操作。还可以尝试使用您正在使用的集合类型，因为不同类型的集合可能具有不同的性能特征。

我认为使用Parallel.ForEach()以及ConcurrentDictionary或ConcurrentBag会更快

下面是Parallel.ForEach()的例子

        var primeNumbers = new ConcurrentBag<T>();
        Parallel.ForEach(numbers, number =>
        {
            if (IsPrime(number))
            {
                primeNumbers.Add(number);
            }
        });

And

 var productImage = new ConcurrentDictionary<int,ResultModel>();
  Parallel.ForEach(pendingActiveImagesBatch, pictureItem =>
  {                           
     productImage.TryAdd(pictureItem.Id,pictureItem));
  });

引用平行。ForEach 参考ConcurrentDictionary最后

我发现foreach循环迭代列表更快。下面是我的测试结果。在下面的代码中，我分别迭代一个大小为100、10000和100000的数组，使用for和foreach循环来测量时间。

private static void MeasureTime()
    {
        var array = new int[10000];
        var list = array.ToList();
        Console.WriteLine("Array size: {0}", array.Length);

        Console.WriteLine("Array For loop ......");
        var stopWatch = Stopwatch.StartNew();
        for (int i = 0; i < array.Length; i++)
        {
            Thread.Sleep(1);
        }
        stopWatch.Stop();
        Console.WriteLine("Time take to run the for loop is {0} millisecond", stopWatch.ElapsedMilliseconds);

        Console.WriteLine(" ");
        Console.WriteLine("Array Foreach loop ......");
        var stopWatch1 = Stopwatch.StartNew();
        foreach (var item in array)
        {
            Thread.Sleep(1);
        }
        stopWatch1.Stop();
        Console.WriteLine("Time take to run the foreach loop is {0} millisecond", stopWatch1.ElapsedMilliseconds);

        Console.WriteLine(" ");
        Console.WriteLine("List For loop ......");
        var stopWatch2 = Stopwatch.StartNew();
        for (int i = 0; i < list.Count; i++)
        {
            Thread.Sleep(1);
        }
        stopWatch2.Stop();
        Console.WriteLine("Time take to run the for loop is {0} millisecond", stopWatch2.ElapsedMilliseconds);

        Console.WriteLine(" ");
        Console.WriteLine("List Foreach loop ......");
        var stopWatch3 = Stopwatch.StartNew();
        foreach (var item in list)
        {
            Thread.Sleep(1);
        }
        stopWatch3.Stop();
        Console.WriteLine("Time take to run the foreach loop is {0} millisecond", stopWatch3.ElapsedMilliseconds);
    }

更新

在@jgauffin建议后，我使用了@johnskeet代码，发现使用数组的for循环比下面的更快，

Foreach循环与数组。 For带列表的循环。 Foreach循环与列表。

请看下面我的测试结果和代码，

private static void MeasureNewTime()
    {
        var data = new double[Size];
        var rng = new Random();
        for (int i = 0; i < data.Length; i++)
        {
            data[i] = rng.NextDouble();
        }
        Console.WriteLine("Lenght of array: {0}", data.Length);
        Console.WriteLine("No. of iteration: {0}", Iterations);
        Console.WriteLine(" ");
        double correctSum = data.Sum();

        Stopwatch sw = Stopwatch.StartNew();
        for (int i = 0; i < Iterations; i++)
        {
            double sum = 0;
            for (int j = 0; j < data.Length; j++)
            {
                sum += data[j];
            }
            if (Math.Abs(sum - correctSum) > 0.1)
            {
                Console.WriteLine("Summation failed");
                return;
            }
        }
        sw.Stop();
        Console.WriteLine("For loop with Array: {0}", sw.ElapsedMilliseconds);

        sw = Stopwatch.StartNew();
        for (var i = 0; i < Iterations; i++)
        {
            double sum = 0;
            foreach (double d in data)
            {
                sum += d;
            }
            if (Math.Abs(sum - correctSum) > 0.1)
            {
                Console.WriteLine("Summation failed");
                return;
            }
        }
        sw.Stop();
        Console.WriteLine("Foreach loop with Array: {0}", sw.ElapsedMilliseconds);
        Console.WriteLine(" ");

        var dataList = data.ToList();
        sw = Stopwatch.StartNew();
        for (int i = 0; i < Iterations; i++)
        {
            double sum = 0;
            for (int j = 0; j < dataList.Count; j++)
            {
                sum += data[j];
            }
            if (Math.Abs(sum - correctSum) > 0.1)
            {
                Console.WriteLine("Summation failed");
                return;
            }
        }
        sw.Stop();
        Console.WriteLine("For loop with List: {0}", sw.ElapsedMilliseconds);

        sw = Stopwatch.StartNew();
        for (int i = 0; i < Iterations; i++)
        {
            double sum = 0;
            foreach (double d in dataList)
            {
                sum += d;
            }
            if (Math.Abs(sum - correctSum) > 0.1)
            {
                Console.WriteLine("Summation failed");
                return;
            }
        }
        sw.Stop();
        Console.WriteLine("Foreach loop with List: {0}", sw.ElapsedMilliseconds);
    }

For有更简单的逻辑实现，所以它比foreach快。

在我的Windows Mobile项目中，我为控件集合使用了for循环。20个控制需要100毫秒!foreach循环只使用了4毫秒。这是一个性能问题……

Jeffrey Richter在techhed 2005上说:

"I have come to learn over the years the C# compiler is basically a liar to me." .. "It lies about many things." .. "Like when you do a foreach loop..." .. "...that is one little line of code that you write, but what the C# compiler spits out in order to do that it's phenomenal. It puts out a try/finally block in there, inside the finally block it casts your variable to an IDisposable interface, and if the cast suceeds it calls the Dispose method on it, inside the loop it calls the Current property and the MoveNext method repeatedly inside the loop, objects are being created underneath the covers. A lot of people use foreach because it's very easy coding, very easy to do.." .. "foreach is not very good in terms of performance, if you iterated over a collection instead by using square bracket notation, just doing index, that's just much faster, and it doesn't create any objects on the heap..."

按需网络直播: http://msevents.microsoft.com/CUI/WebCastEventDetails.aspx?EventID=1032292286&EventCategory=3&culture=en-US&CountryCode=US