Array bounds check efficiency in .net 4 and above

Question

I\'m interested in how efficient low-level algorithms can be in .net. I would like to enable us to choose to write more of our code in C# rather than C++ in the future, but

Answer 1

First of all, I would like to thank everyone who spoken out in this post, from original OP to the guys who provided extremely detailed and insightful explanations. I really, really enjoyed reading the existing answers. Since there is already plentiful of theory of how and why the loops work in the way they do, I would like to offer some empirical (by some definition authoritative) measurements:

Conclusions:

• Foreach loop is faster than For loop.
• Local variable is faster than the array .Length property.
• GC-pinning using unsafe fixed is not faster than normal For loop.

Benchmarking code:

using System;
using System.Diagnostics;
using System.Runtime;

namespace demo
{
    class MainClass
    {
        static bool ByForArrayLength (byte[] data)
        {
            for (int i = 0; i < data.Length; i++)
                if (data [i] != 0)
                    return false;
            return true;
        }

        static bool ByForLocalLength (byte[] data)
        {
            int len = data.Length;
            for (int i = 0; i < len; i++)
                if (data [i] != 0)
                    return false;
            return true;
        }

        static unsafe bool ByForUnsafe (byte[] data)
        {
            fixed (byte* datap = data)
            {
                int len = data.Length;
                for (int i = 0; i < len; i++)
                    if (datap [i] != 0)
                        return false;
                return true;
            }
        }

        static bool ByForeach (byte[] data)
        {
            foreach (byte b in data)
                if (b != 0)
                    return false;
            return true;
        }

        static void Measure (Action work, string description)
        {
            GCSettings.LatencyMode = GCLatencyMode.LowLatency;
            var watch = Stopwatch.StartNew ();
            work.Invoke ();
            Console.WriteLine ("{0,-40}: {1} ms", description, watch.Elapsed.TotalMilliseconds);
        }

        public static void Main (string[] args)
        {
            byte[] data = new byte[256 * 1024 * 1024];
            Measure (() => ByForArrayLength (data), "For with .Length property");
            Measure (() => ByForLocalLength (data), "For with local variable");
            Measure (() => ByForUnsafe (data), "For with local variable and GC-pinning");
            Measure (() => ByForeach (data), "Foreach loop");
        }
    }
}

Results: (uses Mono runtime)

$ mcs Program.cs -optimize -unsafe
For with .Length property               : 440,9208 ms
For with local variable                 : 333,2252 ms
For with local variable and GC-pinning  : 330,2205 ms
Foreach loop                            : 280,5205 ms