.NET boxing / unboxing vs casting performance

Question

I'm trying to understand which of two solutions are preferred from a performance perspective. For example, I have two pieces of code:

1) Boxing / Unboxing

int val = 5;
Session["key"] = val; 
int val2 = (int)Session["key"];

2) Casting (IntObj has int Value property to store int)

IntObj val = new IntObj(5);  
Session["key"] = val;
int val2 = ((IntObj )Session["key"]).Value;

What is the memory management difference between these examples? Is there a faster way to perform such operations?

NOTE: Session is just for example, it can be any Dictionary<string, object>

Answer 1

It looks like what you are really doing here is comparing manual boxing with inbuilt boxing. The inbuilt boxing has been highly optimised - so I wouldn't expect to see a huge difference here, but we can check. Importantly, note that both have the same memory impact: one heap object containing one int field, per int boxed/wrapped.

The following shows pretty-much identical times for the two approached; I would say, therefore, just box it the direct / inbuilt way.

Note: run it in release mode, without a debugger (ideally at the command line). Note the first call is there to pre-JIT everything.

using System;
using System.Diagnostics;
public sealed class IntObj
{
    public readonly int Value;
    public IntObj(int value)
    {
        Value = value;
    }
}
static class Program
{
    static void Main()
    {
        Run(1, 0, false);
        Run(100000, 500, true);
        Console.ReadKey();
    }
    static void Run(int length, int repeat, bool report)
    {
        var data = new object[length];

        int chk = 0;
        var watch = Stopwatch.StartNew();
        for (int j = 0; j < repeat; j++)
        {
            for (int i = 0; i < data.Length; i++)
            {
                data[i] = i;
                chk += i;
            }
        }
        watch.Stop();
        if(report) Console.WriteLine("Box: {0}ms (chk: {1})", watch.ElapsedMilliseconds, chk);
        chk = 0;
        watch = Stopwatch.StartNew();
        for (int j = 0; j < repeat; j++)
        {
            for (int i = 0; i < data.Length; i++)
            {
                chk += (int) data[i];
            }
        }
        watch.Stop();
        if (report) Console.WriteLine("Unbox: {0}ms (chk: {1})", watch.ElapsedMilliseconds, chk);

        chk = 0;
        watch = Stopwatch.StartNew();
        for (int j = 0; j < repeat; j++)
        {
            for (int i = 0; i < data.Length; i++)
            {
                data[i] = new IntObj(i);
                chk += i;
            }
        }
        watch.Stop();
        if (report) Console.WriteLine("Wrap: {0}ms (chk: {1})", watch.ElapsedMilliseconds, chk);
        chk = 0;
        watch = Stopwatch.StartNew();
        for (int j = 0; j < repeat; j++)
        {
            for (int i = 0; i < data.Length; i++)
            {
                chk += ((IntObj)data[i]).Value;
            }
        }
        watch.Stop();
        if (report) Console.WriteLine("Unwrap: {0}ms (chk: {1})", watch.ElapsedMilliseconds, chk);
    }


}