Consider the following code:
using System;
using System.Collections.Generic;
using System.Diagnostics;
namespace ListAllocationPerformance
{
class Program
{
const int count = 100000000;
public static object Memory { get; private set; }
static void Main(string[] args)
{
Console.WriteLine(string.Format("count: {0}", count));
MeasureFunction(FillListWithoutAllocation, "without allocation");
MeasureFunction(FillListWithAllocation, "with allocation");
MeasureFunction(FillArray, "array");
MeasureFunction(FillUnmanagedArray, "unsafe array");
string input = Console.ReadLine();
}
static void MeasureFunction(Action function, string name)
{
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
function();
stopwatch.Stop();
Console.WriteLine(string.Format("Function {0} finished after \t {1}ms", name, stopwatch.ElapsedMilliseconds, count));
}
static void FillListWithoutAllocation()
{
List<int> list = new List<int>();
for (int i = 0; i < count; i++) list.Add(i);
}
static void FillListWithAllocation()
{
List<int> list = new List<int>(count);
for (int i = 0; i < count; i++) list.Add(i);
}
static void FillArray()
{
int[] array = new int[count];
for (int i = 0; i < count; i++) array[i] = i;
}
static void FillUnmanagedArray()
{
unsafe
{
int[] array = new int[count];
fixed(int* ptr = array)
for (int i = 0; i < count; i++) *ptr = i;
}
}
}
}
The result of the release-built is stunning:
count: 100000000
Function without allocation finished after 871ms
Function with allocation finished after 684ms
Function array finished after 168ms
Function unsafe array finished after 91ms
The array outperforms even the pre-allocated list by far! I thought that the list was basically an array under the hood -- but how come that the results differ so much?
