Fast way to convert a two dimensional array to a List ( one dimensional )

Well, you can make it use a “blit” sort of copy, although it does mean making an extra copy 🙁

double[] tmp = new double[array.GetLength(0) * array.GetLength(1)];    
Buffer.BlockCopy(array, 0, tmp, 0, tmp.Length * sizeof(double));
List<double> list = new List<double>(tmp);

If you’re happy with a single-dimensional array of course, just ignore the last line 🙂

Buffer.BlockCopy is implemented as a native method which I’d expect to use extremely efficient copying after validation. The List<T> constructor which accepts an IEnumerable<T> is optimized for the case where it implements IList<T>, as double[] does. It will create a backing array of the right size, and ask it to copy itself into that array. Hopefully that will use Buffer.BlockCopy or something similar too.

Here’s a quick benchmark of the three approaches (for loop, Cast<double>().ToList(), and Buffer.BlockCopy):

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        double[,] source = new double[1000, 1000];
        int iterations = 1000;

        Stopwatch sw = Stopwatch.StartNew();
        for (int i = 0; i < iterations; i++)
        {
            UsingCast(source);
        }
        sw.Stop();
        Console.WriteLine("LINQ: {0}", sw.ElapsedMilliseconds);

        GC.Collect();
        GC.WaitForPendingFinalizers();

        sw = Stopwatch.StartNew();
        for (int i = 0; i < iterations; i++)
        {
            UsingForLoop(source);
        }
        sw.Stop();
        Console.WriteLine("For loop: {0}", sw.ElapsedMilliseconds);

        GC.Collect();
        GC.WaitForPendingFinalizers();

        sw = Stopwatch.StartNew();
        for (int i = 0; i < iterations; i++)
        {
            UsingBlockCopy(source);
        }
        sw.Stop();
        Console.WriteLine("Block copy: {0}", sw.ElapsedMilliseconds);
    }


    static List<double> UsingCast(double[,] array)
    {
        return array.Cast<double>().ToList();
    }

    static List<double> UsingForLoop(double[,] array)
    {
        int width = array.GetLength(0);
        int height = array.GetLength(1);
        List<double> ret = new List<double>(width * height);
        for (int i = 0; i < width; i++)
        {
            for (int j = 0; j < height; j++)
            {
                ret.Add(array[i, j]);
            }
        }
        return ret;
    }

    static List<double> UsingBlockCopy(double[,] array)
    {
        double[] tmp = new double[array.GetLength(0) * array.GetLength(1)];    
        Buffer.BlockCopy(array, 0, tmp, 0, tmp.Length * sizeof(double));
        List<double> list = new List<double>(tmp);
        return list;
    }
}

Results (times in milliseconds);

LINQ: 253463
For loop: 9563
Block copy: 8697

EDIT: Having changed the for loop to call array.GetLength() on each iteration, the for loop and the block copy take around the same time.