c# copying from 2d array including cast

Question

I know if I want to copy an array of the same type, I have at least 3 Options, I can use a double for loop, use Array.copy or Buffer.BulkCopy. The two copy methods are much faster. See for instance here: https://stackoverflow.com/a/33030421 .

Both copy methods allow you to only copy parts of the 2d array but Array.copy needs rank of source and destination to be equal while bulk.copy does not.

I get data from a com interface and doubles or int come through as object. Lets say I want to include a cast on the copy. I can do this:

Stopwatch watch = new Stopwatch();
const int width = 2;
const int depth = 10 * 1000000;
Random r = new Random(100);
object[,] objdata = new object[width, depth];
for (int i = 0; i < width; i++)
{
    for (int j = 0; j < depth; j++)
    {
        objdata[i, j] = r.Next();
    }
}


        int[,] arr2dint = new int[width, depth];
        watch.Reset();
        watch.Start();
        Array.Copy(objdata, 0, arr2dint, 0, objdata.GetLength(0) * objdata.GetLength(1));
        watch.Stop();
        Console.WriteLine("ArrayCopy to 2 dimensional array including cast took {0}", watch.ElapsedMilliseconds);
        watch.Reset();

        var bufferloopcast = new int[width, depth];
        watch.Start();
        for (int i = 0; i < width; i++)
        {
            for (int j = 0; j < depth; j++)
            {
                bufferloopcast[i, j] = (int)objdata[i, j];
            }
        }
        watch.Stop();
        Console.WriteLine("Loop-copy to 2 dimensional array including cast took {0} ms", watch.ElapsedMilliseconds);

Now the copy method is slower. Also it comes with the limitation that source and destination rank have to be equal so I cannot use it to copy only part of an array (say only the first row).

I cannot make the Buffer.BulkCopy to work, error must be primitive type. I tried this, in wain:

 int[,] buff2dint = new int[width, depth];
        watch.Reset();
        watch.Start();
        int sizeo = Marshal.SizeOf(objdata[0, 0]);
        Buffer.BlockCopy(objdata, 0, buff2dint, 0, objdata.GetLength(0) * objdata.GetLength(1)* sizeo);
        watch.Stop();
        Console.WriteLine("BufferCopy to 2 dimensional array including cast took {0}", watch.ElapsedMilliseconds);

SO, why did array.copy become so slow? And what is the best method to copy an 2d array, or parts of it, if you need to include a cast?

as a small side note, do you know you can use separators for int litterals? Seems more readable IMO : const int depth = 10_000_000; — Pac0
– Pac0, Commented Apr 28, 2020 at 15:56

Aaron G · Accepted Answer · 2020-04-28 15:54:30Z

1

What about using an array of arrays instead of a 2d-array?

int [][] buff2dint = new int[width][];
for(int i = 0; i < width; ++i)
{
   buff2dint[i] = new int[depth];
   Buffer.BlockCopy(srcArray, 0, buff2dint[i], depth);
}

answered Apr 28, 2020 at 15:54

Aaron G

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1 Comment

nik Over a year ago

thanks but if the source is object[,] and the destination int[] this would still fail, no?

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