C# copy array of structs to array of doubles

Question

[StructLayout(LayoutKind.Sequential)]
public struct Demo
{
    double X;
    double Y;
}


var data = new Demo[128];

FillWithMeaningfulValues(data);

double[] doubles;

Copy(data, out doubles); // ?

How do I copy the demo array into the doubles array without having to for(...) through each element? In C++, I would use memcpy, but in C# I did not find what I need in Marshal.Copy.

void MethodIDoNotWantToUse(Demo[] demo, out double[] doubles)
{
    doubles = new double[demo.Length * 2];
    for(int i = 0, j = 0; i < demo.Length; ++i)
    {
        doubles[j++] = demo[i].X;
        doubles[j++] = demo[i].Y;
    }  
}

void MethodIWouldPreferToUse(Demo[] demo, out double[] doubles)
{
    doubles = new double[demo.Length * 2];
    memcopy(doubles, demo, demo.Length * 2 * sizeof(double));
}

You can use simple LINQ query to achieve the result: `double[] doubles = data.SelectMany(d => new[] { d.X, d.Y }).ToArray();`. — Ulugbek Umirov, May 14 '14 at 08:33
@UlugbekUmirov That will still iterate through them one-by-one, though. He's trying to copy a whole block at once. — Tim Goodman, May 14 '14 at 08:36
@TimGoodman You're right, but I don't see much performance benefit of bulk copy if there are only 128 elements. — Ulugbek Umirov, May 14 '14 at 08:36
That's just an example. My real data will be much larger (and also more complex structs, but still only composed of blittable types). I've asked because I know the C++ way but didn't know the C# way to achieve an efficient copy. However, you're right that for many cases, your solution-in-comment would be a perfectly viable answer. — Wilbert, May 14 '14 at 08:39

score 7 · Answer 1 · answered May 14 '14 at 08:32

You'll do something like this. Marshal.Copy do provides you what you need.

Demo[] array = new Demo[2];
array[0] = new Demo {X = 5.6, Y= 6.6};
array[1] = new Demo {X = 7.6, Y = 8.6};
GCHandle handle = GCHandle.Alloc(array, GCHandleType.Pinned);
try
{
    IntPtr pointer = handle.AddrOfPinnedObject();
    double[] copy = new double[array.Length*2];//This length may be calculated
    Marshal.Copy(pointer, copy, 0, copy.Length);
}
finally
{
    if (handle.IsAllocated)
        handle.Free();
}

Yes. I've used this technique for shuffling audio around in memory. +1 — spender, May 14 '14 at 08:34

score 6 · Accepted Answer · answered May 14 '14 at 08:33

Since the struct is blittable, the array of the struct is blittable. Therefore you can pin the array of struct and copy into the double array with Marshal.Copy.

void CopyDemoArrayToDoubleArray(Demo[] demo, out double[] doubles)
{
    doubles = new double[demo.Length * 2];
    GCHandle gch = GCHandle.Alloc(demo, GCHandleType.Pinned);
    try
    {
        IntPtr demoPtr = gch.AddrOfPinnedObject();
        Marshal.Copy(demoPtr, doubles, 0, doubles.Length);
    }
    finally
    {
        gch.Free();
    }
}

You might do well to benchmark this against the simpler for loop that you want to avoid. It is plausible that the for loop will perform perfectly adequately.

Matthew Watson · Answer 3 · 2014-05-14T09:14:40.570

It's possible to write a generic method that can convert arrays of any compatible type (by "compatible" I mean "elements must be value types and the size of the elements must be compatible").

You can use P/Invoke to call the Windows API CopyMemory() method.

However, bear in mind that there may not be any performance advantage to doing it this way; you should perform careful timings to be sure.

[DllImport("kernel32.dll", EntryPoint = "CopyMemory", SetLastError = false)]
public static extern void CopyMemory(IntPtr dest, IntPtr src, uint count);

public TOut[] ConvertArray<TIn, TOut>(TIn[] input) where TIn:struct where TOut:struct
{
    if (input == null)
        throw new ArgumentNullException("input");

    int sizeTIn   = Marshal.SizeOf(typeof(TIn));
    int sizeTOut  = Marshal.SizeOf(typeof(TOut));
    int sizeBytes = input.Length*sizeTIn;

    if ((sizeBytes % sizeTOut) != 0)
        throw new ArgumentException("Size of input type is not compatible with size of output type.");

    int sizeOut = sizeBytes/sizeTOut;

    var output = new TOut[sizeOut];

    GCHandle inHandle  = GCHandle.Alloc(input,  GCHandleType.Pinned);
    GCHandle outHandle = GCHandle.Alloc(output, GCHandleType.Pinned);

    try
    {
        IntPtr inPtr  = inHandle.AddrOfPinnedObject();
        IntPtr outPtr = outHandle.AddrOfPinnedObject();

        CopyMemory(outPtr, inPtr, (uint)sizeBytes);
    }

    finally
    {
        outHandle.Free();
        inHandle.Free();
    }

    return output;
}

For your example, you could call this like so:

Demo[] test = new Demo[10];

for (int i = 0; i < 10; ++i)
    test[i] = new Demo {X = i, Y = i};

var result = ConvertArray<Demo, double>(test);

for (int i = 0; i < 20; ++i)
    Console.WriteLine(result[i]);

C# copy array of structs to array of doubles

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