OpenCvSharp3のMatに、OpenCV2系でデコードした画像データをコピーする

MyDecoder.cs

using System;
using System.Runtime.InteropServices;
using OpenCvSharp; // OpenCvSharp3

static class MyDecoder
{
    public static unsafe Mat FromImageDataByOpenCV2(
        [NotNull] byte[] imageData,
        ImreadModes modes = ImreadModes.Color)
    {
        if (imageData == null)
            throw new ArgumentNullException(nameof(imageData));
            
        fixed (byte* pImageData = imageData)
        {
            CvMat src;
            cvInitMatHeader(&src, imageData.Length, 1, CV_8U, pImageData, CV_AUTO_STEP);

            CvMat* decoded = null;
            try
            {
                int isColor = (int)modes;
                decoded = cvDecodeImageM(&src, isColor);

                var dst = new Mat(decoded->rows, decoded->cols, decoded->type);
                long srcLength = decoded->step * decoded->rows;
                long dstLength = dst.Step() * dst.Rows;
                Buffer.MemoryCopy(decoded->ptr, dst.Data.ToPointer(), dstLength, srcLength);
                return dst;
            }
            finally
            {
                if (decoded != null)
                    cvReleaseMat(&decoded);
            }
        }
    }

    private const int CV_8U = 0;
    private const int CV_AUTO_STEP = 0x7fffffff;
    private const int CV_LOAD_IMAGE_GRAYSCALE = 0;
    private const int CV_LOAD_IMAGE_COLOR = 1;

    [DllImport("opencv_highgui2410.dll", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
    private static extern unsafe CvMat* cvDecodeImageM(CvMat* buf, int iscolor);

    [DllImport("opencv_core2410.dll", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
    private static extern unsafe CvMat* cvInitMatHeader(CvMat* mat, int rows, int cols, int type, void* data, int step);

    [DllImport("opencv_core2410.dll", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
    private static extern unsafe void cvReleaseMat(CvMat** mat);

    [StructLayout(LayoutKind.Sequential)]
    private unsafe struct CvMat
    {
        public int type;
        public int step;
        public int* refcount;
        public int hdr_refcount;
        public byte* ptr;
        public int rows;
        public int cols;
    }
}