Get bitmap from .DLL, convert to byte[] and to image

I have received an .DLL for testing purposes, this .dll includes the functionality that will later be used to process live images from hardware.

For this simple .dll, I can open an image (load into memory), get the width and height, and get the pixels which need to be converted to a image. Loading, getting width and getting height is fine, but getting the pixels and converting that to a Bitmap or Image is a problem.

C++ example source that I received:

   ApiFunc->OpenImageFile(this->OpenPictureDialog1->FileName.c_str());
   ApiFunc->AllocMemory();

   w = ApiFunc->GetImageWidth();
   h = ApiFunc->GetImageHeight();
   Image1->Width = w;
   Image1->Height = h;

   unsigned char* ptr = ApiFunc->GetImagePixels();
   COLORREF pixel;
   int r,g,b;
   for(int y=0; y<w; y++)
   {
       for(int x=0; x<h; x++)
       {
           r = (int)*(ptr+3*x);
           g = (int)*(ptr+3*x+1);
           b = (int)*(ptr+3*x+2);

           Image1->Canvas->Pixels[y][x] = RGB(r,g,b);
       }
       ptr += 3*h;
   }

Where in ApiFunc, this can be found:

void __fastcall TAPIFunc::LoadDll(HINSTANCE m_hMain)
{
   //some others above
   GET_IMAGE_PIXELS  = (func_GET_IMAGE_PIXELS   )GetProcAddress( m_hMain, "GET_IMAGE_PIXELS");
   //some others below
}
unsigned char* __fastcall TAPIFunc::GetImagePixels(void)
{
   return GET_IMAGE_PIXELS();
}

So now what I have tried so far, I've tried using byte[] as return parameter, but that throw an MarshalDirectiveException.

    [DllImport("ImageTest.dll")]
    public static extern IntPtr GET_IMAGE_PIXELS();

    private void OpenImage(string filename)
    {
        OPEN_IMAGE_FILE(filename);
        ALLOC_MEMORY();
        int width = GET_IMAGE_WIDTH(); //=800
        int height = GET_IMAGE_HEIGHT(); //=600
        IntPtr buffer = GET_IMAGE_PIXELS();
        int size = width * height * 3;//not sure what the size must be, I think this is one of the issues, just following logic of one answer below.

        //but source: https://stackoverflow.com/a/16300450/2901207
        byte[] bitmapImageArray = new byte[size];
        Marshal.Copy(buffer, bitmapImageArray, 0, size);

        Bitmap bitmap = new Bitmap(width, height, PixelFormat.Format24bppRgb);
        BitmapData bmData = bitmap.LockBits(new System.Drawing.Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.ReadWrite, bitmap.PixelFormat);
        IntPtr pNative = bmData.Scan0;
        Marshal.Copy(imageData, 0, pNative,size);
        bitmap.UnlockBits(bmData);
        bitmap.Save(Environment.CurrentDirectory + @"\result.bmp");
            using (var ms = new MemoryStream(bitmapImageArray))
            {
                //both throw exception: Parameter is not valid
                Bitmap bmp = new Bitmap(ms);
                Image bitmapImage = Image.FromStream(ms);
            }
    }

answer sources:

answer 1, using bitmap.LockBits method answers 2 and 3, using memoryStream

Just to make sure I have a valid image to test with, I saved an image in photoshop, with this option:

ugly test image:

result:

Beautiful isn't it? :)

Also tried using a for-loop, and run until it crashes. ran until count = 1441777 and on another image count = 1527793 (same dimensions).

        int count = 0;
        for (int i = 0; i < width * height * 4; i++)
        {
            count++;
            bitmapImageArray[i] = Marshal.ReadByte(buffer, i);
        }