cv::resize() fails with access violation error (caused by )

Question

This is a followup question from this one,
I'm trying to do a simple resize using opencv which ends up crashing!
This is the example code which the access violation occurs:

void Classifier::Preprocess(const cv::Mat& img, std::vector<cv::Mat>* input_channels)
{
    /* Convert the input image to the input image format of the network. */
    cv::Mat sample;
    if (img.channels() == 3 && num_channels_ == 1)
        cv::cvtColor(img, sample, cv::COLOR_BGR2GRAY);
    else if (img.channels() == 4 && num_channels_ == 1)
        cv::cvtColor(img, sample, cv::COLOR_BGRA2GRAY);
    else if (img.channels() == 4 && num_channels_ == 3)
        cv::cvtColor(img, sample, cv::COLOR_BGRA2BGR);
    else if (img.channels() == 1 && num_channels_ == 3)
        cv::cvtColor(img, sample, cv::COLOR_GRAY2BGR);
    else
        sample = img;

    //resize image according to the input
    cv::Mat sample_resized;
    Size size(input_geometry_.width,input_geometry_.height );

    if (sample.size() != input_geometry_)
        cv::resize(sample, sample_resized, size);
    else
        sample_resized = sample;

    //...
}

This is what I get in my C# solution when debugging :

Exception thrown at 0x00007FF8C8D9AA90 (opencv_imgproc310d.dll) in Classification Using dotNet.exe: 0xC0000005: Access violation reading location 0x0000018B000F2000.

If there is a handler for this exception, the program may be safely continued.

When I debug into the opencv code, I can see that sz is a null struct(<struct at NULL>) :

template<typename _Tp> inline
Size_<_Tp>::Size_(const Size_& sz)
    : width(sz.width), height(sz.height) {}

and this clearly causes the access violation!.

What is wrong here and what should I do ?

Update :
More information :
sample.size() is as exactly as img.size() since the last else clause is executed and thus sample = img.
sample.size() = {width=256 height=378 } cv::Size_<int>
num_channels_ is 3
img.channels() is 3 so is sample.channels()
When I use imshow() to display either img or sample, the access violation exception is issued. creating a simple black image and displaying it in imshow() is fine though.

Answer 1

Thanks to @Micka's suggestions in the comments, I found out the cause was first due to the image that was sent to the preprocess method and not the resize method by itself.
Further debugging proved @Micka's suspicion about the faulty code on C# side. I was originally wrote in the C# side :

[DllImport(@"CDll2.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
static extern void Classify_image(byte[] img, uint height, uint width, byte[] out_result, out int out_result_length, int top_n_results = 2);

private string Classify(Bitmap img, int top_n_results)
{
    byte[] result = new byte[200];
    int len;
    var img_byte = (byte[])(new ImageConverter()).ConvertTo(img, typeof(byte[]));

    Classify_image(img_byte, (uint)img.Height, (uint)img.Width,res, out len, top_n_results);

    return  ASCIIEncoding.ASCII.GetString(result);
}

and this was the code on the dll part :

CDLL2_API void Classify_image(unsigned char* img_pointer, unsigned int height, unsigned int width, char* out_result, int* length_of_out_result, int top_n_results)
    {
        auto classifier = reinterpret_cast<Classifier*>(GetHandle());
        cv::Mat img = cv::Mat(height, width, CV_32FC3, (void*)img_pointer);

        std::vector<Prediction> result = classifier->Classify(img, top_n_results);

        //misc code...
        *length_of_out_result = ss.str().length();
    }

This proved to be plain wrong, or at the very least buggy. Using this code, the image on the C++ side seemed to have been initialized properly, the number of channels, the height, and width all seemed fine.
But the moment you attempt to use the image, either by resizing it or even showing it using imshow(), it would crash the application and be giving an access violation exception, the very same error that happened when resizing and is posted in the question.
Looking at this answer, I changed the C# code responsible for handing the image to the dll. the new code is as follows :

//Dll import 
[DllImport(@"CDll2.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
static extern void Classify_Image(IntPtr img, uint height, uint width, int step, byte[] out_result, out int out_result_length, int top_n_results = 2);

//...
//main code 

Bitmap img = new Bitmap(txtImagePath.Text);
BitmapData bmpData = img.LockBits(new Rectangle(0, 0, img.Width, img.Height),  ImageLockMode.ReadWrite,  img.PixelFormat);
result = Classify_Image(bmpData.Scan0, (uint)bmpData.Height, (uint)bmpData.Width, bmpData.Stride, res, out len, top_n_results);
img.UnlockBits(bmpData); //Remember to unlock!!!

and the C++ code in the DLL :

CDLL2_API void Classify_Image(unsigned char* img_pointer, unsigned int height, unsigned int width, int step, char* out_result, int* length_of_out_result, int top_n_results)
    {
        auto classifier = reinterpret_cast<Classifier*>(GetHandle());

        cv::Mat img = cv::Mat(height, width, CV_8UC3, (void*)img_pointer, step);

        std::vector<Prediction> result = classifier->Classify(img, top_n_results);

        //...
        *length_of_out_result = ss.str().length();
    }

and doing so rectified all access violations I was getting previously.