call opencv function from c++ dll

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I created a c++ dll, that uses Opencv functions, in a c# program. When I try to call a function from the dll containing:

std::vector<cv::Mat> bgChannels(3);
cv::split(bg, bgChannels);

or

cv::Mat mask;
imwrite("masks img.jpg",mask);

it crashes without displaying any error. Any help please? thanks in advance