is my code but is can run in vs2019 but cant in vs2015

i use opencvsharp4 in vs2015 but i cant find Mat.At function

but it run vs2019 on same code

how i can use it in vs2015?

          // At<float> Cant use in vs2015
          // At<float> Can use in vs2015
          // At is OpenCVSharp4 Mat Function

     for (int p = 0; p < prob.Rows; p++)
    {
            Mat prob;
            float centerX = prob.At<float>(p, 0) * image.Width;  // At<float>, At is Error
            float centerY = prob.At<float>(p, 1) * image.Height;
            float width = prob.At<float>(p, 2) * image.Width;
            float height = prob.At<float>(p, 3) * image.Height;
            
            labels.Add(classNames[classes]);
            scores.Add(probability);
            bboxes.Add(new Rect((int)centerX - (int)width / 2, (int)centerY - (int)height / 2, (int)width, (int)height));
    }

[in vs2015 Error Message]

Severity Code Description Project File Line Suppression State Error 
CS7085 By-reference return type 'ref T' is not supported.