Check out example codes for "c# resize image keep aspect ratio". It will help you in understanding the concepts better.

Code Example 1

using System.Drawing;
public static Size ResizeKeepAspect(this Size src, int maxWidth, int maxHeight, bool enlarge = false)
{
    maxWidth = enlarge ? maxWidth : Math.Min(maxWidth, src.Width);
    maxHeight = enlarge ? maxHeight : Math.Min(maxHeight, src.Height);

    decimal rnd = Math.Min(maxWidth / (decimal)src.Width, maxHeight / (decimal)src.Height);
    return new Size((int)Math.Round(src.Width * rnd), (int)Math.Round(src.Height * rnd));
}

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