c# - Find the nearest point on a line

I have a line that exists between two latlon positions, and a point on a certain latlon position. What I want to know is what the nearest point on that line is (in respect to the other point). So a latlon position on that line.

I know how to do the basic math, but I just can't get my head around the latlong calculation. Can anyone help me or give me some pointers on how to do it?

Thanks