Math.Atan in c#

Question

Hye 

I am having a stupid problem.

As It is Illustrated Tan Inverse(1)==45

In C# It Should Return Math.Atan(1 * Math.PI/180) == 45

but it is not

Its is returning something 0.01743.......

I know the degree to radians etc. so please help me out as it is simple question? what is problem behind it, i never noticed and trusted C# calculation, but i cost me a big problem.