free API to find latitude and longitude from pin code or city in c#.

Hi All,

        we are working on a project where we need to do some task depending on the distance calculation between two areas or cities.

for this we are searching for an open source or free API which will calculate the latitude and longitude which in turn will  calculate the distance. 

I found Geolocation api but it is a paid service. Any help will be appreciated.

Thank you in advance