I am holding a C type Schengen visa received from Italy, valid from August 2017 to February 2018. On November 2017, I received another visa from Italy, this time D type National visa (not Schengen); it's for work purposes and it is valid for 1 year. Did my C type visa lose its validity after the issue of the National visa?
It does not matter.
Your D visa gives you extended rights to stay in Italy and has for all practical purposes the same validity as a C visa in all other Schengen countries.