I want to use QGIS to write polyline coordinates in a file. I did already successfully created .shp file. But It is hard to code(in C#) to fetch the coordinates. I need to fetch the coordinates in my tool to perform different calculations. QGIS is a very easy and free way to avoid creating a gui just of creating polyline files.
I think GeoJSON would be your best format.
If you want to host it online and use a web gui to make your edits, there is GeoJSON.io
But if you want to stick to QGIS desktop, I think you're going to have to use a .SHP to create and edit your lines (or PostGIS if you've got the banwidth), then convert the .SHP to GeoJSON (right-click layer, save as, choose GeoJSON format, coordinate system - which will work with a PostGIS layer in QGIS as well...). That's a manual way to do it, but you might script it somehow as well...