text: where & how we use interface in .net application, are we implementing interfaces when importing system.data? why we need to use interfaces in .net application when we can do the same using other oops concepts, r we using/implementing .net frameworks interfaces when using namespaces like system.data, system.data.sqlclient  when to use abstract class & when to use interface you can only inherit from a single class.  you can implement at many different interfaces into a class as you want/need.  interfaces allow unlike classes to bahave the same way.