title: Interfaces separated from the class implementation in separate projects?
body: <p>We work on a middle-size project (3 developers over more than 6 months) and need to make following decision: We'd like to have interfaces separated from concrete implementation. The first is to store the interface in a separate file. </p>
  
  <p>We'd like to go further and separate the data even more: We'd like to have one project (CSPROJ) with interface in one .CS file plus another .CS file with help classes (like some public classes used within this interface, some enums etc.).  Then, we'd like to have another project (CSPROJ) with a factory pattern, concrete interface implementation and other "worker" classes. </p>
  
  <p>Any class which wants to create an object implementing this interface must include the first project which contains the interfaces and public classes, not the implementation itself.</p>
  
  <p>This solution has one big disadvantage: it multiplies the number of assemblies by 2, because you would have for every "normal" project one project with interace and one with implementation. </p>
  
  <p>What would you recommend? Do you think it's a good idea to place all interfaces in one separate project rather than one interface in its own project?</p>