c# - dotNet should a class library only expose an interface or an implementation too -

I am reading myself because I go and am the only developer in our organization. I have started separating specific functions in class libraries so that the need for organization increases, I can reuse that code.

I have for example emailing tingi and mailing in the cloud library when I add library reference to both resolutions.

I also feel that this reduces the repetition of the code but some hoopers jumping (for example daily mailing ting rules and monthly mailing rules)

Is this class in the library Is better practice to implement code only on interface and project basis?

I am using C #netnet but I think architectural decision language at this level is neutral ...

  SolutionRoot1 --ClassLibrary --- - IMailing ---- Mailing - Project 1 ---- References - Project 2 - --- Reference   
 or  
  SolutionRoot1 --ClassLibrary ---- IMailing - Project 1 ---- context ---- mailing - project 2 ---- context ---- mailing    
 
 
 Implementation is better to share if code is used again if you want to Avsayik rules are due to jump through hoops, so should you inherited so that you can define to share the basics and uniqueness of each project.  
 The best practice is to share an interface and share an implementation so that you can choose the best solution to the problem.