title: Build a library dll in multiple frameworks
body: <p>I have seen a lot of libraries that are released in multiple frameworks.  In any framework you choose, the library namespaces are the same, but the dll is compiled in .net 2.0, 3.5 or 4.0.</p>
  
  <p>What is the standard or best practice way of having the same code built in the various frameworks?  I'm curious if there is code duplication going on or if there is a better way.</p>