title: Mixing & Calling .Net 4.5 from .Net Framework 4.0
body: <p>I have an application the is written and targeting .NET Framework 4.0. 
  In one of the Base Project I have to add a reference that is done in .NET 4.5. So, in order to build this project I had to switch it is target framework to 4.5. </p>
  
  <p>Now, the problem that VS is complaining about each project in the application that is referencing this base project. </p>
  
  <ol>
  <li><p>Is there a way I can make this base project target framework 4.5 without having to changing all the project's that is referencing it.</p></li>
  <li><p>Can I mix both frameworks and call from a .NET 4 project to a .NET 4.5 project.</p></li>
  <li><p>The best solution would be if I can use the new external reference built in .NET 4.5 (that cause all this trouble) from within the base project without having to change the base project Target to .NET 4.5. Is that possible?</p></li>
  </ol>
  
  <p>Basically I would like to use this reference built for 4.5 without having to change the whole application target framework.</p>