Can an application target to both 3.5 and 4.5 .Net version?

Question

I write a WPF app, that all source code compatible with both .NET 3.5 and 4.5. That mean if I change .NET target in the Properties to either 3.5 or 4.5, it will complie and run well without any error.
I wonder there is anyway to my my app file can run on both 3.5 and 4.5 .NET pc (example: Windows 7 and Windows 8.1), without complie 2 version for each .NET? Thank!

Edit:
When I copy the .NET 3.5 application in question to a Windows 8.1 pc, Windows refuse to run my app, and require I install .NET 3.5. So if only complie with .NET 3.5, it dont make app can run in .NET 4.5 pc.

Answer 1

Build with .NET 3.5 and add these lines to App.config and it worked for me:

<configuration>
  <startup>
    <supportedRuntime version="v4.0" />
    <supportedRuntime version="v2.0.50727"/>
  </startup>
</configuration>

Answer 2

have your project target 3.5. then add another project to the solution that targets 4.5 and add links to the files from the first project.