How to force WPF app for .NET Framework 3.5 use newer L&F?

Question

I'm thinking of making a simple self-contained .NET application with WPF that can run on any modern Windows system without requiring the end-user to download and install .NET Framework manually.

As far as I know user just need to click on .exe and wait some time until required libraries are installed.

But applications that were compiled for 3.5 use old L&F on most recent Windows.

Is there any way to make applications look like this:

and not like this (if user already has .NET Framework 4+):

Answer 1

Finally I came up with simple solution. My assumption was wrong and it appears .NET Framework actually uses L&F of currently running version, not the targeting one.

1. Create App.config (Project -> Add -> New Item... -> App configuration (.config))

2. Paste following:

   <?xml version="1.0" encoding="utf-8" ?>
   <configuration>
     <startup>
       <!-- Use 4.0 -->
       <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
       <!-- Use 2.0 - 3.5 -->
       <supportedRuntime version="v2.0.50727" />
     </startup>
   </configuration>
   

.NET Framework will scan <supportedRuntime> until it finds available. List of supported runtimes can be found here.