System.Environment.OSVersion returns wrong version

Windows 10 returns that string unless you declare that your application is compatible using a manifest. To do so add an app.manifest (right click your project -> Add -> New Item -> Application Manifest File) then uncomment the following line:

<supportedOS Id="{8e0f7a12-bfb3-4fe8-b9a5-48fd50a15a9a}" />

You can do the same thing for Windows Vista to Windows 10. All are in the same section:

<application>
  <!-- A list of the Windows versions that this application has been tested on and is
       is designed to work with. Uncomment the appropriate elements and Windows will 
       automatically selected the most compatible environment. -->

  <!-- Windows Vista -->
  <!--<supportedOS Id="{e2011457-1546-43c5-a5fe-008deee3d3f0}" />-->

  <!-- Windows 7 -->
  <!--<supportedOS Id="{35138b9a-5d96-4fbd-8e2d-a2440225f93a}" />-->

  <!-- Windows 8 -->
  <!--<supportedOS Id="{4a2f28e3-53b9-4441-ba9c-d69d4a4a6e38}" />-->

  <!-- Windows 8.1 -->
  <!--<supportedOS Id="{1f676c76-80e1-4239-95bb-83d0f6d0da78}" />-->

  <!-- Windows 10 -->
  <supportedOS Id="{8e0f7a12-bfb3-4fe8-b9a5-48fd50a15a9a}" />

</application>

And now when you run your application it’ll report the correct 10.0.*.0 version