Your comments

It would be great to have a demo doing this! Both for WPF and SharpDX...

For WPF, I have the following links
The issue is now at GitHub - and it may be better to discuss the implementation there :-)
Right, if you still want to set camera properties outside the 'valid' range, the coercing solution may not be good.
I think we need some real life examples/demos to solve this issue. I see a lot of possible requirements to what should be restricted (camera position, direction, field of view etc) and it would be great if we could cover them all...
Sorry, I don't understand the problem desription and the link is invalid.
Have a look at the "Transparency" demo in the ExampleBrowser!
For selection by rectangle see the "RectSelection" example in the ExampleBrowser!
AnimateOpacity is used by the HelixViewport3D to animate the opacity of the view cube viewport.

Tip: Use "Find Usages" (Shift+F12 with my shortcut layout) in Visual Studio when you want to find examples!
@przem, @smischke or @der_meister - can you reply on this?