How far along does software compiled for .NET 3.5 get before crashing on a system that only has .NET 2.0 installed?
The application I am developing uses WPF and requires .NET 3.5, but I would like to display a user-friendly dialog (rather than crashing) if the user does not have it installed.
Are there any standard ways to do this, or official Microsoft documentation on it?
It (probably) wont crash until it tries to use a dll that needs 3.5. If the executing app can check the version before using any 3.5 specific dlls, you can display a winform dialog and you should be ok. Your safest bet would be to make the exe be a 2.0 assembly and make all of your 3.5 stuff in a separate dll that is compiled against 3.5. You could do your check in the 2.0 dll before it loads any of your 3.5 assemblies.
.NET 3.5 uses .NET 2.0 runtime, so an app will start perfectly fine (however, it will fail when it will attempt to load 3.5 assemblies). You can check Environment.Version to see if it's .NET 3.5 you're running on and present user with standard MessageBox.Show if not.
Have you considered using ClickOnce deployment? The agent will check for and install any prerequisite items you specify, including .Net. It also makes pushing upgrades of your application fairly painless.