If you need modularity, you’d want to look at Prism. Prism has some elements that can help you with MVVM (DelegateCommand and CompositeCommand, for example), but I think it’s more complete with another MVVM framework.
There was a question a few days ago about how to model Prism. Check that out for a detailed explanation of how to consider Prism’s functionality.
High Level Modelling Advice for Prism MVVM
Unity is an implementation of an inversion of control container and it’s definitely good, but Prism has the ability to use other containers. It has builtin support for MEF (which, in turn, is builtin to .NET 4.0), but it’s not your only choice. Look at some of the samples included in Prism and decide what approach you like better. Unity is not complete on its own for UI compositing in my opinion. MEF might be a closer choice if you wanted to try to composite a UI with the MVVM Framework + IoC Framework approach.
MVVM Light is actually a complimentary framework to Prism. Other MVVM frameworks to consider:
- MVVM Foundation (very lightweight… good for small projects)
- Caliburn (very robust framework)
- Caliburn Micro (shares a name and an author from Caliburn, but it’s akin to MVVM Light with some nice conventions)
- ReactiveUI (Formerly “ReactiveXAML”. This is a bit of a brain melter, but if you learn Reactive Extensions for .NET (Rx), this framework is simply amazing… magical in my opinion.)
If I were going to start a new project: I would go with Prism and ReactiveUI.
Prism because you have to have modularity with large projects and I like the ability to remove and add large units of functionality to an app just by deleting or adding DLLs (and you don’t have to implement the DLL sniffing feature like you would with just an IoC + MVVM approach). Easier to test, easier to debug, easier to develop separately. Nice all around.
ReactiveUI because with UI programming these days, most of your time is spent managing your time on the UI thread. Blocking is a no-no… users don’t want to see a UI freeze; they want to see that animated GIF wait symbol spinning so they know they can work on something else while your data is loading. In addition, so much of the value applications provide these days is taking data from disparate systems and putting them together in the UI… not only will you need a good compositing system (Prism), but you will also need a good MVVM framework that treats asynchronous operations as its bread and butter… ReactiveUI is it.