Why are MvvmLight.Command _and_ MvvmLight.Extras.WP7 both needed?

The Extras assembly exists because EventToCommand requires a reference to System.Windows. Interactivity, while ButtonBaseExtensions, RelayCommand, Messenger etc do not need it. Some people are reluctant to add references to assemblies if they can avoid it.

So for those people who don't need EventtoCommand, they onlu use the base assembly, and the others who want the whole program can add Extras.

The MvvmLight.Extras. WP7 assembly provides a WP7-specific assembly that contains the "Extras", i.e. Those things that you may or may not want to use, which includes EventToCommand.

The MvvmLight. WP7 assembly is the WP7-specific assembly that provides the core functionality, which includes ButtonBaseExtensions. It just so happens that in your scenario, both classes are in the same namespace because they both relate to commands.

Unfortunately, the . NET Framework does not provide a mechanism to reference the same namespace from two different assemblies, hence the duplicate xmlns definitions that you need to make.

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