The app I am working on have 2 states, and on each state the user will see a different set of buttons and menu items on the app bar.

I didnt want to write the app bar completely in C#, for I always choose XAML over C# when possible. Otherwise we would just write it all in C#! 

I started by creating the buttons as app resources in APP.XAML. Note that some buttons overlap both app bars, so this approach avoids having 2 buttons that do the same thing.

<Application.Resources>
<shell:ApplicationBarIconButton x:Key="ChooseButton" IconUri="/Images/ChooseButton.png" Text="choose">
...
</shell:ApplicationBarIconButton>

 I then added both app bars to APP.XAML resource as well, referencing the buttons as StaticResources

<shell:ApplicationBar x:Key="AppBarState1" IsVisible="True" Opacity="0.5">
<shell:ApplicationBar.Buttons>
<StaticResource ResourceKey="ChooseButton" />
...
</shell:ApplicationBar.Buttons>
</shell:ApplicationBar>
 

Now on the page code behind comes the inevitable C#. I feel this is perfectly OK as it has to do with procedure and intelligence rather then declaration.

var chooseButton = (ApplicationBarIconButton)App.Current.Resources["ChooseButton"];
chooseButton.Click += ChooseButtonClick;
...

After setting up the button handlers we will decide which app bar to show and assign it to the page's ApplicationBar property:

ApplicationBar = (IApplicationBar)App.Current.Resources["AppBarState1"]; // choose based on app state
 

 I hope this helps some folks. If you think of a more elegant or appropriate way of achieving the same , please let us know.

My WP7 app used C# to change the app bar buttons (see example on StackOverflow), but your solution is much more elegant.  Well done!

Herbie

I am not really sure if it will work on  WP7. Hopefully it will. If you try let us know the results.