I've seen recently quite a few requiests from the developers externally and internally asking about the ComboBox control for Windows Phone 7 applications. The answer has been that the ComboBox control is not a part of the platform, however if you want to use it you can get one from the Silverlight toolkit (it's not styled for Metro UI though). Here's the quote from the UI Design and Interaction Guide:
"Although requirements and implementations will vary from application to application, utilizing Metro styled elements will create a more consistent and fluid overall UI experience for users."
The reality is that you will not be able to find Combobox in any of the documentation that specify WP7 UX and design. Indeed the Combobox is coming from the desktop UX culture which is driven by the precision of the mouse selection and does not really fit into the "finger touch" paradigm. The general Metro UI guideline is that if a user needs to select an item from the list of potential options, developer could use the List Picker (which I implemented here). However the List Picker can have up to 5 items. If your list is bigger than 5 items than the Picker Box should be used. You could see the samples of the Picker Box usage in the Settings Applet in the WP7 emulator. In the "theme" changing dialog it's the one that changes the accent color:
In the "region and languages" settings page there are a few items that use Picker Box: Region Format, Short Date, First Day of the Week etc..
As you can see from the samples Picker Box essentially is a combination of a button and a dialog with a header and a list box that is customized depending on the data. In the next post I am going to show you how you can implement the Picker Box functionality in your applications.