When you’re importing Windows Phone view file (XAML + its class file) into a project, it’s shown in Visual Studio as separate files. Compared to when you’re creating a new view file, its class file (code-behind) is grouped behind the XAML file.

To ‘merge’ these two files, you need to edit the project settings file (*.csproj file)

<Compile Include="MainPage.xaml.cs"/>

to

<Compile Include="MainPage.xaml.cs">
    <DependentUpon>MainPage.xaml</DependentUpon>
</Compile>

Reference: http://stackoverflow.com/questions/18121494/how-to-merge-a-xaml-file-and-its-code-behind-with-visual-studio