C# – WPF: Execute a Command Binding in a search field when pressing the enter button

I think what you're asking is you want a code-behind file for a ResourceDictionary. You can totally do this! In fact, you do it the same way as for a Window:

Say you have a ResourceDictionary called MyResourceDictionary. In your MyResourceDictionary.xaml file, put the x:Class attribute in the root element, like so:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:Class="MyCompany.MyProject.MyResourceDictionary"
                    x:ClassModifier="public">

Then, create a code behind file called MyResourceDictionary.xaml.cs with the following declaration:

namespace MyCompany.MyProject
{
    partial class MyResourceDictionary : ResourceDictionary
    { 
       public MyResourceDictionary()
       {
          InitializeComponent();
       }     
       ... // event handlers ahead..
    }
}

And you're done. You can put whatever you wish in the code behind: methods, properties and event handlers.

== Update for Windows 10 apps ==

And just in case you are playing with UWP there is one more thing to be aware of:

<Application x:Class="SampleProject.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:rd="using:MyCompany.MyProject">
<!-- no need in x:ClassModifier="public" in the header above -->

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>

                <!-- This will NOT work -->
                <!-- <ResourceDictionary Source="/MyResourceDictionary.xaml" />-->

                <!-- Create instance of your custom dictionary instead of the above source reference -->
                <rd:MyResourceDictionary />

            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>

</Application>

It sounds like you want a StackPanel where the final element uses up all the remaining space. But why not use a DockPanel? Decorate the other elements in the DockPanel with DockPanel.Dock="Top", and then your help control can fill the remaining space.

XAML:

<DockPanel Width="200" Height="200" Background="PowderBlue">
    <TextBlock DockPanel.Dock="Top">Something</TextBlock>
    <TextBlock DockPanel.Dock="Top">Something else</TextBlock>
    <DockPanel
        HorizontalAlignment="Stretch" 
        VerticalAlignment="Stretch" 
        Height="Auto" 
        Margin="10">

      <GroupBox 
        DockPanel.Dock="Right" 
        Header="Help" 
        Width="100" 
        Background="Beige" 
        VerticalAlignment="Stretch" 
        VerticalContentAlignment="Stretch" 
        Height="Auto">
        <TextBlock Text="This is the help that is available on the news screen." 
                   TextWrapping="Wrap" />
     </GroupBox>

      <StackPanel DockPanel.Dock="Left" Margin="10" 
           Width="Auto" HorizontalAlignment="Stretch">
          <TextBlock Text="Here is the news that should wrap around." 
                     TextWrapping="Wrap"/>
      </StackPanel>
    </DockPanel>
</DockPanel>

If you are on a platform without DockPanel available (e.g. WindowsStore), you can create the same effect with a grid. Here's the above example accomplished using grids instead:

<Grid Width="200" Height="200" Background="PowderBlue">
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
    <StackPanel Grid.Row="0">
        <TextBlock>Something</TextBlock>
        <TextBlock>Something else</TextBlock>
    </StackPanel>
    <Grid Height="Auto" Grid.Row="1" Margin="10">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="100"/>
        </Grid.ColumnDefinitions>
        <GroupBox
            Width="100"
            Height="Auto"
            Grid.Column="1"
            Background="Beige"
            Header="Help">
            <TextBlock Text="This is the help that is available on the news screen." 
              TextWrapping="Wrap"/>
        </GroupBox>
        <StackPanel Width="Auto" Margin="10" DockPanel.Dock="Left">
            <TextBlock Text="Here is the news that should wrap around." 
              TextWrapping="Wrap"/>
        </StackPanel>
    </Grid>
</Grid>