Aligning controls on both left and right side in a stack panel in WPF

I have the following code:

<DockPanel>
    <StackPanel DockPanel.Dock="Top" Orientation="Horizontal">
        <RadioButton Content="_Programs" 
                    IsChecked="{Binding Path=ProgramBanksSelected}" IsEnabled="{Binding Path=ProgramsEnabled}" Margin="8" />
        <StackPanel>
            <Label Content="Master" Height="28" Name="MasterFileStatus" VerticalContentAlignment="Center"/>
        </StackPanel>
    </StackPanel>
    ...

The radio button should be placed on the left side in the stack panel (I removed some buttons for not cluttering the example) and the label (which I put temporarily in a nested StackPanel) should be on the right side.

I tried already lots of combinations of alignments but I cannot get the label on the right side. What should I add to accomplish this?

Just do not use a StackPanel, StackPanels stack. They do, for obvious reasons, not allow alignment in the direction in which they stack. Use a Grid, with column definitions like so:

<Grid.ColumnDefinitions>
	<ColumnDefinition Width="Auto" />
	<ColumnDefinition Width="*" />
	<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>

Even though this is old, should someone come across this like I did, here's a simple solution.

Make a new grid and inside that grid put two stack panels with different Horizontal Alignment.

<Grid>
    <StackPanel Orientation="Horizontal" HorizontalAlignment="Left">
        <!--Code here-->
    </StackPanel>
    <StackPanel Orientation="Horizontal" HorizontalAlignment="Right">
        <!--Code here-->
    </StackPanel>
</Grid>

The possible issue is that now without extra handling the two could overlap with each other.

As you have set the StackPanel's orientation to Horizontal, the HorizontalAlignment property won't work on child-elements. You can keep the StackPanel if you need additional controls, though I would recommend switching to a Grid (among other things) to build the layout you want.

Also, the Grid will allow you to control the actual width of each column:

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="50" />
        <ColumnDefinition Width="150" />
    </Grid.ColumnDefinitions>

    <RadioButton
        Grid.Column="0"
        ...
    />

    <Label
        Grid.Column="1"
        ...
    />
</Grid>

User @pasx is right. You should use DockPanel and dock the RadioButton to the left side, and your StackPanel with the label to the right side.

<DockPanel>

    <DockPanel 
        DockPanel.Dock="Top" 
        LastChildFill="False" >

        <RadioButton 
            DockPanel.Dock="Left" 
            Content="_Programs" 
            IsChecked="{Binding Path=ProgramBanksSelected}"
            IsEnabled="{Binding Path=ProgramsEnabled}" 
            Margin="8" />

        <StackPanel
            DockPanel.Dock="Right">

            <Label 
                Content="Master" 
                Height="28" 
                Name="MasterFileStatus" 
                VerticalContentAlignment="Center"/>

        </StackPanel>

    </DockPanel>
    ...

Use the Stackpanel orientation type in a nested fashion as shown in the sample. The Grid Width is set to a parent width in order to achieve full width.

<StackPanel x:Name="stackBlah" Orientation="Vertical">
	<StackPanel Orientation="Horizontal">
		<Grid Width="{Binding ActualWidth, ElementName=stackBlah}" >
			<Grid.ColumnDefinitions>
				<ColumnDefinition Width="*" />
				<ColumnDefinition Width="Auto" />
			</Grid.ColumnDefinitions>
			<Grid.RowDefinitions>
				<RowDefinition />
			</Grid.RowDefinitions>

			<StackPanel Grid.Column="0" HorizontalAlignment="Left" >
				<Button Content="Some button" />
			</StackPanel>
			<StackPanel Grid.Column="1" HorizontalAlignment="Right">
				<ToggleSwitch Header="Some toggle" AutomationProperties.Name="ToggleNotifications"/>
			</StackPanel>
		</Grid>
	</StackPanel>
</StackPanel>