Wpf – How to align side by side in stackpanel

Use Margin or Padding, applied to the scope within the container:

<StackPanel>
    <StackPanel.Resources>
        <Style TargetType="{x:Type TextBox}">
            <Setter Property="Margin" Value="0,10,0,0"/>
        </Style>
    </StackPanel.Resources> 
    <TextBox Text="Apple"/>
    <TextBox Text="Banana"/>
    <TextBox Text="Cherry"/>
</StackPanel>

EDIT: In case you would want to re-use the margin between two containers, you can convert the margin value to a resource in an outer scope, f.e.

<Window.Resources>
    <Thickness x:Key="tbMargin">0,10,0,0</Thickness>
</Window.Resources>

and then refer to this value in the inner scope

<StackPanel.Resources>
    <Style TargetType="{x:Type TextBox}">
        <Setter Property="Margin" Value="{StaticResource tbMargin}"/>
    </Style>
</StackPanel.Resources>

Well, you already found the solution ;) StackPanels won't fill any remaining space by default because their size is always equal to the combined required size of their child elements. Grid is a great way to go because it will dynamically resize when the browser size changes. Mark's answer of using a DockPanel works fine too. The only other method would be to manually size the elements when the parent control's size changes. In general through, stick with Grids for the outer layout and they fill them up with StackPanels and other controls and you should be set.