Answer by brunnerh for Aligning controls on both left and right side in a stack panel in WPF

Just do not use a StackPanel, StackPanelsstack. 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>