What is the easy way to set spacing between items in StackPanel?

I use a transparent separator, which works well:

<Separator Opacity="0" Height="20"/>

You can of course use margins but then if you want to change the margins you have to update all of the elements.

The separator can even be styled in a static resource.

An attached property could do it too but I think it's overkill.

if all the controls are the same then do as IanR suggested and implement a Style that catches that control. if it's not then you can't create a default style to a base class because it just won't work.

the best way for situations like these is to use a very neat trick - attached properties (aka Behaviors in WPF4)

you can create a class that has an attached property, like so:

public class MarginSetter{    public static Thickness GetMargin(DependencyObject obj)    {        return (Thickness)obj.GetValue(MarginProperty);    }    public static void SetMargin(DependencyObject obj, Thickness value)    {        obj.SetValue(MarginProperty, value);    }    // Using a DependencyProperty as the backing store for Margin.  This enables animation, styling, binding, etc...    public static readonly DependencyProperty MarginProperty =        DependencyProperty.RegisterAttached("Margin", typeof(Thickness), typeof(MarginSetter), new UIPropertyMetadata(new Thickness(), CreateThicknesForChildren));    public static void CreateThicknesForChildren(object sender, DependencyPropertyChangedEventArgs e)    {        var panel = sender as Panel;        if (panel == null) return;        foreach (var child in panel.Children)        {            var fe = child as FrameworkElement;            if (fe == null) continue;            fe.Margin = MarginSetter.GetMargin(panel);        }    }}

now, to use it, all you need to do is to attach this attached property to any panel you want, like so:

<StackPanel local:MarginSetter.Margin="10">    <Button Content="hello " />    <Button Content="hello " />    <Button Content="hello " />    <Button Content="hello " /></StackPanel>

Completely reusable of course.

The accepted answer doesn't work anymore. But I used that answer and the blog the author of that answer (Elad Katz) to make a working code (tested in .Net Core) that I reproduce here:

    public static class EstablecedorMargen {     public static Thickness GetMargen(DependencyObject objeto) => objeto != null ? (Thickness)objeto.GetValue(PropiedadMargen) : new Thickness();    public static void SetMargen(DependencyObject objeto, Thickness value) => objeto?.SetValue(PropiedadMargen, value);    public static readonly DependencyProperty PropiedadMargen         = DependencyProperty.RegisterAttached("Margen", typeof(Thickness), typeof(EstablecedorMargen), new UIPropertyMetadata(new Thickness(), Cambió));    public static void Cambió(object sender, DependencyPropertyChangedEventArgs e) {        if (!(sender is Panel panel)) return;        panel.Loaded += new RoutedEventHandler(EstablecerMargenControlesHijos);    }     public static void EstablecerMargenControlesHijos(object sender, RoutedEventArgs e) {        if (!(sender is Panel panel)) return;        foreach (var hijo in panel.Children) {            if (!(hijo is FrameworkElement feHijo)) continue;            feHijo.Margin = GetMargen(panel);        }    } } 

Then you use:

   <StackPanel local:EstablecedorMargen.Margen="10" >                      <Button Content="1" />            <Button Content="2" />            <Button Content="3" />    </StackPanel>