All you wanted to know about WPF (and Silverlight) StackPanel..

StackPanel is a bit like a ToolBar that instead of going from left to right or right to left, goes top to bottom (in the default case). Although a StackPanel is actually more flexible..as it can also go from left to right, simply by setting the attribute Orientation="Horizontal". As common with WPF controls, this one has been around since .NET 3.0 and is also included in Silverlight. It inherits from System.Windows.Controls.Panel which inherits from System.Windows.FrameworkElement (from PresentationFramework.dll).  Interestingly, the same inheritance structure is true also for Silverlight! However, the dll in which is class is contained is quite different (System.Windows.dll).