why some WPF controls do not appear in the toolbox

Some WPF control do 开发者_开发百科not appear in the toolbox panel in the visual studio 2008 sp1, like webbrowser. Is this by design?

Some controls were added in WPF version 4 (Visual Studio 2010). For example, WebBrowser. That is why it does not show up in Visual Studio 2008 (which supports only upto WPF 3.5 SP1).