Fill Space Inside a Stackpanel

I want to put a textbox on the left of a combobox and have the combobox fill the rest of the empty, horizontal space.  When I use a Stackpanel and make the stackpanel's horizontalalignment = stretch, the combobox only gets sized to its content, not the width of the stackpanel.  I got this to work by using a Dockpanel instead.  Is using Dockpanel the best way to do this, or is there a more preferred way?  Thanks.

-Shane