c# - How to stretch and extend a grid up to a parent margin -

i have simple structure (semplification of more complicated one) made this:

so xaml is:

<grid > <grid.columndefinitions>   <columndefinition width="auto" />   <columndefinition width="*" /> </grid.columndefinitions> <border x:name="border1tab3" borderbrush="gainsboro" borderthickness="5" width="200" margin="10,10,10,10" > </border> <border x:name="border2tab3" borderbrush="gainsboro" background="{x:null}"  minwidth="100" borderthickness="5" grid.column="1" margin="10,10,10,10"  >   <stackpanel name="sptab3" margin="0" background="red" verticalalignment="stretch" horizontalalignment="stretch">  <------red      <grid name="grd7tab3" visibility="visible" background="blue"  showgridlines="true"  verticalalignment="stretch"  margin="10">  <----blue     </grid>    </stackpanel> </border> 

so can see have put red colour in stack panel , blue in grid. extend blue grid red stack panel effect grid doesn't show up.

and if put in red stack panel gets size of inside element instead getting parent size.

i newbie @ wpf speaking if set margin=0 , stretch schouldn't ui element take spake of parent?

thank patrick

simply change stackpanel grid, seems more appropriate case

 <border x:name="border2tab3" borderbrush="gainsboro" background="{x:null}"  minwidth="100" borderthickness="5" grid.column="1" margin="10,10,10,10"  >         <grid name="sptab3" margin="0" background="red" verticalalignment="stretch" horizontalalignment="stretch">             <grid name="grd7tab3" visibility="visible" background="blue"  showgridlines="true"  verticalalignment="stretch"  margin="10">              </grid>         </grid>     </border>