WPF TextBox Height Equals Parent Height minus 50 pixels?

I'm successfully getting a 开发者_StackOverflowTextBox to resize with a parent Border height but I need the TextBox to actually be 50 pixels smaller in height than the parent Border.

Any ideas how to achieve this?

The code I'm using is

<Border VerticalAlignment="Stretch" HorizontalAlignment="Stretch" >
     <TextBox x:Name="txtActivityNotes" HorizontalAlignment="Stretch" Height="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Border}}, Path=ActualHeight}" AcceptsReturn="True" VerticalContentAlignment="Top" TextWrapping="WrapWithOverflow" VerticalScrollBarVisibility="Auto" />
</Border>

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Can't you just set a bottom margin of 50?

<TextBox Margin="0,0,0,50" />

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I've tried following and it works. add following to text box in xaml:

------
VerticalAlignment="Stretch" 
HorizontalAlignment="Stretch" 
Height="{Binding RelativeSource={RelativeSource FindAncestor, *AncestorType*={x:Type *Grid*}}, Path=ActualHeight}"
------

Here, AncestorType is type of container containing the textBox. In my case it was 'Grid'. And also add margin like,

  Margin="0,0,0,50"

to keep distance from border below.

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oops sorry i posted on the same page.!

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how about using a converter on the binding to minus 50 off the height

heres an example of using a converter