>Still feeling brain dead on XAML :(
>
>Simple control:
>
>

><UserControl x:Class="cdsThemes.TestControl2"
>    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
>    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>    Height="300" Width="300">
>    <Grid Background="BlanchedAlmond">
>  <TextBox Height="23"
>             Margin="25,103,0,0"
>             Name="textBox1"
>             VerticalAlignment="Top"
>             HorizontalAlignment="Left"
>             Width="120">Blah Blah Blah</TextBox>
>  </Grid>
></UserControl>

>
>And I use it in another simple control:
>
>

><UserControl x:Class="cdsThemes.TestControl1"
>    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
>    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>    xmlns:local="clr-namespace:cdsThemes"
>    Height="300" Width="300">
>    <Grid>
>    <local:TestControl2 x:Name="testControl2"></local:TestControl2>
>  </Grid>
></UserControl>
>

>
>Lets say I want to change the margin on textBox1 inside of TestControl2 from the XAML in TestControl1. How do I do that?
>
>My actual situation may be harder because textBox1 in that case is a custom control based on a TextBox defined in another assembly. But I can't even get this one to work, so I figured I'd start with the simpler problem.
>
>I can do it easy in C#
>
>

>testControl2.textBox1.Margin = new Thickness(1, 1, 1, 1);
>

>
>just can't seem to work out the XAML.

For a rough and ready solution you can create a public property on the contained control. e.g:

public Thickness TextBoxMargin
        {
            get { return textBox1.Margin; }
            set { textBox1.Margin = value; }
        }

Then, in containing controls XAML you can use:

<local:TestControl2 x:Name="testControl2" TextBoxMargin="1,1,1,1"></local:TestControl2>

But I think creating a DependencyProperty is the 'correct' answer.
HTH,
Viv