Is there an existing way to use arbitrary C# expressions in XAML?

I'd like to be able to use arbitrary C# expressions in XAML. Usually this would be to compute a property for a UI element based on two bound values.

For example calculating the width of a UI element based on two other properties.

This is a contrived example of what I'd like the XAML to look like:

<TextBox
   x:Name="textBox1"
   />

<TextBox
   x:Name="textBox2"
   />

<Rectangle
    Height={Double.Parse(textBox1.Text) + Double.Parse(textBox2.Text)}
    />

Of course there is no built-in way of doing this in XAML.

I know that I could use a MultiBinding combined with a custom converter and this is usually the way I do this kind of thing. However it seems to me that it would be so much simpler to just include some C# code in the XAML and I was wondering if anyone out there had already solved this problem with a XAML extension or something else.