Programming WPF, 2nd Edition

Limitations of WPF/HWND Interop

As useful as WPF's integration with other presentation technologies is, because WPF's approach to rendering and composition is radically different from the way Win32 UIs have previously worked, there are some limitations to how you can mix the two. Essentially, the new features that WPF offers do not translate back into the old world. The old UI technologies still have the same limitations they always did, even when being hosted by a WPF application.

Airspace

The most important thing to understand is the principle of airspace: within any single top-level window, each pixel belongs to exactly one technology. So, a single pixel must belong either to WPF or to Win32.^[125]

One of the upshots of the airspace principle is that the clip region of a UI element does not apply to any HWND-based children. This is a nonobvious restriction, and it applies because clipping is a composition feature. (Internally, it depends on a UI element and all its children being rendered, and then having the rendered output clipped. A Win32 element cannot render into the internal intermediate buffers that WPF uses to perform clipping.) For example, here we're nesting three Windows Forms controls inside a clipped WPF Grid:

<Grid ...>
  ...

  <Grid.Clip>
    <StreamGeometry>M174,0 348,174 174,348 0,174 z</StreamGeometry>
  </Grid.Clip>
  <!-- properly clipped -->
  <Rectangle ...>...</Rectangle>

  <!-- properly clipped -->
  <TextBlock ...>
    WPF<LineBreak />
    WPF<LineBreak />
    WPF
  </TextBlock>