Support for Windows Forms in Hosts and Add-Ins

We’ve been receiving a lot of questions recently regarding the support for Windows Forms based add-ins and hosts in a multi-AppDomain environment. Now that we’ve released .NetFX 3.5 with the support for passing WPF controls across AppDomains the lack of direct support from Windows Forms becomes more noticiable.

The good news is that it’s actually possible to use Windows Forms controls in both your hosts and add-ins and have them contribute directly to each others UI. The bad news is that Windows Forms, currently, doesn’t have any dirrect support for this feature and so you’ll have to go through WPF to do this.

In 3.5 the WPF team created the helper functions, in the System.AddIn.Pipeline.FrameworkElementAdapters class, which will convert to/from FrameworkElements and InativeHandleContract. It does this by wrapping the FrameworkElements in an HWND on one side and then parenting this HWND to a FrameworkElement on the other. This means that in actuality the FramworkElement and its children never actually get passed across the bounary.

If, in addition to the FrameworkElementAdapters class, you use the WindowsFormsHost and ElementHost classes to host WindowsForms controls in WPF elements and vice-versa then both your hosts and your add-ins can use WindowsForms controls and you can logically pass them across the AppDomain boundary without actually doing so. The downside to this approach is that this requires that you load WPF into your process. If you already have some WPF code running in your process then WPF requirment is very minimal, but it can be significant overhead in the case of a pure WindowsForms host with pure WindowsForms add-ins and you’ll have to decide for yourself whether it is worth it.

There were also a few lingering questions about whether the above technique was even supported by the WindowsForms teams but there is good news there. We were recently able to clarify this question and the answer is that it is supported because it manages to avoid actually passing across and WindowsForms controls across the AppDomain boundaries: