NetControlDesigners - Developing Custom Controls for the new WinForms Designer

The new Windows Forms (WinForms) .NET Designer needs a new SDK for authoring Custom ControlDesigners. The necessary migration of Custom ControlDesigners from .NET Framework to .NET is straight forward. Only UI-intensive design time experience, such as custom dialogs requires custm code. This blog post shows, what work is necessary to migrate WinForms ControlDesigners from .NET Framework designer model to .NET designer model.

The Demo scenario – the WinForms TileRepeater Control

If you ever developed in WPF, then you probably like, and in WinForms, miss a feature, which is really useful in list binding scenarios: DataTemplates and DataTemplateSelectors. Imagine, you have a list of elements as a data source, each deriving from the same base type. For example, an ImageItem, which holds a path to an image on disk. And from those you derive an PortraitImageItem and a LandscapeImageItem. Now, you bind this list to a List control, which will pick as the renderer for each item a WinForms user control based on the type of the item you want to bind. In the sample, on binding this control will instantiate a respective user control for showing images in portrait format, and user controls for showing images in landscape format. By choosing the right inheritance hierarchy, even introducing a Separator item for grouping those user controls, for example by months, is easily possible.

This is, what this sample is about. And using this control at runtime looks like this:

The challenge

For making the TileRepeater control work at design time in a useful way, we need proper WinForms Designer support. Especially for this control: with the new .NET Designer which comes with separate processes for the Visual Studio .NET Framework client-based UI functionality on the one side and the actual .NET Forms and Control instantiation, management and rendering in a dedicated Server process on the other side, authoring ControlDesigners has become a bit tricky. It is compared to the Framework Designer a breaking change when you need to implement custom .NET type editors. And here is why:

When we want to show a Dialog for example, which allows the user to pick the types for the data template selection (so, which item type of the list to bind should result in what UserControl to render), we have to deal with two different processes: The Visual Studio Process runs in .NET Framework. But the actual control, which we are showing the custom UI for, runs in the dedicated .NET server process: If you target .NET Core 3.1, it hosts .NET Core 3.1, if you target 6.0, it hosts 6.0, and so on. That’s necessary, because you need types only the specific version of .NET knows about. The Visual Studio .NET Framework based client process is simply not able to deal with all the new .NET types. It doesn’t know them. So, from that fact arises the actual challenge: Since the Control Designer’s dialogs are also running in the context of .NET Framework, it cannot simply search for the types (in our example neither both the items to bind and the resulting UserControl types) it is supposed to offer the user in that dialog. Rather, the .NET Framework part of the type editor needs to ask the .NET Process for those types, and then it uses helper transport classes to get those types cross-process back to the Framework based Visual Studio process. It can now process the user’s input and send the results back to the server process. And yes, that’s a change from the previous .NET Framework-only Control Designers, and it involves indeed some refactoring of the design time code, but only if there is an actual UI which needs to be shown on top of the UI that is presented in the context of your actual control. Here is what that means exactly:

• If you just have a UI, which is based on a type-converter and therefore shown in the context of the Property grid (like Enums or dedicated items to show in a property grid’s property grid cell ComboBox), your UI will be supported by the new designer model out of the box.

• If you have a UI, which is part of the control (like custom painted adorners or Action Lists), then you would need to write your control library against the WinForms Designer SDK, but you don’t need to roundtrip data to the Server process. Everything from the Developer’s perspective seems to actually be done server-side, and you can reuse most of the existing Control Designer Code, but just need to target it against the Windows Designer SDK.

• If you have custom type editors, however, which are displaying dedicated modal dialogs, then there is some rewriting effort involved for roundtripping the required data between the two processes.

Two sample Versions and the why

The sample app contains those two versions: One Control plus its Designer, which is the simplified version of the sample, and which just comes with a custom type converter for assigning just one TileContent user control as the renderer for each item. This one is called SimpleTileRepeater. It just needs its Control Designer code next to the actual Control’s implementation in one assembly/project.

The second Control is the actual full blown TileRepeater control and has next to everything the SimpleTileRepeater has, also the UI for the Collection Editor, which allows the user to make a list of TypeAssignments. And then, the UI for doing one of the assignments of that list of course is again a dedicated Type Editor, which needs to be implemented in the way just described.

To make the sample compile

Please note: To make this demo compile, you need to add the output of the package project, which gets build into the folder,

NetControlDesigners\src\TileRepeater\NuGet\BuildOut

as a package source: