upgrade-assistant | A tool to assist developers in upgrading NET Framework applications to NET 6 and beyond | DevOps library

Over a year ago I began writing a .NET Framework 4.6.1 app using Windows Forms. At the time I knew about WPF, yet was familiar with Windows Forms and it had most of the controls I needed. For the missing controls, I wrote two in Windows Forms and one in WPF. All of these coexisted fine, with the WPF control containerized within an element host.

This week I began the migration process to .NET Core 3.1. My tests with a copy of the project were positive, as well as initial results with the actual migration. After minor refactoring, the solution built and ran without issue. Then the gremlin appeared after opening the main UI form in the WinForms Designer. Back in .NET Framework, all of my custom controls appeared inside the Designer's Toolbox, providing easy drag-and-drop onto the form. In .NET Core, only my WinForms controls appeared in the Toolbox, not my WPF control. Because the Designer could not see that control, it stripped it from the Form's designer code, leaving an empty element host behind.

Here's the kicker. After reverting the Designer's changes, any direct manual edits to the Form's designer code is accepted, and building the project succeeds and runs fine. So for some reason the Designer does not like WPF controls in WinForms.

Things I've tried:

• During my testing I discovered that the main WinForms UI needed both "UseWindowsForms" and "UseWPF" set to "true" for the project to compile. I then added the "UseWindowsForms" parameter to the WPF user control library. This caused the control to appear in the Designer's Toolbox, yet attempting to add the control resulted in this error: "Failed to create component ... Microsoft.DotNet.DesignTools.Client.DesignToolsServerException ... Make sure the type implements IComponent and provides an appropriate public constructor. Appropriate constructors either take no parameters or take a single IContainer parameter." And the existing WPF control in the code was still removed.
• I copied the WPF control from the library to the main UI project, edited the namespace, and removed the library project reference. Same result as above.
• Created a new Windows Forms User Control library, added "UseWPF" to the project, and copied the WPF control to this library. Same result as above.
• Back to the test copy of my project, I followed Microsoft's guides for "try-convert" and "upgrade-assistant". The latter seemed promising at first, as it replaced, modified, or removed outdated referenced and packages. But, no success.
• Tried the above migration steps with both .NET Core 3.1 and .NET 5. Same results.

The point I'm at now is to keep manually editing the Form's designer code. Not ideal for large changes, and also not sustainable if/when this project is passed to another developer. Any thoughts? Should I attempt porting the Windows Forms UI to WPF? Or is this simply a maturity issue with the still relatively new .NET Core Windows Forms Designer?

Visual Studio version: Community 2019 16.9.3

Screenshot of IComponent error: enter image description here