The WinForms designer in .NET Core is still in its preview and currently it misses key features like support for user controls or libraries of third party vendors. We are closely working with Microsoft to determine the best possible implementation approach. At the moment we cannot provide you with a time estimate for a possible release or preview of the feature.
As for the designer, please note that even the Microsoft implementation is still limited and only available as a preview in Visual Studio. If you need to target .NET Core and have complex designers you may consider trying the workaround as explained here: https://github.com/dotnet/winforms/blob/master/Documentation/winforms-designer.md . With this approach you can still target .NET Core and at the same time achieve the design-time experience you are already used to.
Additionally, you can also check some of our resources on this topic:
I hope this information helps. If you need any further assistance please don't hesitate to contact me.
Dess | Tech Support Engineer, Sr.
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