I don't understand what you mean; we have tons of custom controls. I am looking at a .NET 6 project right now where I have a custom control that shows up in the

I don't understand what you mean; we have tons of custom controls. I am looking at a .NET 6 project right now where I have a custom control that shows up in the toolbox under my project's group, and when you add it to the designer it renders (at design time) as it did before. It's clearly my OnPaint method drawing, etc. I wonder if you're talking about a previous version of .NET Core? .NET 6, very recently released, is the first version where legacy WinForms apps can start to upgrade. There were blockers in previous .NET Core versions. If you're maybe talking about .NET Core 3.1, WinForms was not at all ready in that version. It was almost there in .NET 5 but they made a mistake with the default font handling that broke legacy apps, but they both fixed that bug and finished the designer for .NET 6.