With thanks to Panagiotis Kanavos ( answering my recent similar duplicate question )

First, switch to .NET 6. .NET 5 reaches End Of Line on May 2022, 3 months away. .NET 6 is the Long Term Support version, with the new fully functional Windows Forms designer. Second, you can't use .NET Old code with .NET (Core) 5 or 6. You can't share any UI-related code, because .NET Old only supports up to .NET Standard 2.0, which doesn't support Windows Forms

With thanks to Panagiotis Kanavos ( commenting on my recent duplicate question )

First, switch to .NET 6. .NET 5 reaches End Of Line on May 2022, 3 months away. .NET 6 is the Long Term Support version, with the new fully functional Windows Forms designer. Second, you can't use .NET Old code with .NET (Core) 5 or 6. You can't share any UI-related code, because .NET Old only supports up to .NET Standard 2.0, which doesn't support Windows Forms