What causes this error: Designer load encountered an error. Please ensure that the project can be built successfully.

I did an in-place upgrade of my .NET Framework 4.8.1 VB .NET project to a.NET 9.0 project. I am unable to open the Windows Forms Designer for any of the forms in the project.

I created a new Windows Form in the project to compare the new form's designer source code to the upgraded source code of existing forms. The new (just added) Windows form won't load in the forms designer either.

In both cases, the error message is simply: "Designer load encountered an error. Please ensure that the project can be built successfully." Clicking on Help with this error" starts up the Help Viewer app which displays "No results were found." Clicking on "Search the MSDN Forums for posts related to this error" goes to https://zcusa.951200.xyz/en-us/answers/.

Next, I created a new VB .NET 9.0 Windows Forms project. The default form created by that process does open in Windows Forms designer. I compared the designer source code from the new project's default form to the designer source code from the upgraded project and am unable to see any difference.

So...... There is something about the upgraded project that is causing this issue. The issue affects existing forms as well as newly created forms. That leads me to believe the problem is something in the project file that the upgrade inserted or left behind. But what????

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My Project / Application / General
Output Type: Windows Application
Target Framework: .NET 9.0
Target OS: Windows
Target OS Version: 10.0.17763.0
Windows Forms: Enabled
Windows Presentation Foundation: DIsabled

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