Opening Visual Studio project file gets an error

1. are you using any 3rd party tools like plugins. first check those plugins are missing.
2. create new forms with same name and copy the working forms and replace with newly created forms. then open the vs might be work.

The error indicates that Visual Studio cannot find any class in the file that meets the Windows Forms Designer requirements.

Use these steps as next actions:

1. Confirm the class is designer‑compatible
   • Ensure the class you want to design inherits from a designable base class, such as System.Windows.Forms.Form or System.Windows.Forms.UserControl.
   • If the class does not derive from a base class that supports designers, it cannot be opened in the designer. In that case, edit it only in Code view.
2. Make sure the designer is using the correct class
   • Visual Studio requires the first class in the file to be the one used by the designer.
   • If the designable class is not the first class in the file, move its code so that it is the first class, then reopen the designer.
3. Build the solution and check base classes
   • Build the entire solution so that all referenced assemblies and base classes are compiled.
   • If the file’s class inherits from another form/control in a different project or assembly, ensure that project is built and referenced.
   • If the base form is in a separate assembly compiled as x64 or Itanium, recompile that assembly as AnyCPU so the 32‑bit Visual Studio designer can load it.
4. When a file truly cannot be designed
   • If the base class does not support any designers, or the class is not intended for visual design, use View Code instead of Design view.

If, after these steps, the class still cannot be designed, the file is likely not suitable for the Windows Forms Designer and should be edited only in code.

References:

• Windows Forms Designer error page
• Cannot Display Inherited Form in Form Designer When Base Form Defined in 64-bit Assembly
• Class Designer errors
• MSB5009 diagnostic code