Enhance BC30738 error documentation with Windows Forms and My.MyApplication guidance (dotnet#48539)

* Initial plan

* Enhance BC30738 error documentation with Windows Forms and My.MyApplication guidance

Co-authored-by: BillWagner <[email protected]>

* Apply suggestions from code review

* Use xref syntax for My.MyApplication and events

Co-authored-by: BillWagner <[email protected]>

* Apply suggestions from code review

Co-authored-by: Genevieve Warren <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BillWagner <[email protected]>
Co-authored-by: Bill Wagner <[email protected]>
Co-authored-by: Genevieve Warren <[email protected]>

Author: 3 people

docs/visual-basic/misc/bc30738.md

@@ -11,14 +11,44 @@ ms.assetid: c3212d87-6af3-4120-9e06-4d85fa910de2
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 # 'Sub Main' is declared more than once in '\<namespace>': \<message>
 
-`Sub Main` can only be declared once inside a namespace.  
-  
- **Error ID:** BC30738  
-  
-## To correct this error  
-  
-- Make sure there is only a single `Sub Main` procedure in your project.  
-  
+`Sub Main` can only be declared once inside a namespace.
+
+**Error ID:** BC30738
+
+## Cause
+
+This error often occurs in Windows Forms applications when you define your own `Sub Main` procedure while the Visual Basic Application Framework is enabled. The application framework automatically generates a `Main` procedure, creating a conflict when you add another one.
+
+## To correct this error
+
+Choose one of the following approaches:
+
+### Option 1: Use the application framework (recommended for most Windows Forms apps)
+
+- Remove your custom `Sub Main` procedure.
+- Configure your startup form and initialization code using the application framework:
+  - Set the **Startup form** in **Project Properties** > **Application** tab.
+  - Use the `My.MyApplication` events for custom startup logic.
+
+### Option 2: Disable the application framework
+
+If you need programmatic control over application startup (for example, to select which form to display based on command-line arguments):
+
+- In **Project Properties** > **Application** tab, uncheck **Enable application framework**.
+- Set **Startup object** to your module or class containing `Sub Main`.
+- Implement your own `Sub Main` procedure to control application startup.
+
+### Option 3: Remove duplicate Main procedures
+
+- Ensure there is only a single `Sub Main` procedure in your entire project.
+
+## Access `My.MyApplication.Main`
+
+If you need to access the automatically generated startup code, you can work with the <xref:Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase> object and its events such as <xref:Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Startup>, <xref:Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.StartupNextInstance>, and <xref:Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Shutdown>. The application framework provides these events specifically for customizing application behavior without defining your own `Main` procedure.
+
 ## See also
 
+- [Main Procedure in Visual Basic](../programming-guide/program-structure/main-procedure.md)
+- [Overview of the Visual Basic Application Model](../developing-apps/development-with-my/overview-of-the-visual-basic-application-model.md)
+- [-main (Visual Basic)](../reference/command-line-compiler/main.md)
 - [Procedures](../programming-guide/language-features/procedures/index.md)