Start VB.NET GUI app using Sub Main or form startup object?

The primary reason for using Main() in VB .NET 1.x was for adding code that needed to run before any forms were loaded. For example, you might want to detect whether an instance of your Windows Forms app was already loaded. Or you might want to intercept any unhandled exception for the AppDomain:

AddHandler AppDomain.CurrentDomain.UnhandledException, AddressOf MyExceptionFilter

But the next version of VB and Visual Studio 2005 introduced a new Application model that made Main() unnecessary in most scenarios. You can now intercept the My.Application.Startup event to add code that needs to run before any forms are loaded.

Note that the code for the Startup event handler is stored in the ApplicationEvents.vb file, which is hidden by default.

You can do it either way, but you should really only keep code in the form that is directly related to the operations and user interface elements on that form. Application startup code isn't related to UI, normally concerned with splash screens, checking network connectivity, verifying a single instance only, setting up user configuration settings, and so on.

After the above items (or the appropriate initialization code for your app) are complete, Sub Main can create an instance of the main form, then show it so the user can begin interacting with your application.

This separates startup code from your form code. Later, when you're maintaining the application, you'll be glad you separated the two.

Yes, and I have done it a few times.

One reason is, that if your app is COM EXE (speaking now from a VB6 point of view) then you want to be able to detect in what context the EXE is being called (being launched or being spoken to by some other app).

For example:

Sub Main()
    If App.StartMode = vbSModeAutomation Then
        ...
    Else
        ...
    End If
End Sub

Another is if you want your app to be able to handle any command line parameters.

For example:

Sub Main()
    If App.PrevInstance Then End
    If InStr(Command, "/s") > 0 Then
        Form1.Show
    ElseIf InStr(Command, "/p") > 0 Then
        LoadPicture ("c:\windows\Zapotec.bmp")
    End If
End Sub

(from one of my attempts to make a screen saver)