Consider the following VB6 code:

        Dim frm As Form
        For Each frm In Forms
            frm.Caption  = ""
        Next

and its VB.NET translation:

        Dim frm As VB6Form
        For Each frm In Forms6
            frm.Caption = ""
        Next

As you see, references to the Form and MDIForm classes are converted to VB6Form and VB6MdiForm, respectively. Interestingly, this VB.NET code works correctly even if you add standard .NET forms to the project, because the Forms6 collection contains instances of the VB6Form and VB6MdiForm classes, but not standard .NET forms that you might have created in Visual Studio.

You can force the Forms6 collection to include native .NET forms by setting the Forms6.IncludeNetForms property to True:

        Sub Main()
        '## IncludeStatement Forms6.IncludeNetForms = True
            ...
        End Sub

Typically, when you set the Form6.IncludeNetForms property to True you also need a project-wide ChangeType pragma that tells VB Migration Partner that Form variables should be converted as System.Windows.Forms.Form variables rather than VB6Form variables:

        '## project:ChangeType Form, Form

If you omit this pragma, the controlling variable in For loops is rendered as a VB6Form variable and causes a TypeMismatch exception when it is assigned a reference to native .NET forms.