OO Question - Superclass / Subclass

[GottaGetITDone]

Hi Everybody!

Im brand new to .Net and I've got to say Im lovin it so far!

Got a question though...

I have an MDI Child and I want to refer to it like so....

Private Sub DisplayForm(ByRef frm as System.Windows.Forms.Form)
{

}

I may be passing multiple different MDI children to this subroutine. When I turn on Option Strict it says it cannot perform an implicit conversion.

Please correct me but I was under the impression that OO polymorphism (Superclass <-> Subclass) would take care of this?

How would I delcare the subroutine so it could accept my child forms.

Thanks for your input.

[Kasracer]

Each one of your forms inherits System.Windows.Forms.Form

When you create your Form, it's based on .Net's Form but then you modify it making it different.

So it you have 3 forms called FormAbout, FormMain, and FormYippy, they are not System.Windows.Forms.Forms anymore. It would be like trying to pass a Double to a function that is waiting for an Int.

Depending on what you want to do, you may be able to cast it but if they're very different it may be a better idea to re-do your logic.

[GottaGetITDone]

WOW That was quick!

Hmmm your probably right, a logic redesign may be the way to go. This is what I'm trying to do...

Private WithEvents frmCRpt As frmCampRpt
    Private WithEvents frmCMan As frmCampMan
 
    Private Sub tbMain_ButtonClick(ByVal sender As System.Object, ByVal e As System.Windows.Forms.ToolBarButtonClickEventArgs) Handles tbMain.ButtonClick
 
        Select Case tbMain.Buttons.IndexOf(e.Button)
            Case 0
                DisplayForm(frmCRpt, 0)
            Case 1
                DisplayForm(frmCMan, 1)
            Case 2
                'DisplayForm(x, 2)
        End Select
    End Sub
 
    Private Sub DisplayForm(ByRef frm As Windows.Forms.Form, ByVal iBtnIndex As Integer)
 
        Try
            If frm Is Nothing Then
                Select Case iBtnIndex               'Select Form Type
                    Case 0
                        frm = New frmCampRpt
                    Case 1
                        frm = New frmCampMan
                End Select
                
                frm.MdiParent = Me
                frm.Show()                          'Show new instance
 
                tbMain.Buttons(iBtnIndex).Pushed = True
            Else
                If frm.WindowState = FormWindowState.Minimized Then
                    frm.WindowState = FormWindowState.Normal
                End If
                frm.BringToFront()                  'Show existing instance
            End If
        Catch
        End Try
 
    End Sub
 
    Private Sub frmCRpt_Closed(ByVal sender As Object, ByVal e As    System.EventArgs) Handles frmCRpt.Closed
        tbMain.Buttons(0).Pushed = False
        frmCRpt = Nothing
    End Sub

So I’m trying to determine if the MDI Child form has been displayed and if so just bring it to the front without creating another instance. This is so a user can enter some info into a form, go to another form, then return to the original form in the state they left it. As you can see this is hanging off a toolbar control which I am mimicking the behavior of a task bar, hence the 'tbMain.Buttons(iBtnIndex).Pushed = True' line of code.

I assumed that I would be able to refer to the Superclass like I was able to back in the days of C++ 6.0.

The code above works, but if wanting to be type safe, it breaks...

I suppose the obvious logic redesign is to pack all this logic into the Toolbar's Click event. This would eliminate the need to pass the form. Conversely there would be alot of repeated code in each Case statement...

Any thoughts, ideas or suggestions?

[jmcilhinney]

What you're doing is all correct except for one problem. You are passing the first parameter by reference. It's not the fact that you are passing an instance of a derived class into the method that's the problem. It's the fact that you're passing an instance of the base class back out again. Change the ByRef to ByVal and watch the error disappear. The reference you pass out is type Form and you're assigning it to variable of a derived class, and that's the implicit conversion that's not allowed. You could convert the procedure to a function, pass the parameter by value and then return a reference to the new form, which you could cast as the correct type:

Private WithEvents frmCRpt As frmCampRpt
    Private WithEvents frmCMan As frmCampMan
 
    Private Sub tbMain_ButtonClick(ByVal sender As System.Object, ByVal e As System.Windows.Forms.ToolBarButtonClickEventArgs) Handles tbMain.ButtonClick
 
        Select Case tbMain.Buttons.IndexOf(e.Button)
            Case 0
                frmCRpt = DirectCast(DisplayForm(frmCRpt, 0), frmCampRpt)
            Case 1
                frmCMan = DirectCast(DisplayForm(frmCMan, 1), frmCampMan)
            Case 2
                'DisplayForm(x, 2)
        End Select
    End Sub
 
    Private Function DisplayForm(ByVal frm As Windows.Forms.Form, ByVal iBtnIndex As Integer) As Form
 
        Try
            If frm Is Nothing Then
                Select Case iBtnIndex               'Select Form Type
                    Case 0
                        frm = New frmCampRpt
                    Case 1
                        frm = New frmCampMan
                End Select
 
                frm.MdiParent = Me
                frm.Show()                          'Show new instance
 
                tbMain.Buttons(iBtnIndex).Pushed = True
            Else
                If frm.WindowState = FormWindowState.Minimized Then
                    frm.WindowState = FormWindowState.Normal
                End If
                frm.BringToFront()                  'Show existing instance
            End If
        Catch
        End Try
 
        Return frm
    End Function

That's quite inelegant though. I'd suggest making your child forms singletons and then you simply refer to their Instance properties at all times. The forms themselves will take care of creating an instance if one is required.

[jmcilhinney]

Here's what a basic singleton class looks like:

Public Class Singleton
 
    'The one and only instance.
    Private _instance As Singleton
 
    'Make the constructor private so that an instance cannot be created from the outside.
    Private Sub New()
 
    End Sub
 
    Public ReadOnly Property Instance() As Singleton
        Get
            If Me._instance Is Nothing Then
                'No instance exists so create one.
                Me._instance = New Singleton
            End If
 
            'Return a reference to the one and only instance.
            Return Me._instance
        End Get
    End Property
 
End Class

[GottaGetITDone]

"I see!" said the Blind Man.

You are correct changing the parameter pass type to 'ByVal' removes the compile error.

Singleton - This is the sort of answer I was looking for. I know nothing of this Singleton but your explanation shows its the way to go. I will spend some time researching this solution and post when properly developed.

I currently have 3 VB.Net & 1 Crystal Reports.Net text books on order...man I cant wait.

Thank You jmcilhinney - Much appreciated

[jmcilhinney]

That code was not quite correct. The '_instance' variable and 'Instance' property should be Shared. Here's code for a VB 2005 singleton form class:

Public Class SingletonForm
 
    Private Shared _instance As SingletonForm
 
    Public Shared ReadOnly Property Instance() As SingletonForm
        Get
            If _instance Is Nothing OrElse _instance.IsDisposed Then
                _instance = New SingletonForm
            End If
 
            Return _instance
        End Get
    End Property
 
    Private Sub New()
        ' This call is required by the Windows Form Designer.
        InitializeComponent()
 
        ' Add any initialization after the InitializeComponent() call.
 
    End Sub
 
End Class

and here's how you would use it:

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'Make the form visible in case it is not already.
        SingletonForm.Instance.Show()
 
        'Focus the form in case it was already visible.
        SingletonForm.Instance.Activate()
    End Sub

There's no need for class level variables for the forms because you simply refer to the Shared Instance property all the time. Note that the IsDisposed part in the Instance property takes care of creating a new instance if the old instance was closed.