Dynamically create an object where the type is not known till runtime. VB.net 2005 Generics?

Can I do this if the type only has constructors as:

Sub New()
End Sub

Or can I call a constructor that takes params if necessary, I might night need this though.

This is my method as follows:

Private Sub HandleShowChild(ByVal type As Type)
        ResetStatus()

        If IsChildInMemory(type) Then
            the found form is activated in IsChildInMemory
        Else 'Create a new instance and show it
            Dim newForm = CType(System.Activator.CreateInstance(type), type)
            newForm.Show()
        End If
    End Sub

I would like to constraing the Type permissable to this function to only objects which inherit from Form. Also I don't like:

Dim newForm = CType(System.Activator.CreateInstance(type), type)
newForm.Show()

As newForm is not typed haven't tested if this will work yet as object. Ideally want something like:

Dim newForm as typeof(type) = CType(System.Activator.CreateInstance(type), type)
newForm.Show()

> Or can I call a constructor that takes params if necessary
Yes, you can:
  CreateInstance ( type As Type, ParamArray args As Object() )

> I would like to constraing the Type permissable to this function to only objects which inherit from Form.
You can use:
  if type.BaseType == typeof(System.Windows.Form) then ...
Sorry for C# sysntax

>Ideally want something like
I don't it is possible w/o some switch and one-by-one checking.

OK I have got this far:

Public Sub SwitchToFormAndDisposeMe(ByVal sender As Form, ByVal toFormType As Type)
        If sender IsNot Nothing Then
            sender.Dispose()
        End If
        HandleShowChild(toFormType)
End Sub

Private Function IsChildInMemory(ByVal childType As Type) As Boolean
        Dim frmChild As Form
        For Each frmChild In Me.MdiChildren
            If frmChild.GetType() Is childType Then
                Me.ActivateMdiChild(frmChild)
                Return True
            End If
        Next
        Return False
    End Function

Private Sub HandleShowChild(ByVal type As Type)
        ResetStatus()

        If IsChildInMemory(type) Then
            'Ignore the provided form, just activate the found form
        Else 'Create a new instance of this type and show it
            Dim newForm = System.Activator.CreateInstance(type)
            PrepareAndShowForm(newForm)
        End If
    End Sub

I can show the new Form yah! However I am sure how best to call

SwitchToFormAndDisposeMe(..,..)

On another form I am calling as follows:

SwitchToFormAndDisposeMe(Me, frmXXX.GetType()) where frmXXX refers to the class name not an instance

but this actually calls the Sub New of form frmXX I want a solution that does not instantiate the object I just need to pass an objects type information for compare to the Switch function, the main controlling MDI will handle instantiation of new Forms. I am so close I can feel it!

Thanks

I don't want to use My.Forms.frmXXX as this creates a new instance of that Form as I use it. I want to get a type object as pass it to the Switch method.

craigdev,
Sorry, I can't understand what you want.
Can you please elaborate?

> SwitchToFormAndDisposeMe(Me, frmXXX.GetType()) where frmXXX refers to the class name not an instance
How can you call GetType() on class name?

Ok I am a litte confused now myself.

I am on FormA, I want to switch to FormB. I don't want to instantiate an instance of FormB inside class A, I want this all to handled in my Main class. I know the type I want to switch to, but I don't want to make an instance here.


Public Class FormA

Private Sub Button1_Click( ByVal sender As System.Object,  ByVal e As System.EventArgs) Handles Button1.Click
        Main.SwitchToFormAndCloseMe(Me, FormB.GetType()) 'Or just pass the class, not sure how to wire this, maybe the Switch should not take a type param
End Sub

End Class


How am I best to get the type of a class that I want, pass it to switch, let it handle it appropriately?

Thanks

I am fairly new to VB.net been coding in C# mainly so maybe when I am suing FormB.GetType where FormB is the class name and I have not created an instance, in fact an instance is actually created when I use FormB?

I am coding in C# too :)
I can recommend the following:
If you call FormA.ShowDialog() from Main class, so Main class knows when FormA is closed and can handle it.
If you call FormA.Show() from Main class, you can add OnClose event handler to FormA in your Main class and handle this event as well.
If from some reason you don't want to create an instance of FormB in FormA (although FormA decides which Form should be opened after itself), you can add some property FormToOpenAfterMe to FormA. Main class will use this property an open whatever you want.
I hope I understood you :)

By the way, if FormB is a class name, how can you call FormB.GetType()
Is it allowed in VB? It is probably like typeof(FormB)?
Any case, an instance should not be created here (IMHO).

if you know the object you are passing is a form - forma and formb are inherrited from system form.

in SwitchToFormAndCloseMe(Me,FormB) where SwitchToFormAndCloseMe(a as form,b as form)

http://msdn2.microsoft.com/en-us/library/zyy863x8.aspx <- here is a link talking about ctype,directcast, and trycast

public sub SwitchToFormAndCloseMe(a as form, b as form)

'//simple - dont care because its a form
a.close
b.show

if b.gettype= gettype(FormB) then
  with directcast(FormB)
    '//do what you need
  end with
endif

end sub

youll have to excuse me if that was a bit jumbled - just woke up

I want to able write code in FormA button click like

Main.SwitchToFormAndDisposeMe(Me, typeof(FormB))

and let main dispose A and create a new instance of FormB and show it,

I am not sure of the VB.net syntax for this, at the moment I am getting an erro telling me to "Insert the missing Is"

Also not sure what param my Main.SwitchToFormAndDisposeMe(.. should take whether it should be a "Type" or an object and inside the method call that object to get type, actually I don't want an object just type information.

So close but just not quite getting it.

Thanks heaps, you have been excellent help.

I think when I am typing FormB it is being treated as an instance which I don't want, how do I refer to the Class and pass that to the method or just that Class's type. I never want to create an instance because quite a bit of work is done in the constructor.

im not 100% sure on this but i think anytime you refer to a class in vb the new sub fires.

you could move your processing to the load event - that will not fire till show is called.

If anyone can confirm this that would be awesome, it's just that this is an existing applicaiton I have to work on with quite a few forms, it would take some time to refactor the solution and shift all processing to Load. Plus I think it's a cleaner solution if I don't have to create an instance, I shouldn't need to.

Thanks

This is code now (I just don't know how to call it):

Private Sub HandleShowChild(ByVal providedType As Type)
        ResetStatus()

        If IsChildInMemory(providedType) Then
            'Ignore the provided form, just activate the found form, done in IsChildInMemory
        Else 'Create a new instance of this type and show it

            If providedType.BaseType() is GetType(Windows.Forms.Form)
                Dim newForm = System.Activator.CreateInstance(providedType)
                PrepareAndShowForm(newForm)
            Else
                Throw New ArgumentException("Type must inherit form System.Windows.Form, provided type inherits from: " & providedType.BaseType.ToString())
            End If            
        End If
    End Sub

Also:

If providedType.BaseType() is GetType(Windows.Forms.Form)


fails for forms where I am using Visual Inheritance on Forms, so the baseType of the passed form is actually MyBaseForm, so I need to check

If providedType.BaseType() is GetType(Windows.Forms.Form) or providedType.BaseType().BaseType is GetType(Windows.Forms.Form)

Obviously this would only ever work for 2 levels of Inherited forms, what is a good solution for this, if you recursively kept call baseType you would get all the way to System.Object, how do you know when to stop or is there some other attribute on Type I should be testing against?

Thanks

You can check without recursion:
if ( (typeof(Windows.Forms.Form)).IsInstanceOfType(t) )

>>
im not 100% sure on this but i think anytime you refer to a class in vb the new sub fires.
<<

In VB (up to 6) and in VB.NET 2005, there is a "default instance" of any Class based on Form: this was not the case in VB.NET 2002 or 2003.

What this means (I haven't followed the full thread by the way, so it may not be appropriate to your current needs) is that you can, anywhere in the Namespace within which a Form's class file exists, simply  refer to that TYPE and it will (if one does not already exist) create an instance.  If you only want one instance of that TYPE then you have no need expressly to instantiate it.  If you want more that one instance, you will need to go down the "normal" route - like

    Dim myForm1 As New FormType

- for other instances, and keep track of those instances.  But for the "default instance" just referring to the TYPE is enough.

Roger

The thing is I don't want an instance, I do not want the Form's Sub New to be called. I want to call code like the following with the type of the class, and let this method create an intance of that class. I do not want the Constructor of that class to called before I pass the params to this method.

Private Sub HandleShowChild(ByVal providedType As Type)
        ResetStatus()

        If IsChildInMemory(providedType) Then
            'Ignore the provided form, just activate the found form, done in IsChildInMemory
        Else 'Create a new instance of this type and show it

            If providedType.BaseType() is GetType(Windows.Forms.Form)
                Dim newForm = System.Activator.CreateInstance(providedType)
                PrepareAndShowForm(newForm)
            Else
                Throw New ArgumentException("Type must inherit form System.Windows.Form, provided type inherits from: " & providedType.BaseType.ToString())
            End If            
        End If
End Sub

Have a look at the last post in this thread.  

http://www.experts-exchange.com/Programming/Programming_Languages/Dot_Net/VB_DOT_NET/Q_22080422.html

I still haven't mastered everything in this thread but, from what I have read, my feeling is that the approach outlined there might meet your needs

Roger

Just read it, doesn't quite fit my requirements. Calling:

ShowForm(thatForm)

Will create an instance of thatForm, I do not want this. I want to pass the Type of thatForm to the Main module and let it create the Type if it does not already exist in my MDIChild collection else show the form. I do not want it to create an instance in the callingForm as the Constructor to thatForm does quite a bit of work, there is alot of code to refactor if I had to shift processing from New() -> Load


Kind Regards

gena17

IsInstanceOfType I am having some trouble with.


If (GetType(System.Windows.Forms.Form)).IsInstanceOfType(providedType) Then
                Dim newForm = System.Activator.CreateInstance(providedType)
                PrepareAndShowForm(newForm)
Else
                Throw New ArgumentException("Type must inherit form System.Windows.Form, provided type inherits from: " & providedType.BaseType.ToString())
End If

I am getting the argException thrown as providedType.Base Type is System.Windows.Form not System.Windows.Forms.Form

If I try and check for:

(GetType(System.Windows.Forms.Form)).IsInstanceOfType(providedType) or (GetType(System.Windows.Form)).IsInstanceOfType(providedType)

VS 2005 tells me "System.Windows.Form" is not defined. I have check my base forms they all inherit from System.Windows.Forms.Form

this is a strange one!

>>
Calling:

ShowForm(thatForm)

Will create an instance of thatForm, I do not want this.
<<

I beg to differ.  If you are calling your sub HandleShowChild it means you DO want an instance of your form type.  

The question that sub is answering is whether it already exists or not.  If it does exist (in its default instance) then the call with reference to its Type will not create a new instance.  If it does not exist then you will need to create it in order to show it.  I am not clear why creating it in calling the sub rather than in the sub itself makes any operational difference.  

I think the point is that - although the call to the HandleShowChild sub may be made within "the calling form", it does not (EMPHASIS added) "create an instance IN the callingForm".  Because you are calling the default instance by TYPE it is (on my understanding) created "at large".  If you then add that default instance to (e.g.) an MDI parent's children or expressly make its parent or its owner the calling form then it will exhibit the behaviour appropriate to those relationships.  But that does not mean that, as the default instance, just because it was first referred to in one particular form it is created IN that form.

Let me stress that I am not saying that the alternative approach being discussed in this thread is wrong.  It's just that, unless I have misunderstood the requirements, it seems more complicated than the existance of default instances requires.

Roger

Ok so I need to just be sure on this. Also I am not quite sure what you mean by "default instance" If I have a form called FormA and FormB, and their will be many switchings between these MDIChild forms I only every want to call the constructor of each form once for the life of the application. If I change to use your ShowForm method to take a form and if my usage is as follows:

Public Class FormA
    Inherits System.Windows.Forms.Form

Private Sub btnShowFormB_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnShowFormB.Click
       Main.ShowForm(Me, FormB) 'not explicitly creating an instance
End Sub

End Class


Public Class FormB
    Inherits System.Windows.Forms.Form

Private Sub btnShowFormA_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnShowFormA.Click
       Main.ShowForm(Me, FormA) 'not explicitly creating an instance
End Sub

End Class


Then are you saying that a defualt instance of each form gets constructed only once, or this will happend everytime the the button is clicked, I don't want Sub New to called over and over. Form Load can be called multiple times, this is not a problem

Thanks

Is it possible to achieve it the way I was suggestion using Generics in some some elegant implementation.

Yes, that's what I'm saying.  It will, of course, depend on what you do with FormA while you're not using it.  If you .Close it, then the "default instance" will no longer exist, so when you call ShowForm(FormA) again, its .New will be fired again.  So, to avoid re-firing, an unused form will need to be hidden rather than disposed of.  And, as I say, it will only work if you only want ONE instance of FormA, ONE instance of FormB, etc.  And it won't work with VB.NET 2002 or 2003.

To test it, stick (temporary) debugging code in your child forms' .New subs on these lines

        Debug.WriteLine("FormA.New")

As to a more elegant, Generics approach, I'm not sure.  As this one works for me, I haven't tried it ;-)

Roger

You are right Roger I just tested it. Cool it works as you described I will implement in this fashion from here in. The constructors only get called once. Wierd concept of MS but helpful.

Also some of the MDIChildren call ShowDialog for some small forms to get users data in a textbox etc, very simple forms. Yet this dialog which comes up is not linked at all to the MDIChild or Parent so if I click minimise on the parent this Dialog (MDIChild) still remains center screeen and active. Of you hit "Show Desktop" on the Windows QuickLaunch the MDIParent is minimised but the dialog still floats above all other applications, even on alt-tabing to other running applications.

Any idea how do I fix this?

What is my best practice for showing Modal Dialogs off an MDIChild?

Thanks

Glad the ShowForm thing works OK.

>>
What is my best practice for showing Modal Dialogs off an MDIChild?
<<

I recognise the description re that from your current question in VB.NET (which is really my "home" TA).  I didn't chip in there because I don't think there's an easy answer.  I think you'll have to "self-code" the modality.  This is because the two built-in elements - MDI parent/children on the one hand and modal forms (i.e. used with showdialog) on the other - seem to me to be incompatible.  The MDI concept has everything working WITHIN the MDI frame (in program terms, not just visually).  Modal forms function OUTSIDE that frame.  The whole point of ShowDialog is that the user cannot do anything else until s/he has properly responded to the form concerned.

The EFFECT of modality is obviously achievable - e.g. consider the MS Word Find/Replace "dialog" in its MDI environment - but I haven't found a straightforward way of doing it just with the "normal" VB.NET mechanisms.

Here's a PAQ which might help

http://www.experts-exchange.com/Programming/Programming_Languages/Dot_Net/VB_DOT_NET/Q_21960589.html

It's the Accepted Answer post near the end that might be relevant: there was a lot of "round the houses" stuff before we got to an answer in that case.

Roger

I am not sure where to start with this. I have normal form transitions going really well now.

I am at the stage at several leafs within the from menu/wizard hierarchy where the existing code has about 10 buttons on a form, each one opens a model ShowDialog() allows the user to edit some grids etc click Ok, Cancel. The container form then checks the DialogResult of that model form, it OK update(dataset) If cancel RejectChanges is being called.

So now I am presented with scenario I would have to change all the forms to call show, put the dataset update code inside the OK click handle, move the RejectChanges into the OnClose of the form, test for DialogResult Cancel if true Reject Changes?

Any other suggestion or do I just have to refactor all these forms to conform to this scenario, I can achieve modal likeness, by calling Show on the current Dialogs and also setting the FormStyleBorder as None and Dock to Fill. So you only can ever work on one form at a time. This is the way the application intends the user to drive it anyway.

Regards

Also a lot of the in code execution logic relies on the the code blocking at the point ShowDialog() is called, wait for a result then carry on. Any way to achieve this with some .Net statement?

I think you can keep your data-processing on the calling form.  Get the calling form to listen for a (self-coded) event that is raised by the called form (the "dialog") with a Save or Cancel argument depending which button is pressed.  And the same approach - waiting for the firing of an event from the "dialog" - could at the same time implement the "wait for a result then carry on" requirement that you mention.  In the latter context, I would envisage the calling form's calling code ending with the showing of the "dialog", and then separate code to save or reject the changes when the called form fires its event.

This is brainstorming - which means I've done no testing ;-).  And it's not ideal, even in brainstorming terms.  But whatever you do is going to have to be a compromise, and this approach might hit a better balance between saving as much as you can of what you've got at the moment and having extensively to refactor for the new environment.

I've written in general terms, because details would depend on minutiae of your set-up which I don't know.  If the general idea appeals but you need to discuss details, it might be helpful if you posted sample code for the "dialog calling" and "data processing" operations of a calling form and the subs from a "dialog" form.

Roger

Sounds good, how would I force the calling form to "wait" for this event?

Also the client is very picky about the look and feel of the application (they are a media house). I need to paint the MDI Parent container with a gradient effect. I have tried handling the Paint event of the MDI Parent and doing it here, but it is not working. Ideally I would paint this effect in the child container, set all MDI Child form's background to transparent and the app would look good, all the labels would have to support transparency as well.

What is my best and most efficient (the current forms a quite flickery (I think due to the bind of the syncfusion grids)), to implement this requirement? or should I open another question?

Regards

Ok I have sorted the MDI Parent paint using the solution found at the bottom here:

http://www.bobpowell.net/mdiback.htm it works really well.

However I am trying to do this for child forms:

Private Sub PrepareAndShowForm(ByRef childForm As Form)
        PrepareForm(childForm)
        childForm.BackColor = color.Transparent
        childForm.Visible = True
        childForm.Show()
End Sub

As I set my MDI child windows to have FormStyleBorder as None and Dock to Fill, they function really like a panel, I'd like each MDI child to support transparency and pick up the gradient background form the MDI Parent, also to support transparent lables on the child. Is this acheivable? or do I need to change all my existing form to inherit from panel instead of Form, what would be the consequences of this?

Thanks

>>
how would I force the calling form to "wait" for this event?
<<

Put a sub to handle it in the calling form.  And then either declare the called form WithEvents (but that means you would have to declare it at form level) or add a handler when you do declare it

        AddHandler CalledForm.Event, AddressOf CallingFormEventHandler

On the painting side, it sounds as though you're now happy with what you've got but, if not, another question would be better.  Display issues are not my forte.

If, for display purposes, you decide to replace your forms by panels you wouldn't be able to use the MDI parent's child forms collection to organise / control them.  And you'd have to self-code Events (e.g. like FormClosing) that exist in Forms but not in Panels.  But, as I remarked before, you're in a "having to compromise" situation, and you're really the only one that can decide where the best compromise is.

Roger

I still not qutie getting you. In calling form

dialogForm.Show(Me)
'Need to Wait here till I get an event from dialogForm only once I have that result step down to the Process() method

'Do some other processing
Process()

This behaviour works if I dialogForm.ShowDialog(Me), I need to mimic the blocking of ShowDialog for Show()

Thanks

in dialogForm the property Me.Owner will hold the reference to the caller - because you use show(Me)

so that will allow you contorl over that form - and i think there is an ownedforms that works the other way