How do I pass a form to a sub to create a child form out of?

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I wanted a sub to create child forms but i am having issues

VB

Private Sub CreateChild(frm As Form)

    Dim frmChild As New frm
    frmChild.MdiParent = My.Forms.frmNavigation
    frmChild.Show()
    frmChild.Tag = My.Forms.frmNavigation.intTabCounter
    frmChild.FormBorderStyle = FormBorderStyle.None
    frmChild.ControlBox = False
    frmChild.MaximizeBox = False
    frmChild.MinimizeBox = False
    frmChild.ShowIcon = False
    frmChild.Text = ""
    frmChild.Dock = DockStyle.Fill

End Sub

Wondered if anyone can nudge me in the right direction.

I feel it's because i am trying to Dim as New but i am passing it the form i want to Dim a New of.

What I have tried:

I thought maybe i could pass the Forms name through as a string and then somehow loop through my forms for one matching that name, then Dim as New. Problem seems to be i can only loop through open forms.

Posted 8-Jun-17 7:59am

BeginnerCoderPete

Updated 8-Jun-17 8:30am

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Richard Deeming · Accepted Answer · 2017-06-08T08:30:00

Solution 1

Try using a generic method:

VB.NET

Private Sub CreateChild(Of TForm As {Form, New})()
    Dim frmChild As New TForm()
    ...
End Sub

To call it, you need to specify the type of form you want to create:

VB.NET

CreateChild(Of YourCustomForm)()

The type must inherit from Form, and must have a public parameterless constructor.

Generic Types in Visual Basic (Visual Basic) | Microsoft Docs[^]

Posted 8-Jun-17 8:30am

Richard Deeming

Updated 8-Jun-17 10:13am

v2

Comments

BeginnerCoderPete 8-Jun-17 16:11pm

I seem to be getting an error on ,New) saying it is not a valid identifier. I am currently reading through the link you provided. Thank you

Richard Deeming 8-Jun-17 16:14pm

Sorry, I misread the link. You need curly braces around the constraints when you have multiple constraints:

(Of TForm As {Form, New})

BeginnerCoderPete 8-Jun-17 17:50pm

I have a public variable on the form i am opening as a child, would it be possible to also pass that into this sub to pass to the child form that's created?

Richard Deeming 9-Jun-17 6:43am

You'd need to declare the property in a base class or an interface, and have your forms inherit from the base class or implement the interface.

Then you'd add the base class / interface as a constraint on the method:

(Of TForm As {BaseClass, New})
(Of TForm As {Form, IInterface, New})

You'll then have access to the members of that base class / interface within the method.

BeginnerCoderPete 9-Jun-17 8:08am

Thank you for your reply, at the moment i'm finding it difficult to get my head around so i will have to spend sometime reading up on this. Really appreciate the help.

BeginnerCoderPete 20-Jun-17 17:23pm

I ran this past my IT manager when i got back to work and he came up with this

    Private Sub CreateChild1(Of TForm As {Form, New})(intVariableName As String, intVariableValue As Integer)
        Dim frmChild As New TForm()
        frmChild.MdiParent = My.Forms.frmNavigation
        CallByName(frmChild, intVariableName, CallType.Set, intVariableValue)
        frmChild.Show()
        frmChild.Tag = My.Forms.frmNavigation.intTabCounter
        frmChild.FormBorderStyle = FormBorderStyle.None
        frmChild.ControlBox = False
        frmChild.MaximizeBox = False
        frmChild.MinimizeBox = False
        frmChild.ShowIcon = False
        frmChild.Text = ""
        frmChild.Dock = DockStyle.Fill
    End Sub

The CallByName() part is what he has used to pass the value through. I just wondered if this was alright really, as it's different to your suggestion.

Richard Deeming 21-Jun-17 14:42pm

CallByName[^] is a VB-specific late-bound call.

If it works for what you need, then it's fine.

It'll be very slightly slower than using a base class or an interface, but unless you're calling it in a tight loop, you probably won't notice the difference. :)

BeginnerCoderPete 21-Jun-17 15:40pm

Ok :) thank you, for your reply

BeginnerCoderPete 8-Jun-17 16:21pm

Excellent, thank you very much.