[2008] How do I make a new instance based on a variable's type?

[Tbird761]

Hey guys. I've been programming for a long time, but there's something I've never figured out how to do in all this time. Surely there is a way to turn the following code into a sub so I don't have the same boilerplate show form code in use a thousand times in a program:

Example:

Public InstanceOfFrmGeneric As frmGeneric

If InstanceOfFrmGeneric Is Nothing Then
    InstanceOfFrmGeneric = New frmGeneric()
    InstanceOfFrmGeneric.Show()
Else
    If InstanceOfFrmGeneric.IsDisposed Then
        InstanceOfFrmGeneric = New frmGeneric()
        InstanceOfFrmGeneric.Show()
    Else
        InstanceOfFrmGeneric.Focus()
    End If
End If

Basically, I need a way to do this provided the input of the form's instance variable. Ideally something like:

Private Sub showForm(ByRef InstanceOfForm As Form)
    'Pseudo-code

    If InstanceOfForm Is Nothing Then
        InstanceOfForm = New Magic Thing Letting Me Make New Instance Of InstanceOfForm Based on Its Type Or Whatever
        InstanceOfForm.Show()
    Else
        If InstanceOfForm.IsDisposed Then
            InstanceOfForm = New New Magic Thing Letting Me Make New Instance Of InstanceOfForm Based on Its Type Or Whatever
            InstanceOfForm.Show()
        Else
            InstanceOfForm.Focus()
        End If
    End If
End Sub

Anyone know how to do this? It's probably a simple Typing thing or something, but that's not an area I'm good at since it doesn't seem to flow with the normal language.

[gep13]

Hey,

Can you not implement a Singleton pattern in the Form definition.

Have a look here for details:

http://www.ondotnet.com/pub/a/dotnet...singleton.html

Gary

[Negative0]

You could use a generic:

    Private Sub showForm(Of t As Form)(ByRef InstanceOfForm As t)
        'Pseudo-code

        If InstanceOfForm Is Nothing Then
            InstanceOfForm = DirectCast(Activator.CreateInstance(Of t)(), t)
            InstanceOfForm.Show()
        Else
            If InstanceOfForm.IsDisposed Then
                InstanceOfForm = DirectCast(Activator.CreateInstance(InstanceOfForm.GetType), t)
                InstanceOfForm.Show()
            Else
                InstanceOfForm.Focus()
            End If
        End If
    End Sub


    Dim f As Form2
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        showForm(Of Form2)(f)
    End Sub

[Shaggy Hiker]

I tend to like Negative0s suggestion, but I thought I might suggest that you also look at GetType. The name property would hold the type name, though the only thing I can really think to do with that is to use it for a switch, which would be fine for just a few forms, but wouldn't be very extensible.

[Tbird761]

Thank you very much, folks. Two different and interesting solutions to accomplish the same goal. It works!

Activator.CreateInstance...I'm going to have to remember that one.