Show single instance of form in other project - vb.net

Question

I have multiproject solution where one project is "main", startup project.
From this project I starting forms in other projects which all are referenced in startup project.
Problem is that I can start those forms as instances like this:
Dim ka As New otherproject.frm_thatform
With ka
.BringToFront()
.Show(Me)
End With
That opens new "thatform" every time but I would like to to start "thatform" like single instance every time and that "thatform" comes to front.
How to do that?
I try like this:
Dim ka As otherproject.frm_thatform
With ka
.BringToFront()
.Show(Me)
End With
... but that don't work (Object reference not set to an instance of an object).

Answer 1

The trick is to not Dim the form and use it as a Shared object.
Otherproject.frm_thatform.Show()
Otherproject.frm_thatform.BringToFront()
As long as you dont close this window you can call it the same way as a Dimmed object.
Instead of ka you just type Otherproject.frm_thatform
As soon as you close the window it will loose everything in it.
EDIT: Apparently this only works with forms inside the project. My bad :(
What you need to do instead is keep a list of Forms inside your main project.
Make sure you give the form a name, and when you click the button to open the form simply loop through the list to find the set name.
Something like this:
Private FormList As New List(Of Form)
Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
Dim theForm As Form = Nothing
For Each f As Form In FormList
If f.Name = "Selected form name" Then
theForm = f
End If
Next
If theForm Is Nothing Then
theForm = New Otherproject.frm_thatform()
theForm.Name = "Selected form name"
FormList.Add(theForm)
End If
End Sub

Answer 2

A couple of options -
create a static/ shared form variable
Shared form As SingletonformEx.Form1
If form IsNot Nothing Then
form.BringToFront()
Else
form = New SingletonformEx.Form1()
form.Show()
End If
Extend the form class and implement singleton pattern on it.
Public Class SingletonForm Inherits Form
Private Shared m_instance As SingletonForm
Private Sub New()
'InitializeComponent();
End Sub
Public Shared ReadOnly Property Instance() As SingletonForm
Get
If m_instance Is Nothing Then
m_instance = New SingletonForm()
End If
m_instance.BringToFront()
Return m_instance
End Get
End Property
End Class
note: this is C# code converted to vb.net so i am not sure if this is perfect

Answer 3

In your original code, move the form declaration out to class (form) level:
Private ka As otherproject.frm_thatform = Nothing
Then check to see if it is Nothing or has been disposed, and recreate it as necessary:
If (ka Is Nothing) OrElse ka.IsDisposed Then
ka = New otherproject.frm_thatform
ka.Show(Me)
End If
If ka.WindowState = FormWindowState.Minimized Then
ka.WindowState = FormWindowState.Normal
End If
ka.BringToFront()