VB - Find child form from parent - vb.net

Question

I am in a project with multiple form.
I create a TicTacToe form here :
Private Sub MenuTicTacToe(ByVal sender As Object, ByVal e As System.EventArgs)
Dim page As Form = New TicTacToe
page.Show(Me)
End Sub
Here is a TicTacToe form:
Public Class TicTacToe
Public opponent as String
'Some code where user set opponent
Public Function Receive(S As String)
if string = opponent
'Some code
End Function
End Class
I would like to call my function Receive in my main form
If i do:
TicTactoe.Receive(S)
It call a instance of Receive where opponent does not exist.
I would like to find the oppened form of TicTacToe and call Receive
Thanks

Answer 1

Comments in line
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
page.Receive("Joe")
End Sub
'A form level variable to hold a reference to the instance of TicTacToe
'Although vb.net can use default instances, you have created an explicit
'instance of TicTacToe so you need to keep a reference if you want to
'refer to this instance.
Private page As TicTacToe
Private Sub MenuTicTacToe(ByVal sender As Object, ByVal e As System.EventArgs)
page = New TicTacToe()
page.Show(Me)
End Sub
Partial Public Class TicTacToe
Inherits Form
Public opponent As String
'Functions must be declared as a Type
'If you do not need a return value use a Sub
Public Function Receive(S As String) As String
Dim someString As String = ""
If S = opponent Then
'Do something
End If
'There must be a return Value
Return someString
End Function
End Class

Answer 2

Use this to show the form
Dim page As TicTacToe
Private Sub MenuTicTacToe(ByVal sender As Object, ByVal e As System.EventArgs)
page = New TicTacToe
page.Show(Me)
End Sub
Then you can use
page.Receive(S)
Edit
To use multiple forms
For Each f As TicTacToe in Application.OpenForms().OfType(Of TicTacToe)
f.Receive (S)
Next

Answer 3

In C#, you'd need a new instance, but as you are in VB, the compiler already does that for you.
What you are currently doing, is creating a new instance of the TicTacToe form and showing it:
Private Sub MenuTicTacToe(ByVal sender As Object, ByVal e As System.EventArgs)
Dim page As Form = New TicTacToe
page.Show(Me)
End Sub
But you don't save that instance anywhere. Then, in your next piece of code, you are using a different instance, which is the static one created by the compiler:
TicTacToe.Receive(S) // TicTacToe is the static instance
Therefore, you end up calling two different instances, which explains why there is no opponent set.
To get around this problem, do not create a new instance. In your Private Sub MenuTicTacToe, just use the instance created by the compiler, and you won't have this problem, just like this:
Private Sub MenuTicTacToe(ByVal sender As Object, ByVal e As System.EventArgs)
TicTacToe.Show(Me)
End Sub
Hope this helps.