Visual Basic

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Dim output As String = "Open forms: " & vbCrLf
    For Each frm As Form In My.Application.OpenForms
        output &= frm.Text & vbCrLf
    Next
    MsgBox(output)
End Sub

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim frm As Form = New Form2
    frm.Show()
    me.hide
End Sub

Public Class Form1
    Dim frm As Form2
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'Create a new instance of Form2 if we don't have one
        'Notice I passed this instance in the constructor
        If frm Is Nothing OrElse frm.IsDisposed Then frm = New Form2(Me)

        frm.Show()
        Me.Hide()
    End Sub
End Class

Public Class Form2
    Private m_Caller As Form    'The calling form

    Public Sub New(ByVal caller As Form)
        InitializeComponent()

        m_Caller = caller
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        m_Caller.Show()
        Me.Hide() 'You could also call me.close instead which may be more appropriate
    End Sub
End Class

Public Class Form1
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Form2.Show()
        Me.Hide()
    End Sub

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        'Set the text to the time of creation so we can tell this
        'we are dealing with the same instance
        Me.Text = Now.ToLongTimeString
    End Sub
End Class

Public Class Form2

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Form1.Show()
        Me.Hide()
    End Sub

    Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'Set the text to the time of creation so we can tell this
        'we are dealing with the same instance
        Me.Text = Now.ToLongTimeString
    End Sub
End Class

B Journey wrote:

The most practical way should be the set the caller form as parent form and then in 2nd form redirect user to parent form.

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim frm As New Form2
    frm.Show(Me)
End Sub

B Journey wrote:

This cannot be applied for forms because a form instance can be triggered from different procedures at same time. Therefore, sticking with creation time is not appropriate. Think about two timer components. Each timer shows the same form in every second because the each timer works independently.