1. Сначала вы создаете MDIParent, основную форму, код модуля
2. Введите код в модуль
Module MainModule
Public Forms1 As New List(Of frmMain)()
Public Sub New1Form()
Dim form2a = New frmMain
Forms1.Add(form2a)
Forms1.Item(Forms1.Count - 1).Text = "Form" & Forms1.Count - 1
Forms1.Item(Forms1.Count - 1).Show()
End Sub
Public Sub NewForm()
Dim form2a = New frmMain
Forms1.Add(form2a)
Dim form2b = New frmMain
Forms1.Add(form2b)
Forms1.Item(Forms1.Count - 1).Text = "Form" & Forms1.Count - 1
Forms1.Item(Forms1.Count - 1).Show()
Forms1.Item(Forms1.Count - 2).Text = "Form" & Forms1.Count - 2
Forms1.Item(Forms1.Count - 2).Show()
End Sub
Public Sub CloseOldForm()
If (Forms1.Count - 3) >= 0 Then
Forms1.Item(Forms1.Count - 3).Close()
End If
End Sub
End Module
3. Введите код в MDIParent1
Private Sub NewToolStripButton_Click(sender As Object, e As EventArgs) Handles NewToolStripButton.Click
MainModule.New1Form()
End Sub
4. Введите код в основной форме
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
MainModule.NewForm()
If (MainModule.Forms1.Count - 3) >= 0 Then
Me.Hide()
End If
End Sub
5. Скачать исходный код
https://www.tummosoftware.com/p/how-to-close-existing-running-program.html