Imports System.Windows.Forms
 
Public Class MDIParent1
 
    Private Sub ShowNewForm(ByVal sender As Object, ByVal e As EventArgs) _
      Handles NewToolStripMenuItem.Click, NewToolStripButton.Click, _
      NewWindowToolStripMenuItem.Click
 
        Dim ChildForm As New Form1
        ChildForm.MdiParent = Me
        ChildForm.Show()
 
    End Sub
 
End Class

Public Class Form1
 
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles _
      Button1.Click
        Dim obj As New Form2
        obj.ShowDialog()
    End Sub
 
    Public Sub aa(var1 As String)
        TextBox1.Text = var1
    End Sub
 
End Class

Public Class Form2
 
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles _
      Button1.Click
        Form1.aa("test")
    End Sub
 
End Class