The interface of my MDI Program:

My MDI program code source:
¡® This is the MDI main window (the container), use it¡¯s menu option to open the child window as many as you want.
Public Class frmMdi

    Private Sub mNewSub1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles mNewSub1.Click
        Dim f As New frmSub1
        f.MdiParent = Me
        f.Show()
    End Sub

    Private Sub mNewSub2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles mNewSub2.Click
        Dim f As New frmSub2
        f.MdiParent = Me
        f.Show()
    End Sub
End Class

¡® Child window 1. From here, want to call the function or excute the procedure in child window 2.
Public Class frmSub1

    ' Call the function named fSum in child window 2.
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        For Each f As Form In frmMdi.MdiChildren
            If TypeOf f Is frmSub2 Then ¡® see Note
                ' I want to call fSum in f, something like this: textbox1.text=f.fSum(4)
                ' but it is no fSum object in f. How can explore the fSum object in frmSub2?
            End If
        Next
    End Sub
¡®Note: In practice, I have to use another IF statement within this IF statement to locate one of the frmSub2

    ' Excute the procedure named pSum in child window 2.
    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        ' Same problem, how to explore the pSum object in frmSub2 and can excute it here.
    End Sub
End Class

¡® Child window 2. have the function and procedure being called or excuted by child window 1.
Public Class frmSub2

    Friend Sub pSum(ByVal j As Integer)
        Dim i As Integer, sum As Integer

        For i = 1 To j
            sum = sum + i
        Next
        TextBox1.Text = sum
    End Sub

    Friend Shared Function fSum(ByVal j As Integer) As Integer
        Dim i As Integer

        For i = 1 To j
            fSum = fSum + i
        Next
    End Function

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        TextBox1.Text = fSum(Val(TextBox1.Text))
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Call pSum(TextBox1.Text)
    End Sub
End Class