Zitat:

Klar. In jeder Childform ist .MdiParent die Hauptform.

Dim clientcount As Int32 = 1
    Protected Friend Property counter() As Int32
        Get
            Return clientcount
        End Get
        Set(ByVal value As Int32)
            clientcount = value
        End Set
    End Property
Private Sub ts_btn_newfrm_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles ts_btn_newfrm.Click
        Dim ChildForm As New frm_client()
        clientcount += 1
        With ChildForm
            .MdiParent = Me
            .Text = "Kindform (" & clientcount & ")"
            .Show()
        End With
End Sub

Private Sub frm_client_FormClosing(ByVal sender As Object, ByVal e As _
  System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
        'dim frm as form = MdiParent
        'frm.clientcount -= 1
'########geht nicht########
  'MdiParent.clientcount -= 1 ### geht auch nicht
    End Sub