ให้โชว์ ShowDialog ใน Mdiparent ได้ป่าวครับ ขอวิธีหน่อยครับ

   If ExistsForm("yourFormName") Then
      Return
   End If

    'Check  Mdi form is open
    Private Function ExistsForm(ByVal f As String) As Boolean
        Dim fc As FormCollection = Application.OpenForms
        Dim ret As Boolean = False
        For Each fe As Form In fc
            If fe.Name.Equals(f, StringComparison.CurrentCultureIgnoreCase) Then
                fe.Activate()
                ret = True
            End If
        Next
        Return ret
    End Function

    Private Sub ShowForm(ByVal f As Form, Optional ByVal IsMdiChildren As Boolean = True, Optional ByVal IsModual As Boolean = False)
        If IsMdiChildren Then
            If Not f.IsMdiContainer Then
                f.MdiParent = Me
            End If
            Call ShowFormX(f, IsModual)
        Else
            Call ShowFormX(f, IsModual)
        End If
    End Sub

    Private Sub ShowFormX(ByRef frm As Form, ByVal isModual As Boolean)
        If isModual Then
            Using x As Form = frm
                x.ShowDialog()
            End Using
        Else
            frm.Show()
        End If
    End Sub

 if (this.MdiChildren.Count() == 0)
{
//Do Something
}