Namespace myComboBox
    Public Class popUpComboBox
        Inherits System.Windows.Forms.ComboBox
 
        Protected Overrides Sub OnDropDown(ByVal e As System.EventArgs)
            Dim grid As New System.Windows.Forms.DataGridView
 
            grid.Width = 1000
            grid.Height = 1000
            grid.Left = Me.Left
            grid.Top = Me.Top + Me.Height
 
            grid.Show()
        End Sub
 
    End Class
End Namespace