Show hide div using codebehind

Question

I have a DropDownList for which I am trying to show a div OnSelectedIndexChanged but it says OBJECT REQUIRED.

I a

Answer 1

vb.net code

  Protected Sub DropDownList1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles DropDownList1.SelectedIndexChanged
    If DropDownList1.SelectedIndex = 0 Then
        OK1.Style.Add("display", "none")
    Else
        OK1.Style.Add("display", "block")
    End If
End Sub