How to hide Selected Item of DropDownList in ASP.Net

HTML

<asp:DropDownList ID="ddlHideSelectedValues" runat="server">
            <asp:ListItem Text="A" />
            <asp:ListItem Text="B" />
            <asp:ListItem Text="C" />
            <asp:ListItem Text="D" />
        </asp:DropDownList>
        <asp:CheckBox ID="chkHideSelected" runat="server" Text="Hide Selected" AutoPostBack="true"
            OnCheckedChanged="HideSelected" />

C#:

protected void HideSelected(object sender, EventArgs e)
        {
            if (this.chkHideSelected.Checked)
            {
                this.ddlHideSelectedValues.SelectedItem.Text = string.Empty;                
            }
            else
            {
                this.ddlHideSelectedValues.SelectedItem.Text = this.ddlHideSelectedValues.SelectedItem.Value;
            }
        }

Thank You.