How to disable a Dropdownlist once someone has selected an item in the Dropdownlist

<asp:DropDownList id='DropDownList1' runat='server'>
	<asp:ListItem Value='Red'>Red</asp:ListItem>
	<asp:ListItem Value='Blue'>Blue</asp:ListItem>
	<asp:ListItem Value='Green'>Green</asp:ListItem>
</asp:DropDownList>

VB.NET

DropDownList1.Attributes.Add('onChange','this.disabled=true;' )

C#

DropDownList1.Attributes.Add('onChange','this.disabled=true;' );