This link points to code that shows you how to use the context key on an asp.net autocompleteextender control

example

sample C# and asp code

 aspx page
   <cc1:AutoCompleteExtender runat="server" ID="AutoCompleteExtender1" TargetControlID="txtAccount"
                    ServiceMethod="GetAccount" ServicePath="ProductsWS.asmx" MinimumPrefixLength="1"
                    ContextKey="Admin" UseContextKey="true"
                    CompletionInterval="200" EnableCaching="true">
                </cc1:AutoCompleteExtender>

aspx code behind

 if (!IsPostBack)
        {
            //set key to user name unless user is admin

            if (Convert.ToBoolean(Session["isMasterAccount"].ToString()) == true)
            {
                AutoCompleteExtender1.ContextKey = "Admin";
            }
            else
            {
                AutoCompleteExtender1.ContextKey = Session["Name"].ToString();
            }

        }

asmx page

 [WebMethod]
    public string[] GetAccount(string prefixText, int count, string contextKey)
    {
        //filter list of accounts by person logged in.
        DAL dLayer = new DAL();
        string uName = contextKey;

        return dLayer.GetAccount(prefixText, uName).ToArray();

    }