i have autocomplete extender as

<asp:TextBox ID="txtCommodity" runat="server" CssClass="TextBox" AutoPostBack="true"></asp:TextBox>

<ajaxToolkit:AutoCompleteExtender ID="autoComplete" runat="server" TargetControlID="txtCommodity"

CompletionInterval="100" CompletionSetCount="20" EnableCaching="false" ServicePath="~/AutoCompleteList.asmx"

MinimumPrefixLength="1" ServiceMethod="GetCompletionList" />

in one of aspx.cs pages

and my webservice is 

[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]

public class AutoCompleteList : WebService

string[] ret = new string[] { "A", "B", "C" };

 Did you register the webservice with the scriptmanager?

yes my master page has this

<ajaxToolkit:ToolkitScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">

<Services>

<asp:ServiceReference Path="~/AutoCompleteList.asmx" />

</Services>

</ajaxToolkit:ToolkitScriptManager>

Hi,

I have tested your code locally to resolve this issue. What my thought is the format of the ServiceMethod is not correct.

Please change it to this: 

        [System.Web.Services.WebMethod]
        [System.Web.Script.Services.ScriptMethod]
        public string[] GetCompletionList(string prefixText, int count)//, string contextKey
        {
            string[] ret = new string[] { "A", "B", "C" };

            return ret.ToArray();
        }

http://www.asp.net/AJAX/AjaxControlToolkit/Samples/AutoComplete/AutoComplete.aspx
How Do I:Use the ASP.NET AJAX AutoComplete Control

Have my suggestions helped?

Best regards,

Zhi-Qiang Ni