问无法在自动完成文本框中调用webmethod

EN

<ajax:ScriptManager ID="ScriptManager1"  runat="server">
 <Services >
  <ajax:ServiceReference Path="MyService.asmx" />
 </Services>
</ajax:ScriptManager>

 <asp:TextBox ID="txtMaterialNo"  
             Width="100%" runat="server" ></asp:TextBox>


       <cc1:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server" CompletionInterval="20"
        MinimumPrefixLength="1" ServiceMethod="GetMaterialId"
         ServicePath="MyService.asmx" TargetControlID="txtMaterialNo"> </cc1:AutoCompleteExtender>

    [ScriptService]
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class MyService : System.Web.Services.WebService
 public MyService()
    {

        //Uncomment the following line if using designed components 
        //InitializeComponent(); 
    }

    [System.Web.Services.WebMethod]
    public string[] GetMaterialId(string prefixMaterial)
    {
...... code
.... return
    }
  }