Ajax Autocomplete extender is not working in vb .net

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ASP.NET

VB12

Ajax-toolkit

Hi experts,
i want to add ajax autocomplete extender in my project that has been created in vb.
but service method is not calling. Its not working properly please help me solve this issue.

my code is

<asp:TextBox ID="txtSearch" runat="server" AutoCompleteType="FirstName" ></asp:TextBox>
<asp:AutoCompleteExtender ID="ACEtxtSearch" runat="server" MinimumPrefixLength="0"
ServiceMethod="GetCompletionList" Enabled="True" TargetControlID="txtSearch" 
EnableCaching="true" FirstRowSelected="true"
CompletionInterval="10" CompletionSetCount="10" CompletionListCssClass="completionList" CompletionListItemCssClass="listItem"
CompletionListHighlightedItemCssClass="itemHighlighted">
</asp:AutoCompleteExtender>

vbcode

<WebMethod()> _
Function GetCompletionList(ByVal prefixText As String, ByVal count As Integer) As String()

End Function

please help me solve this issue
thanks in advance
(Keerthi Kumar)