Implement Sorting of GridView inside AJAX UpdatePanel in ASP.Net

Please compare your code and my reply code. You can also use AsyncPostBackTrigger.

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:GridView ID="GridView1" runat="server" AllowSorting="True" OnSorting="GridView1_Sorting"
            AutoGenerateColumns="False" CellPadding="5">
            <Columns>
                <asp:BoundField DataField="ProductID" SortExpression="ProductID" HeaderText="Product ID"
                    HeaderStyle-HorizontalAlign="Left" />
                <asp:BoundField DataField="ProductName" SortExpression="productname" HeaderText="Product"
                    HeaderStyle-HorizontalAlign="Left" />
                <asp:BoundField DataField="UnitPrice" SortExpression="unitprice" HeaderText="Price"
                    HeaderStyle-HorizontalAlign="Left" />
            </Columns>
        </asp:GridView>
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="GridView1" />
    </Triggers>
</asp:UpdatePanel>

Or you can do this from code behind also in page load.

ScriptManager.GetCurrent(Page).RegisterAsyncPostBackControl(this.GridView1);

Try it.

Thank You.