Hi anjaliagarwal5,

Async post back in web forms still proceeds the whole page life cycle as if it's a traditional post. the difference is only the updated content inside target update panel that will be sent in response.

Therefore, it will not affect the content outside the UpdatePanel. If you need to achieve the requirements you mentioned, you need to change it to a full page postback ( PostBackTrigger ).

Page:
<form id="form1" runat="server" method="post">
        <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
                <asp:Panel runat="server" ID="pnlTest" Visible="false">
                    This is a test
                </asp:Panel>
        <asp:UpdatePanel ID="up1" runat="server" >
            <ContentTemplate>
                <asp:TextBox ID="txtTest" runat="server" OnTextChanged="txtestChanged" AutoPostBack="true"></asp:TextBox>
            </ContentTemplate>
            <Triggers>
                <asp:PostBackTrigger ControlID="txtTest" />
            </Triggers>
        </asp:UpdatePanel>
    </form>

Code behind:
 protected void txtestChanged(object sender, EventArgs e)
        {
            pnlTest.Visible = true;
        }

Or put the panel in another UpdatePanel and use the update() function.

    <form id="form1" runat="server" method="post">
        <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
        <asp:UpdatePanel runat="server" ID="upPanel" UpdateMode="Conditional">
            <ContentTemplate>
                <asp:Panel runat="server" ID="pnlTest" Visible="false">
                    This is a test
                </asp:Panel>
            </ContentTemplate>
        </asp:UpdatePanel>
        <asp:UpdatePanel ID="up1" runat="server" >
            <ContentTemplate>
                <asp:TextBox ID="txtTest" runat="server" OnTextChanged="txtestChanged" AutoPostBack="true"></asp:TextBox>
            </ContentTemplate>
            <Triggers>
                <asp:AsyncPostBackTrigger ControlID="txtTest" EventName="TextChanged" />
                <%--<asp:PostBackTrigger ControlID="txtTest" />--%>
            </Triggers>
        </asp:UpdatePanel>
    </form>

Code behind:
 protected void txtestChanged(object sender, EventArgs e)
        {
            pnlTest.Visible = true;
            upPanel.Update();
        }

Best regards,

Xudong Peng