ASP.Net get ScriptManager.RegisterClientScriptBlock return on UpdatePanel hiddenField

Question

I am trying to obtain the return of a Javascript function that I call ont server side using ScriptManager.RegisterClientScriptBlock .

Currently, my javascript function save the value in a HiddenField that is contained in an UpdatePanel. When I call the method , I run the ScriptManager.RegisterClientScriptBlock and after that get the value of the HiddenField , but always returns me the value of the previous call. Here I show the code:

My User Control ASPX Side:

<script>

var num = 0;

function getReturn() {

    num = num + 1;
    var hr= document.getElementById('<%= hdf.ClientID %>');
    hRetorno.value = num;

}
</script>

...

<asp:UpdatePanel ID="up1" UpdateMode="Conditional" runat="server">
    <ContentTemplate>
        <asp:HiddenField ID="hdf" runat="server" />
    </ContentTemplate>
</asp:UpdatePanel>

My User Control Server Side Code:

    public String GetReturn()
    {
        return MyControl.jsGetReturn(this.Page, this.hdf);
    }

    private static String jsGetReturn(Page page, HiddenField hid)
    {
        ScriptManager.RegisterStartupScript(page, page.GetType(), "key", "getReturn();", true);

        return hid.Value;

    }

Index Page ASPX Side:

        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>

        <div style="width:720px; height:480px;">
            <uc1:MyControl runat="server" id="MyControl1"/>
        </div>

        <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
            <ContentTemplate>

                <asp:Button ID="btn" runat="server" OnClick="btn_Click" />
                <asp:TextBox ID="txt" runat="server" ></asp:TextBox>

            </ContentTemplate>
        </asp:UpdatePanel>

Index Page Server Side Code:

    protected void btn_Click(object sender, EventArgs e)
    {
        txt.Text = MyControl1.GetReturn();
    }

Answer 1

You can not do these things simultaneously.

1. Set value of hidden field from server side code using javascript via RegisterClientScriptBlock or RegisterStartupScript.
2. And retrieve that hidden filed value in server side code at the same time.

This is because, (changed) hidden field value is available to server-side code only when there is postback, when you set the value from server-side, there is no postback happening, that is why you are always getting previous value , i.e. that old value which is posted back to page.

EDIT

When you invoke RegisterClientScriptBlock or RegisterStartupScript, it won't make JS call instantly, rather it just append a javascript call before or after <form.. tag based on what you used and they are called on document load, so that means in jsGetReturn when you call RegisterStartupScript, it will set value of hidden field in document load, - hid.Value will not have updated value, as it is yet to be incremented via document load.