Get Label value modified using JavaScript at Server Side on PostBack in ASP.Net

Hi DemNorbi,

You are trying to pass Label1 value to Label3 which got lost after Button2 click due to page postback. So you need to use hiddenfield to store the value on Button1 click and on Button2 click assign the hiddenfield value to Label3.

Check this example. Now please take its reference and correct your code.

HTML

<div>
    Label 1:<asp:Label ID="Label1" Text="" runat="server" />
    <br />
    Label 3:<asp:Label ID="Label3" Text="" runat="server" />
    <asp:HiddenField ID="hfValue" runat="server" />
    <br />
    <asp:Button ID="Button1" Text="Button1" runat="server" OnClick="Button1_Click" />
    <asp:Button ID="Button2" Text="Button2" runat="server" OnClick="Button2_Click" />
</div>
<script type="text/javascript">
    function Function1() {
        var Top = document.getElementById('<%= Button1.ClientID %>').offsetTop;
        var TopLabel1 = document.getElementById('<%= Label1.ClientID %>');
        TopLabel1.innerHTML = Top;
        document.getElementById('<%= hfValue.ClientID %>').value = Top;
    }
</script>

Code

C#

protected void Button1_Click(object sender, EventArgs e)
{
    ScriptManager.RegisterStartupScript(this.Page, Page.GetType(), "Positions", "Function1()", true);
}
protected void Button2_Click(object sender, EventArgs e)
{
    Label3.Text = hfValue.Value;
}

VB.Net

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)
    ScriptManager.RegisterStartupScript(Me.Page, Page.[GetType](), "Positions", "Function1()", True)
End Sub

Protected Sub Button2_Click(ByVal sender As Object, ByVal e As EventArgs)
    Label3.Text = hfValue.Value
End Sub

Screenshot