How can I stop event excution in the page load event

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C#

Hi,

I have been working with an application for more that 2 years, we have nearly 60 forms. Initially client requested to redirect to log in page if the session expires. But now they want us to show a log in popup when the user press a control (like button). What I am doing is checking the session on the page load and display a popup using javascript. I just want to stop the execution cycle in the page load itself. please see the below code.

HTML

<asp:Button ID="button" runat="server" onclick="button_Click" Text="Check" /> 

C# Code:

protected void Page_Load(object sender, EventArgs e)
        {
            if (Session["User"] != null)
            {
                //Page load function
            }
            else
            {
                //Here i will call the Jquey Popup that I have created
                //I dont want to continue this execution process by calling button_Click
            }
        }

        protected void button_Click(object sender, EventArgs e)
        {
            //Save contition
        }

is there any way to stop the execution cycle in that else condition. Please give me suggestion for this scenario or please let me know idea to do this.