What is the equivalent of Web Forms "Page_Load" in ASP.Net Core 5, so my code will run before any page loading?

In asp.net core, you can use Action filters to replace the "Page_Load" method in webform.

You can register the global scope of Action filters in the startup to ensure that the Action filters will be executed before each action is executed.

Add the following Action filter in your project :

public class MyActionFilter : IActionFilter
    {
        public void OnActionExecuting(ActionExecutingContext context)
        {

                // Do something before the action executes.
            if (!context.HttpContext.Request.Path.ToString().Contains("Login"))
            {
                if (context.HttpContext.Session.GetString("user") == null)
                {
                    context.Result = new RedirectToRouteResult(
                        new RouteValueDictionary { { "controller", "Login" }, { "action", "Index" } });
                }
            }
            
        }

        public void OnActionExecuted(ActionExecutedContext context)
        {
            // Do something after the action executes.
        }
    }

Then in startup.cs ConfigureServices method, add following code to apply it to all scope:

services.AddControllersWithViews(options =>
            {
                options.Filters.Add(typeof(MyActionFilter));
            });