Change Action Parameter but keep URL same

Question

So I have a C# .NET project.
On certain pages I need to change a parameter passed through the ActionResult but still keep the original URL.

In order to change the parameter to the one I needed, I changed the variable through the ActionContext. But this then changes the URL to the parameter - which is expected but not wanted.

public class CustomController : Controller {
    protected override void OnActionExecuting(ActionExecutingContext filterContext) {
        if (isReplaceType) {
            // replace action parameter to needed one
            filterContext.ActionParameters["variable1"] = replacedVariable;

            // Fake URL code here ??? ----------------
        }
        base.OnActionExecuting(filterContext);
    }
}

I need to change site.com/replacedVariable to site.com/originalVariable in the user's browser URL bar.

How can I do this?

---

EDIT: Route looks like this in Global.asax

routes.MapRoute(
    name: "HomeDefault",
    url: "{variable1}/{action}",
    defaults: new { controller = "home", action = "index" },
    namespaces: new string[] { "Project.Controllers" }
);

Answer 1

After reading through this question: Change URL After Action is Hit MVC? It suggests that the only way to change the URL is to redirect the page - meaning changing an action variable was not the reason why the URL changed to the new variable.

I found that the page redirected in certain cases and this is why it changed. Hence, in order to fix it, I have overridden the Redirect function and replaced the new variable with the old value in the URL.

public class CustomController : Controller {
    protected override RedirectResult Redirect(string url) {
        if (Session["replacedVariable"] == null) {
            return base.Redirect(url);
        }

        string replaceUrl = url.Replace("/" + Session["replacedVariable"], "/originalVariable");
        return base.Redirect(replaceUrl);
    }
}