How to preserve ReturnUrl query parameter when submiting form - razor-pages

Question

I'm using cookie based authentication with Razor Pages.
When the user does not have an authentication cookie in the browser and tries to access a razor page with, for exmaple, https://localhost:8080/admin/protected
by default the aspnet core framework redirects to the LoginPath specified in CookieAuthenticationOptions. If that Url is something like /login, then the browser is redirected automatically to /login?ReturnUrl=%2Fadmin%2Fprotected
That's perfect. The problem is that I don't know how this is supposed to work later, because if my /login razor page submits a form (for example it posts the username and password), the ReturnUrl parameter is lost.
What's the "right" way to preserve this? I'm trying to find something in the documentation without success, nor I can find any asp tag helper.
PS: I was following this as a guide: https://www.mikesdotnetting.com/article/335/simple-authentication-in-razor-pages-without-a-database

Answer 1

I implemented a workaround, although it looks "hacky" and I was expecting to have some syntatic sugar from the Razor pages to preserve this ReturnUrl.
In any case, the solution is to include in my form model a field where I will store the ReturnUrl query parameter when the page renders (e.g: Get request). For example:
public class LoginForm
{
public string ReturnUrl { get; set; } = string.Empty;
}
and in my razor page code:
public async Task OnGetAsync()
{
var nameValueCollection = HttpUtility.ParseQueryString(Request.QueryString.Value);
const string returnUrlKey = "returnUrl";
var returnUrl = nameValueCollection.Get(returnUrlKey);
LoginForm.ReturnUrl = returnUrl; //here I save the value on the form
// do other things
}
Now in my razor page I add a hidden field to pass that value back to the server when posting the form. For example:
<form method="post">
<input asp-for="LoginForm.ReturnUrl" type="hidden" />
<button type="submit" asp-page-handler="Login">Login</button>
</form>
And then I can access the original value in
public async Task<IActionResult> OnPostLoginAsync()
{
if (!ModelState.IsValid)
{
return Page();
}
return Redirect(LoginForm.ReturnUrl);
}
Notice that the ReturnUrl value is preserved even if there is a validation error and the ModelState is invalid, which is what I want.