ASP.NET ModelState.IsValid is validating returnUrl parameter

Question

I've already spent hours finding someone with the same issue, by Googling and it seems like nobody else has the same issue. So I am creating my first post here :)

I have a method inside a controller, which is supposed to log-in the user and afterwards redirect the user to the url which he was previously trying to reach. But ModelState.IsValid is not only validating the model, but also the url parameter and if the user is NOT trying to get redirected (user is at the 'root-page' and clicks the button to login - in other words the returnUrl is null) and then it would always tell me that the validation is unsuccessful.

I do not want to remove this error all the time manually, but find a different solution, where it would not try to validate this parameter.

[AllowAnonymous]
[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Login([Bind("Email,Password")] LoginViewModel data, string returnUrl)
{
    if (!ModelState.IsValid) 
         return View(data);

    // todo throw custom error if user does not exist
    ClassUser? user = _context.Users.FirstOrDefault(x => x.Email == data.Email);
    //...
}

Click here to see ModelState's output

Thank you very much in advance. I've been trying for hours to solve this already.

Answer 1

Just put a question mark at the end of your last parameter datatype like this string? returnUrl or try Nullable<string> returnUrl