An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter. What to do?

Question

public ActionResult Applicant(int JobPostId)
{
    List<JobSeeker> jobSeekers = new List<JobSeeker>();
    var ApplicationList = db.JobApplications.Where(x => x.JobPostId == JobPostId).ToList();

    foreach(JobApplication app in ApplicationList)
    {
        jobSeekers.Add(db.Users.OfType<JobSeeker>().Single(x => x.Id == app.JobSeekerId));
    }
    // var JobseekerForJobPost = db.Users.OfType<JobSeeker>().Where(x => x.JobPostsAppliedFor.Any(y => y.JobPostID == JobPostId)).ToList();

    return View(jobSeekers);
}

@Html.ActionLink("View Applicants", "Applicant", new { id = item.JobPostID }) |


public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.MapRoute(
        name: "Default",
        url: "{controller}/{action}/{id}",
        defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
    );
}

I cannot figure it out why it is giving me error.

Answer 1

Your action method has a value type parameter (an int ):

public ActionResult Applicant(int JobPostId)

But you're not supplying the route with a value for it:

Html.ActionLink("View Applicants", "Applicant", new { id = item.JobPostID })

The error is telling you that you must supply a value for that parameter, because an int can't otherwise be null . If you want this value to possibly be null , make it nullable:

public ActionResult Applicant(int? JobPostId)

Or, if it shouldn't be nullable, provide a value for it:

Html.ActionLink("View Applicants", "Applicant", new { JobPostID  = item.JobPostID })

(From your usage of that value in that action method, it looks like it shouldn't be null . But I suppose it's possible that it could be in your data.)

---

Side note: By C# convention variables should be lowecase:

public ActionResult Applicant(int jobPostId)

and:

Html.ActionLink("View Applicants", "Applicant", new { jobPostID  = item.JobPostID })

Answer 2

Just make your parameter

public ActionResult Applicant(int JobPostId)

optional

public ActionResult Applicant(int? JobPostId)

In the routing it's mapped as optional thats why you have to make it nullable