使用LINQ查询单字符串导致路由

Question

I've now tried for two days by trial, error and google to do the following thing:

When an user enters my MVC site, it loggs with its Windows account. It's working properly. When he does that, MVC connects to db to extract his name (based on his account), which also works well. My task is to make my Index page to contain filtered information by default. But user must also be able to change these filters by himself. My site's search functions are working well, using parameters from the url. I tried to edit my routing by directly typing the username and then by setting it to Parameter.Optional. Both methods result in errors:

public class RouteConfig
{
    public static CaseDBContext db = new CaseDBContext();
    public static string UName()
    {
        return db.Users.AsQueryable().Where(col => col.UNAD == HttpContext.Current.User.Identity.Name.ToString()).Select(col => col.Users).ToList().SingleOrDefault().ToString();
    }
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapRoute(
            name: "",
            url: "{controller}/{action}/{dropDownIssBy}",
            defaults: new { controller = "Cases", action = "Index", dropDownIssBy = UName(), caseStatus = UrlParameter.Optional, sortBy = UrlParameter.Optional }
        );

This one ends in null reference exception. If I use new Func(UName), the site loads but when debugged it doesn't fire UName function at all and just ignores it.

private CaseDBContext db = new CaseDBContext();
public string UName()
{
return db.Users.AsQueryable().Where(col => col.UNAD == User.Identity.Name.ToString()).Select(col => col.Users).ToList().SingleOrDefault().ToString();
}
    public ActionResult Index(int? page, [...parameters...], string caseStatus = "In Progress", string dropDownIssBy = UName(), string SortBy = "Issue Date asc")
{
code here
}

The upper results in "Default parameter value must be compile-time constant.

I also tried this:

public static string UName(string i)
    {
        string str = db.Users.AsQueryable().Where(col => col.UNAD == i).Select(col => col.Users).ToList().SingleOrDefault().ToString();
        return str;
    }
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapRoute(
            name: "",
            url: "{controller}/{action}/{dropDownIssBy}",
            defaults: new { controller = "Cases", action = "Index", dropDownIssBy = UName(HttpContext.Current.User.Identity.Name.ToString()), caseStatus = UrlParameter.Optional, sortBy = UrlParameter.Optional }
        );

Result is "Object reference not set to an instance of an object."

FYI: The first code is connected to the first mentioned tryout. The second code relies on different RouteConfig settings(instead of string dropDownIssBy = UName it is string dropDownIssBy = Parameter.Optional).

I'm not good at these routing problems. I really need to apply the LINQ query to achieve the desired functionality. Could you please give me some advice how to solve the problem?

Thanks in advance!

Answer 1

You're trying to assign the method UName to string dropDownIssBy (rather than it's result) in both cases. You missed off your brackets. ie use UName()

The lines in question should read:

defaults: new { controller = "Cases", action = "Index", dropDownIssBy = UName(), caseStatus = UrlParameter.Optional, sortBy = UrlParameter.Optional }

and

public ActionResult Index(int? page, [...parameters...], string caseStatus = "In Progress", string dropDownIssBy = UName(), string SortBy = "Issue Date asc")

---

If you did intend passing a method then the syntax for the first line would look more like:

defaults: new { controller = "Cases", action = "Index", dropDownIssBy = new Func<string>(UName), caseStatus = UrlParameter.Optional, sortBy = UrlParameter.Optional }

For the second line I don't believe you can set a default method for an optional parameter. You could get around this by having the default as null, and call UName from within the method if it is null. eg

public ActionResult Index(int? page, [...parameters...], string caseStatus = "In Progress", Func<string> dropDownIssBy = null, string SortBy = "Issue Date asc")
{
    if(dropDownIssBy == null)
    {
        dropDownIssBy = UName;
    }
}