MVC : The parameters dictionary contains a null entry for parameter 'k' of non-nullable type 'System.Int32'

It appears that you are using the default route which is defined as this:

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

The key part of that route is the {id} piece. If you look at your action method, your parameter is k instead of id. You need to change your action method to this so that it matches the route parameter:

// change int k to int id
public ActionResult DetailsData(int id)

If you wanted to leave your parameter as k, then you would change the URL to be:

http://localhost:7317/Employee/DetailsData?k=4

You also appear to have a problem with your connection string. In your web.config, you need to change your connection string to this (provided by haim770 in another answer that he deleted):

<connectionStrings>
  <add name="EmployeeContext"
       connectionString="Server=.;Database=mytry;integrated security=True;"
       providerName="System.Data.SqlClient" />
</connectionStrings>

It seems that your action needs k but ModelBinder can not find it (from form, or request or view data or ..) Change your action to this:

public ActionResult DetailsData(int? k)
    {

        EmployeeContext Ec = new EmployeeContext();
        if (k != null)
        {
           Employee emp = Ec.Employees.Single(X => X.EmpId == k.Value);

           return View(emp);
        }
        return View();
    }

I faced this error becouse I sent the Query string with wrong format

http://localhost:56110/user/updateuserinfo?Id=55?Name=Basheer&Phone=(111)%20111-1111
------------------------------------------^----(^)-----------^---...
--------------------------------------------must be &

so make sure your Query String or passed parameter in the right format