How to assign roles to users by a single user

I have been struggling to assign roles to users by a single person on ASP.NET MVC. I have used a view bag on a controller and a dropdown list on the view. When I assign roles, it is not saved on the database. Can anyone help me sort this out? The error message which I get is "UserId and RoleId are required."

 

/* This is the AssignRole View */

@model Microsoft.AspNet.Identity.EntityFramework.IdentityUserRole

@{

ViewBag.Title = "AssignRole";

}

@using (Html.BeginForm())

{

}

/* This is Roles Controller */

// Assign roles

public ActionResult AssignRole()

{

ViewBag.AllUsers = new SelectList(db.Users, "Id", "UserName");

ViewBag.AllRoles = new SelectList(db.Roles, "Id", "Name");

return View();

}

[HttpPost]

public ActionResult AssignRole(IdentityUserRole userRole)

{

if (ModelState.IsValid)

{

db.UserRoles.Add(userRole);

try

{

db.SaveChanges();

}

catch (DbEntityValidationException e)

{

Console.WriteLine(e);

}

return RedirectToAction("UsersWithRoles");

}

ViewBag.AllUsers = new SelectList(db.Users, "Id", "UserName",userRole.UserId);

ViewBag.AllRoles = new SelectList(db.Roles, "Id", "Name",userRole.RoleId);

return View(userRole);

}