缺少自定义角色提供程序命名空间

我正在尝试编写自己的角色提供程序，但它说命名空间丢失了，我做错了什么。我在c#mvc visualstudio2012中编码。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Security.RoleProvider; //this gave me an error
using MyProject.Models;
namespace MyProject.Controllers
{
    public class MyRoleProvider : RoleProvider
    {
        //
        // GET: /RoleProvider/
        private DefaultConnection db = new DefaultConnection();
        public ActionResult Index()
        {
            return View();
        }
        public override bool IsUserInRole(string username, string roleName)
        {
            // Return status defaults to false
            bool ret = false;
            if (RoleExists(roleName))
            {
                {
                    int c = (from m in db.Users
                             where m.userid == username &&
                             m.role == roleName
                             select m).Count();
                    if (c > 0)
                        ret = true;
                }
            }
            return ret;
        }
        public override bool RoleExists(string roleName)
        {
            bool ret = false;
            // If the specified role doesn't exist
            if (!GetAllRoles().Contains(roleName))
                ret = true;
            return ret;
        }
        public override string[] GetAllRoles()
        {
            string[] roles = null;
            roles = (from roleadmin in db.Users
                     select roleadmin.userid).ToArray();
            return roles;
        }
    }
}

using System.Web.Security;