asp.net mvc 4 - Creating a custom role principal to be used with custom role provider, How do I get IIdentity to pass as parameter? -

i have custom role provider mvc4 application working in creates roles, checks role existence, check isuserinrole [authorize(roles = "admin")] still using default system.web.security.roleprincipal.isinrole(string role) method

i have tried create custom roleprincipal overrides isinrole method having problem finding correct parameter constructor , unsure how set in web.config. code follows:

using system; using system.collections.generic; using system.linq; using system.web; using system.configuration.provider; using metalearning.data; using system.web.security;  namespace project.principal {   public class myprincipal : system.web.security.roleprincipal   {     private mycontext dbcontext = new mycontext(system.configuration.configurationmanager.connectionstrings["mycontext"].connectionstring);     private repository<myuser> userrepository;     private repository<role> rolerepository;              public myprincipal()     {         this.userrepository = new repository<myuser>(dbcontext);         this.rolerepository = new repository<role>(dbcontext);     }      public override bool isinrole(string role)     {         role rolecheck = rolerepository.get(r => r.name == role).firstordefault();         myuser user = userrepository.get(u => u.username == httpcontext.current.user.identity.name).firstordefault();         user.roleid = rolecheck.roleid;         userrepository.update(user);         userrepository.savechanges();         return true;     } } 

}

i checked roleprincipal documentation http://msdn.microsoft.com/en-us/library/system.web.security.roleprincipal.aspx , can see basic constructor roleprincipal(iidentity) takes parameter of iidentity. how , can retrieved unsure is? there additional changes required in webconfig?