asp.net - Check if user exist in LDAP before authorization -

i use asp.net identity 2.0 , ldap authentication in mvc project , loing method shown below. @ first step check if user authenticated in ldap, need detect if there no user in ldap or credentials wrong return correct message user. how can this?

[httppost] [allowanonymous] [validateantiforgerytoken] public async task<actionresult> login(loginviewmodel model, string returnurl) {     if (!modelstate.isvalid)     {         return view(model);     }      applicationgroupmanager groupmanager = new applicationgroupmanager();      // validate user using ldap      if (membership.validateuser(model.username, model.password))     {         formsauthentication.setauthcookie(model.username, model.rememberme);         // formsauthentication.setauthcookie(model.username, false);          //code omitted brevity          if (this.url.islocalurl(returnurl) && returnurl.length > 1 && returnurl.startswith("/")                     && !returnurl.startswith("//") && !returnurl.startswith("/\\"))         {             return this.redirect(returnurl);             //return redirecttolocal(returnurl);         }         return this.redirecttoaction("index", "issue");     }     else     {         modelstate.addmodelerror("", "wrong credentials");         return this.view(model);     } }