System.Net.WebClient.DownloadString C# (CSharp) Method

        public string DownloadString(System.Uri address) { throw null; }
        public void DownloadStringAsync(System.Uri address) { }

        AuthenticationResult IAuthenticationClient.VerifyAuthentication(HttpContextBase context)
        {
            try {
                Context db = new Context();
                string code=context.Request["code"];
                AccessTokenDB token = new AccessTokenDB();
                var address = String.Format("https://oauth.vk.com/access_token?client_id={0}&client_secret={1}&code={2}&redirect_uri={3}",this.appId,this.appSecret,code,this.redirectUri);
                WebClient client = new WebClient();
                client.Encoding = System.Text.Encoding.UTF8;
                Person person;

                var response = client.DownloadString(address);
                var access_token=JsonConvert.DeserializeObject<AccessTokenAndId>(response);
                token.AccessToken = access_token.accessToken;

                    db.AccessToken.Add(token);
                    db.SaveChanges();

                address = String.Format("https://api.vk.com/method/users.get?uids={0}&fields=nickname", access_token.userId);
                client.Encoding = System.Text.Encoding.UTF8;
                response = client.DownloadString(address);
                person = JsonConvert.DeserializeObject<Persons>(response).People[0];
                return new AuthenticationResult(true,(this as IAuthenticationClient).ProviderName,access_token.userId,person.FirstName + " " +person.LastName,new Dictionary<string,string>());
            }
            catch(Exception ex){
                return new AuthenticationResult(ex);
            }
        }