public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
        {
            var user = Users()
                .FirstOrDefault(x => x.Name == context.UserName
                                && x.Password == context.Password);

            if (user == null)
            {
                context.SetError("invalid_grant",
                    "Usuário não encontrado ou a senha está incorreta.");
                return;
            }

            var identyUser = new ClaimsIdentity(context.Options.AuthenticationType);
            identyUser.AddClaim(new Claim("sub", context.UserName));
            identyUser.AddClaim(new Claim(ClaimTypes.Role, "user"));
            context.Validated(identyUser);
        }

        public static IEnumerable<User> Users()
        {
            return new List<User>
            {
                new User { Name = "Marcelo", Password = "admin" },
                new User { Name = "Joao", Password = "12345" },

            };
        }