public class ResourceOwnerPasswordValidator : IResourceOwnerPasswordValidator
{
    public Task ValidateAsync(ResourceOwnerPasswordValidationContext context)
    {
        //dapper.net for getting user from DB
        using (IDbConnection db = new SqlConnection("Data Source=.;Initial Catalog=TestDB;Integrated Security=True"))
        {
            var user = db.Query<User>("SELECT * FROM Registration WHERE Username=@Username AND Password=@Password",
                new { Username = context.UserName, Password = context.Password }).SingleOrDefault();

            if (user == null)
            {
                context.Result = new GrantValidationResult(TokenErrors.InvalidRequest, "Username or password is incorrect");
                return Task.FromResult(0);
            }

            #region
            var users = new List<InMemoryUser>
            {
                 new InMemoryUser
                {
                    Subject = "818727",
                    Username = user.Username,
                    Password = user.Password,
                    Claims = new Claim[]
                    {
                        new Claim(JwtClaimTypes.Name, user.FirstName),
                        new Claim(JwtClaimTypes.GivenName, user.FirstName),
                        new Claim(JwtClaimTypes.FamilyName, user.LastName),
                        new Claim(JwtClaimTypes.Email, user.Email)
                    }
                }
            };
            #endregion

            context.Result = new GrantValidationResult(user.Id.ToString(), "Password", users[0].Claims);

            return Task.FromResult(0);
        }
    }
}