public class UserRepository : IDisposable
{
    private readonly UserDataContext context;
    public UserDataContext Context
    {
        get { return context; }
    }

    public UserRepository()
    {
        context = new UserDataContext(UserDataContext.DBConnectionString);
        CreateDatabase();
    }

    private void CreateDatabase()
    {
        if (context.DatabaseExists() == false)
        {
            context.CreateDatabase();
        }
    }

    public User GetByID(int id)
    {
        return context.GetTable<User>().FirstOrDefault(e => e.UserId.Equals(id));
    }

    public User GetByUsername(string username)
    {
        return context.GetTable<User>().FirstOrDefault(e => e.UserName.Equals(username));
    }

    public void Save(User user)
    {
        if (user.UserId > 0)
        {
            context.GetTable<User>().Attach(user, true);
        }
        else
        {
            context.GetTable<User>().InsertOnSubmit(user);
        }

        context.SubmitChanges();
    }

    public List<User> GetAll()
    {
        return context.GetTable<User>().ToList();
    }

    public int Count()
    {
        return context.GetTable<User>().Count();
    }

    public void Dispose()
    {
        if (context != null)
        {
            context.Dispose();
        }
    }
}

[Table]
public class User
{
    [Column(IsPrimaryKey = true, IsDbGenerated = true, DbType = "INT NOT NULL Identity", CanBeNull = false, AutoSync = AutoSync.OnInsert)]
    public int UserId { get; set; }

    [Column]
    public string UserName { get; set; }

    [Column]
    public string Password { get; set; }
}

public class UserDataContext : DataContext
{
    public static string DBConnectionString = "Data Source=isostore:/User.sdf";

    public UserDataContext(string connectionString) : base(connectionString) { }

    public Table<User> Users;
}