Service for determining if a user's trial has expired.

TrialService

class AzureTrialService : ITrialService
{
    private readonly IMobileServiceClient _service;

    public AzureTrialService(IMobileServiceClient service)
    {
        _service = service;
        TrialPeriod = TimeSpan.FromDays(7);
    }

    public TimeSpan TrialPeriod { get; set; }

    public async Task<bool> IsExpiredAsync<TUser>(string userId) where TUser : TrialUser, new()
    {
        bool isExpired = false;

        object dateVal;
        if (ApplicationData.Current.LocalSettings.Values.TryGetValue("trialExpiration", out dateVal) == false)
        {
            // get user from server
            IMobileServiceTable<TUser> table = _service.GetTable<TUser>();

            var users = await table.Where(userInfo => userInfo.Id == userId).ToListAsync();
            var user = users.FirstOrDefault();

            if (user == null)
            {
                // new user, add it
                var trialExpirationDate = DateTimeOffset.Now + TrialPeriod;
                dateVal = trialExpirationDate;
                user = new TUser { Id = userId, TrialExpirationDate = trialExpirationDate.ToUniversalTime() };
                await table.InsertAsync(user);
            }
            else
            {
                // mobile services will deserialize as local DateTime
                dateVal = user.TrialExpirationDate;
            }
            ApplicationData.Current.LocalSettings.Values["trialExpiration"] = dateVal;
        }
        var expirationDate = (DateTimeOffset)dateVal;
        isExpired = expirationDate < DateTimeOffset.Now;

        return isExpired;
    }
}

/// <summary>
/// Represents information for a trial user.
/// </summary>
abstract class TrialUser
{
    /// <summary>
    /// Gets or sets the id for the user.
    /// </summary>
    public abstract string Id { get; set; }

    /// <summary>
    /// Gets or sets the date the trial will expire for the user.
    /// </summary>
    public abstract DateTimeOffset TrialExpirationDate { get; set; }
}

interface ITrialService
{
    /// <summary>
    /// Get or sets the time amount of time the user is allowed to use the trial.
    /// </summary>
    TimeSpan TrialPeriod { get; set; }

    /// <summary>
    /// Gets a value indicating if the trial has expired for the given userId.
    /// </summary>
    /// <param name="userId"></param>
    /// <returns>A task that will complete when the data is retrieved.</returns>
    Task<bool> IsExpiredAsync<TUser>(string userId) where TUser : TrialUser, new();
}