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Async Lazy<t>立即获得结果

lazy-initialization .net asp.net c#

-1

ShaneKm · 技术社区 · 6 年前

我有以下几点:

public class User 
{
        private readonly Lazy<Task<List<ReminderDb>>> _reminders;

        public SmsUserDb()
        {
            // Get _reminderReader from IoC

            _reminders = new Lazy<Task<List<ReminderDb>>>(async () => (List<ReminderDb>)await _reminderReader.Get(UserId));
        }

        public string UserId { get; set; }
        public Task<List<ReminderDb>> Reminders => _reminders.Value;
}

当我这样实例化一个对象时:

    var n = new SmsUserDb {UserId = "123456"};
    var rems = await n.Reminders;

此代码有效,我看到n.reminders=“waiting for activation”直到我点击waiting n.reminders行。

但是,当我这样从缓存查询这个用户对象时:

        var n1 = await _userReader.GetUserFromCache("+17084556675"); // return SmsUserDb
        var n2 = await n1.Reminders;

当它点击getuserfromcache()时,它立即调用_reminderreader.get(userid),后者再次调用cache以获取提醒。然后就超时了。所以懒惰不起作用,很可能导致死锁。

public async Task<SmsUserDb> GetUserFromCache(string phoneNumber)
{
    var hash = CachedObjectType.smsuser.ToString();
    var fieldKey = string.Format($"{CachedObjectType.smsuser.ToString()}:user-{phoneNumber}");
    var result = await _cacheUserService.GetHashedAsync(hash, fieldKey);
    return result;
}

    private async Task<List<ReminderDb>> GetRemindersFromCache(string userId)
    {
        var hash = CachedObjectType.smsreminder.ToString();
        var fieldKey = string.Format($"{CachedObjectType.smsreminder.ToString()}:user-{userId}");
        var result = await _cacheService.GetHashedAsync(hash, fieldKey);
        return result ?? new List<ReminderDb>();
    }

有什么问题?

2 回复 | 直到 6 年前

Matthew Watson 6 年前

Lazy<T>

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace Demo
{
    public class ReminderDb
    {
        public string Value;
    }

    public class ReminderReader
    {
        public async Task<List<ReminderDb>> Get(string userId)
        {
            Console.WriteLine("In Get() for " + userId);
            await Task.Delay(1000);
            Console.WriteLine("Returning from Get()");
            return new List<ReminderDb>{new ReminderDb{Value = userId}};
        }
    }

    public class Cache
    {
        public void Add(string key, SmsUserDb value)
        {
            _cache.Add(key, value);
        }

        public async Task<SmsUserDb> GetHashedAsync(string key)
        {
            await Task.Delay(1000);
            return _cache[key];
        }

        readonly Dictionary<string, SmsUserDb> _cache = new Dictionary<string, SmsUserDb>();
    }

    public class SmsUserDb
    {
        readonly Lazy<Task<List<ReminderDb>>> _reminders;
        readonly ReminderReader _reminderReader = new ReminderReader();

        public SmsUserDb()
        {
            _reminders = new Lazy<Task<List<ReminderDb>>>(async () => (List<ReminderDb>) await _reminderReader.Get(UserId));
        }

        public string                 UserId    { get; set; }
        public Task<List<ReminderDb>> Reminders => _reminders.Value;
    }

    static class Program
    {
        static async Task Main()
        {
            var db = new SmsUserDb(){UserId = "user ID"};
            var cache = new Cache();
            cache.Add("key", db);

            Console.WriteLine("Press RETURN to await cache.GetHashedAsync()");
            Console.ReadLine();
            var result = await cache.GetHashedAsync("key");

            Console.WriteLine("Press RETURN to await Reminders[0].Value");
            Console.ReadLine();
            Console.WriteLine((await result.Reminders)[0].Value);
        }
    }
}

ShaneKm 6 年前

public class User 
{
        private readonly Lazy<Task<List<ReminderDb>>> _reminders;

        public SmsUserDb()
        {
            // Get _reminderReader from IoC

            _reminders = new Lazy<Task<List<ReminderDb>>>(async () => (List<ReminderDb>)await _reminderReader.Get(UserId));
        }

        public string UserId { get; set; }
        public Task<List<ReminderDb>> Reminders => _reminders.Value;
}

public class CacheUser {
    public string UserId { get; set; }
    public List<ReminderDb> Reminders {get; set; }
}