MemoryCache获取/设置并分配给变量

Question

I have been testing some code locally and read through quite a bit of SO posts but I am a bit confused still how MemoryCache works in certain scenarios. So the question relates to the following code:

 class Program
{

    static void Main(string[] args)
    {
        var test = new Test();
        test.TestingMethod();
    }

}

public class Test
{
    private readonly MemoryCache MemoryCache = MemoryCache.Default;
    private CacheItemPolicy policy = null;
    private CacheEntryRemovedCallback callback = null;

    public void TestingMethod()
    {
        var ints = new List<int>();
        for (var i = 0; i <= 1000; i++)
        {
            ints.Add(i);
        }

        callback = this.MyCachedItemRemovedCallback;
        policy = new CacheItemPolicy
        {
            Priority = CacheItemPriority.Default,
            AbsoluteExpiration = DateTimeOffset.Now.AddSeconds(2),
            RemovedCallback = callback
        };

        MemoryCache.Set("ints", ints, policy);
        var intsCached = (List<int>) MemoryCache.Get("ints");
        Task.Delay(TimeSpan.FromSeconds(15)).Wait();
        MemoryCache.Set("ints", new List<int>() {1}, policy);

        foreach (var intCached in intsCached)
        {
            Console.WriteLine(intCached);
        }
        Console.ReadLine();
    }

    void MyCachedItemRemovedCallback(CacheEntryRemovedArguments arguments)
    {
        Console.WriteLine("Expired");
    }
}

In the console output of this TestMethod I am getting Expired 1 - 1000 on separate lines

Why would I get the 1-1000 instead of only the 1 that we set after expiration? Doesn't this list refer to the same ref?

Answer 1

MemoryCache (or any other entity that stores values) will not magically update content of the List when you set entry with the same name to different value. Ie what if you do MemoryCache.Set("ints", 42, policy); instead?

Looks like you expect value ( intsCached ) to be computed long after it is assigned to:

  var intsCached = (List<int>) MemoryCache.Get("ints");

intsCached is computed at that line and no longer know where it comes from.

If you want to have "variable" that remembers where it comes from and always get latest value from that location - use Func to compute it every time:

  Func<List<int>> intsCached = () => (List<int>) MemoryCache.Get("ints");
  ...
  foreach (var intCached in intsCached()) 
  ...