ObjectCache & MemoryCache

• The **ObjectCache** is the primary type for the in-memory object cache resides in System.Runtime.Caching.
  • You can create and manage cache entries in a key/value pair manner.
  • Specify expiration and eviction information.
  • Trigger events that are raised in response to changes in cache entries.
• All implementations of the **ObjectCache** class should disallow null as a value for cached data.
  • This helps avoid confusion between null as a data value and a cache entry that was not found.
• The MemoryCache class is an in-memory object cache implementation of the ObjectCache class.
• Files and Sql databases can be monitored for changes, invalidating the cache when the data of interest is changed.
  • The HostFileChangeMonitor is use to track the state of a file, and SqlChangeMonitor for a database.

public static string GetValue()
{
    const string cacheKey = "CacheKey";
    ObjectCache cache = MemoryCache.Default;
    var cacheValue = cache[cacheKey];
    if (cacheValue == null) // Null value means cache value doesn't exist/expired.
    {
        var value = "Value From External Resource";

        // Create a cache policy that evicts the cache based on the absolute expiration policy.
        var cachePolicy = new CacheItemPolicy
        {
            // Indicates whether a cache entry should be evicted at a specified point in time:
            AbsoluteExpiration = DateTimeOffset.Now.Add(TimeSpan.FromMinutes(60)),
            // Indicates whether a cache entry should be evicted if it has not been accessed in a given span of time:
            SlidingExpiration = TimeSpan.FromMinutes(15)
        };

        // Update the cache based on the policy.
        cache.Set(cacheKey, value, cachePolicy);
    }

    return (string)cacheValue;
}

ObjectCache Methods

• Add: add an item, does not overwrite if it exists.
• Set: add an item, overwrite if exists.
• Get: return an item from the cache
• AddOrGetExisting: add an item, if it exists return what is already there.
• Contains: check if a key has an entry in the cache.
• GetCount: total number of entries in cache.
• GetEnumerator: can be used to iterate over all entries.
• GetValues: return a Set of values (takes an **IEnumerable** of keys).
• Remove: remove a given cache entry.

HostFileChangeMonitor

public static string GetFileContents()
{
    const string cacheKey = "FileContents";
    ObjectCache cache = MemoryCache.Default;    
    if (cache[cacheKey] == null)
    {
        var filesToMonitor = new List<string>
        {
            "C:\\\\.Directory\\\\FileName1.txt",
            "C:\\\\.Directory\\\\FileName2.txt"
        };        

        // Create a cache policy to monitor a set of files.
        // The policy will evict the cache value when any file changed.
        var cachePolicy = new CacheItemPolicy();
        cachePolicy.ChangeMonitors.Add(new HostFileChangeMonitor(filesToMonitor));

        // Update the cache based on the policy.
				var fileContents = File.ReadAllText(filesToMonitor.First());
        cache.Set(cacheKey, fileContents, cachePolicy);
    }
    return cache.Get(key);
}

SqlDependency

• The SqlDependency represents a query notification dependency between an application and an instance of SQL Server.