C# issue with mutiple users trying to access cached object

Question

I'm having to store a Data set in cache for X min and when it expires i have it so that the next request will check if the cache exist and then goes and fetches updated data and set the Cache. The issue is that another user will come-in at this time and try to access the object and causing an exception to be thrown. How do i prevent this from occurring?

Answer 1

You can do this using the lock statement in C#.

You need an object variable that is used as a "lock" so only one thread can access it at a time.

Put the lock around code that updates or retrieves the cache object.

Here is a simple class which demonstrates this:

public class UpdateCache
{
   private static object _myLockObject;

   public static void UpdateCache()
   {
     lock(_myLockObject)
     {
        .. Update cache object
     }
   }

   public static void LoadFromCache(string key)
   {
     lock(_myLockObject)
     {
       .. retrieve data from cache
     }   

   }
}

Answer 2

A typical pattern is to encapsulate the cached object in a property. The property getter can then control all access to the cached object.

private static object _locker = new object();

public static DataSet CachedDataSet
{
    get
    {
        if (Cache["DataSetKey"] == null)
        {
            lock (_locker)
            {
                if (Cache["DataSetKey"] == null)
                {
                    Cache["DataSetKey"] = CreateAndFillDataSet();
                }
            }
        }
        return (DataSet) Cache["DataSetKey"];
    }
}