/// <summary>
/// Get an existing value based on the return type, inserting it if it does not exist, and updating the cache if it does.
/// Use this method for a singleton cache instance where there will only exist a single cached item at a time of this type
/// </summary>
/// <typeparam name="TValue">The return type of the stored value</typeparam>
/// <param name="valueFactory">The zero-parameter function to retrieve the value fresh if it does not currently exist</param>
/// <param name="evictionLengthHours">The length of time (in hours) until this item will be considered stale. If null, will never expire</param>
/// <returns>The existing or newly created value that exists for this type</returns>
public static TValue AddOrUpdate <TValue>(Func <TValue> valueFactory, double?evictionLengthHours) where TValue : class
{
var policy = BuildCacheItemPolicy(evictionLengthHours);
object value = _store.AddOrUpdateSafe(valueFactory, oldValue => valueFactory(), policy);
return((TValue)value);
}
