gistfile1.cs

public class PageTemplateManager : IPageTemplateManager
{
	readonly IReadModelRepository readModelRepository;
	readonly object mutex;
	readonly IDictionary<Guid, IList<PageTemplate>> templates;

	public PageTemplateManager(IReadModelRepository readModelRepository)
	{
		this.readModelRepository = readModelRepository;
		this.templates = new Dictionary<Guid, IList<PageTemplate>>();
		this.mutex = new object();
	}

	public IEnumerable<PageTemplate> All(Guid siteId)
	{
                if(!this.templates.ContainsKey(siteId))
		        this.EnsureTemplatesAreLoaded(siteId);

		return this.templates[siteId];
	}

	void EnsureTemplatesAreLoaded(Guid siteId)
	{
		lock(this.mutex)
		{
			if(!this.templates.ContainsKey(siteId))
			        this.templates.Add(siteId, readModelRepository.Query<PageTemplate>(new { SiteId = siteId}).ToList());
		}
	}
}