Updated ASP.NET caching sample with thread-safety.

ICacheService.cs

using System;

namespace AppNamespace.Caching
{
	interface ICacheService
	{
		T Get<T>(string cacheKey, Func<T> getItemCallback) where T : class;
		T Get<T>(string cacheKey, DateTime absoluteExpiration, TimeSpan slidingExpiration, Func<T> getItemCallback) where T : class;
	}
}

InMemoryCache.cs

using System;
using System.Web;
using System.Web.Caching;

namespace AppNamespace.Caching
{
	public class WebMemoryCache : ICacheService
	{
		private static readonly object WebCacheLock = new object();
		
		public T Get<T>(string cacheKey, Func<T> getItemCallback) where T : class
		{
			return Get(cacheKey, System.Web.Caching.Cache.NoAbsoluteExpiration, TimeSpan.FromHours(1), getItemCallback);
		}

		public T Get<T>(string cacheKey, DateTime absoluteExpiration, TimeSpan slidingExpiration, Func<T> getItemCallback) where T : class
		{
			// Return cached value if exists
			var item = HttpRuntime.Cache.Get(cacheKey) as T;
			if (item != null)
				return item;
			
			// Double-checked locking
			// See: http://stackoverflow.com/questions/39112/what-is-the-best-way-to-lock-cache-in-asp-net
			lock (WebCacheLock)
			{
				// Ensure cache wasn't updated while waiting for lock
				// Return cached value if exists
				item = HttpRuntime.Cache.Get(cacheKey) as T;
				if (item != null)
					return item;
				
				// Generate item
				item = getItemCallback();
				
				// Ensure item is not null
				// Could potentially just return null here, but callers may expect cache to prevent
				// multiple getItemCallback invocations.
				if (item == null)
                        		throw new ArgumentException("Item creation function yielded null, which is not cacheable.", "getItemCallback");
	                    	
                    		// Add item to cache
				HttpContext.Current.Cache.Insert(cacheKey, item, null, absoluteExpiration, slidingExpiration);
			}
			
			return item;
		}
	}
}

Sample.cs

var cachedTime = new WebMemoryCache().Get("CachedDateTimeKey", Cache.NoAbsoluteExpiration, TimeSpan.FromMinutes(5), () =>
	{
		return DateTime.UtcNow;
	});