Example of cache decorator

CacheDecorator.cs

  // Used as an interceptor to provide cache mechanism.
  // ISomeService can be any other service and this is just an example for you own class cache.
  // Should be used as singleton to get benefit from it.

  public class CacheDecorator : ISomeService
  {
      private static ConcurrentDictionary<string, Task<string>> Cache { get; } = new ConcurrentDictionary<string, Task<string>>();
      private ISomeService Implementation { get; }

      public CacheDecorator(ISomeService implementation)
      {
          Implementation = implementation;
      }

      public Task<string> GetAsync(string id)
      {
          string key = id;

          if (Cache.TryGetValue(key, out var item))
          {
              return item;
          }
          else
          {
              return Implementation.GetAsync(key).ContinueWith(task =>
              {
                  Cache.TryAdd(key, task);
                  return task.Result; // we don't block with task.Result because task is already finished
              });
          }
      }
  }

  public interface ISomeService
  {
      Task<string> GetAsync(string id);
  }