ConcurrentDictionary GetOrAdd async

Question

I want to use something like GetOrAdd with a ConcurrentDictionary as a cache to a webservice. Is there an async version of this dictionary? GetOrAdd will be making a web request using HttpClient, so it would be nice if there was a version of this dictionary where GetOrAdd was async.

To clear up some confusion, the contents of the dictionary will be the response from a call to a webservice.

ConcurrentDictionary<string, Response> _cache
    = new ConcurrentDictionary<string, Response>();

var response = _cache.GetOrAdd("id",
    (x) => { _httpClient.GetAsync(x).GetAwaiter().GetResponse(); });

Answer 1

GetOrAdd won't become an asynchronous operation because accessing the value of a dictionary isn't a long running operation.

What you can do however is simply store tasks in the dictionary, rather than the materialized result. Anyone needing the results can then await that task.

However, you also need to ensure that the operation is only ever started once, and not multiple times. To ensure that some operation runs only once, and not multiple times, you also need to add in Lazy:

ConcurrentDictionary<string, Lazy<Task<Response>>> _cache = new ConcurrentDictionary<string, Lazy<Task<Response>>>();

var response = await _cache.GetOrAdd("id", url => new Lazy<Task<Response>>(_httpClient.GetAsync(url))).Value;