async/await "List<> does not contain a definition for 'GetAwaiter'"

Question

I apologize for sounding dense, but I swear I have read up on async/await and played with examples, but I am still stumped at simple things like why the following code is giving me error:

        [HttpGet("{includeInactive}")]
        public async Task<List<Torrent>> GetTorrentsAll(bool includeInactive)
        {
            List<Torrent> torrentsAll = await _cacheStore.GetCachedTorrentsAll();

            //...potentially filter torrentsAll here

            return torrentsAll;
        }

So _cacheStore.GetCachedTorrentsAll() is not an async function. It's just a function that potentially might be taking a while in the database, so I was trying to ensure that it's not blocking. I thought that awaiting it within the async web API was the way to go about that. But I get confusing error under the await _cacheStore.GetCachedTorrentsAll() line that says 'List does not contain a definition for 'GetAwaiter' and no accessible extension method 'GetAwaiter' accepting a first argument of type 'List' could be found (are you missing a using directive or an assembly reference?).

I am very confused by this.

Answer 1

So _cacheStore.GetCachedTorrentsAll() is not an async function. It's just a function that potentially might be taking a while in the database, so I was trying to ensure that it's not blocking.

It is reasonable to make it asynchronous, then.

I guess maybe since _cacheStore.GetCachedTorrentsAll() is not async, it will still block the thread regardless (?)

This is correct. Synchronous methods - by definition - block the calling thread until they complete.

In which case async/await seems like a rabbit hole to me,

It can be. In this case, you have correctly identified an operation that should be asynchronous (i.e., I/O-bound). But just sticking an async on the controller method isn't going to help; that's actually the last step in making this asynchronous.

The first step is to identify the lowest-level API that should be asynchronous. E.g., the database query. Make that call use the asynchronous database API first, use await, and make that method async. Then let async/await grow from that method up through your app. It will eventually end up at the controller method, and then you can make that async.