Question

I was trying out System.Json (Beta) from NuGet. Also, trying to understand this new async/await stuff, just started Tinkering with Visual Studio 2012.

Wondering if using a ContinueWith if the await blocks until the whole thing is complete?

E.g, is this:

JsonValue json = await response.Content.ReadAsStringAsync().ContinueWith<JsonValue>(respTask => JsonValue.Parse(respTask.Result));

The same as:

        string respTask = await response.Content.ReadAsStringAsync();
        JsonValue json = await Task.Factory.StartNew<JsonValue>(() => JsonValue.Parse(respTask));

?

Answer 1

Those are similar but not identical.

ContinueWith returns a Task representing the continuation. So, to take your example:

JsonValue json = await response.Content.ReadAsStringAsync()
    .ContinueWith<JsonValue>(respTask => JsonValue.Parse(respTask.Result));

Consider just the expression:

                       response.Content.ReadAsStringAsync()
    .ContinueWith<JsonValue>(respTask => JsonValue.Parse(respTask.Result));

The result of this expression is a Task representing the continuation scheduled by ContinueWith.

So, when you await that expression:

                 await response.Content.ReadAsStringAsync()
    .ContinueWith<JsonValue>(respTask => JsonValue.Parse(respTask.Result));

You are indeed awaiting the Task returned by ContinueWith, and the assignment to the json variable will not take place until the ContinueWith continuation has completed:

JsonValue json = await response.Content.ReadAsStringAsync()
    .ContinueWith<JsonValue>(respTask => JsonValue.Parse(respTask.Result));

---

Generally speaking, I avoid ContinueWith when writing async code. There’s nothing wrong with it, but it’s a bit low-level and the syntax is more awkward.

In your case, I would do something like this:

var responseValue = await response.Content.ReadAsStringAsync();
var json = JsonValue.Parse(responseValue);

I would also use ConfigureAwait(false) if this were part of a data access layer, but since you’re accessing response.Content directly I assume that you’ll need the ASP.NET context later in this method.

Since you’re new to async / await, you may find my async / await intro helpful.