Is using async/await better then using task.Start() and why?

Question

Compare the following two methods:

static async Task<int> DownloadAsync(string url)
{
    var client = new WebClient();
    var awaitable = client.DownloadDataTaskAsync(url);
    byte[] data = await awaitable;
    return data.Length;
}

usage: Task<int> task = DownloadAsync("http://stackoverflow.com");

static Task<int> Download(string url)
{
    var client = new WebClient();
    var task = client.DownloadDataTaskAsync(url);
    byte[] data = task.Result;
    return Task.FromResult(data.Length);
}

usage:

Task task = new Task(() => Download("http://stackoverflow.com"));
task.Start();

As far as I can see both methods run asynchronously. My questions are:
Is there any difference in behavior between the two methods?
Why do we prefer async-await other then it being a nice pattern?

Answer 1

The two methods you post are completely different.

DownloadAsync is a truly asynchronous method. This means that while the data is downloading, there are no threads blocked on that asynchronous operation.

Download synchronously blocks the calling thread by calling Task.Result. I explain on my blog why Result should not be used with asynchronous Tasks: in the general case, it can cause deadlocks. But let's assume there's no deadlock. You then call it from a TPL task, so it blocks the task thread (most likely a thread pool thread). While the data is downloading, that task thread is blocked on that asynchronous operation.

So, DownloadAsync is more efficient.