Converting a WebClient method to async / await

问题

I have some existing code which I am porting to Windows 8 WinRT. The code fetches data from URL, asynchronously invoking a passed delegate:

private void RequestData(string uri, Action<string> action)
{
  var client = new WebClient();
  client.DownloadStringCompleted += (s,e) => action(e.Result);
  client.DownloadStringAsync(new Uri(uri));
}

Converting to WinRT requires the use of HttpClient and asynchronous methods. I've read a few tutorials on async / await, but am a bit baffled. How can I change the method above, but maintain the method signature in order to avoid changing much more of my code?

回答1:

private async void RequestData(string uri, Action<string> action)
{
    var client = new WebClient();
    string data = await client.DownloadStringTaskAsync(uri);
    action(data);
}

See: http://msdn.microsoft.com/en-us/library/hh194294.aspx

回答2:

How can I change the method above, but maintain the method signature in order to avoid changing much more of my code?

The best answer is "you don't". If you use async, then use it all the way down.

private async Task<string> RequestData(string uri)
{
  using (var client = new HttpClient())
  {
    return await client.GetStringAsync(uri);
  }
}

回答3:

Following this example, you first create the async task wtih, then get its result using await:

Task<string> downloadStringTask = client.DownloadStringTaskAsync(new Uri(uri));
string result = await downloadStringTask;

回答4:

var client = new WebClient();
string page = await client.DownloadStringTaskAsync("url");

var client = new HttpClient();
string page = await client.GetStringAsync("url");

回答5:

await the result of the HttpClient.GetStringAsync Method:

var client = new HttpClient();
action(await client.GetStringAsync(uri));

回答6:

And this piece of code is for UploadValuesAsync:

public class WebClientAdvanced : WebClient
{
    public async Task<byte[]> UploadValuesAsync(string address, string method, IDictionary<string, string> data)
    {
        var nvc = new NameValueCollection();
        foreach (var x in data) nvc.Add(x.Key, x.Value.ToStr());

        var tcs = new TaskCompletionSource<byte[]>();
        UploadValuesCompleted += (s, e) =>
        {
            if (e.Cancelled) tcs.SetCanceled();
            else if (e.Error != null) tcs.SetException(e.Error);
            else tcs.SetResult(e.Result);
        };

        UploadValuesAsync(new Uri(address), method, nvc);
        var result = await tcs.Task;
        return result;
    }
}

来源：https://stackoverflow.com/questions/13240915/converting-a-webclient-method-to-async-await

标签

microsoft-metro

async-await