如何非异步下载数据？

Question

I want to download data non asynchronously in a Windows Phone application. I would make a downloader class, and have a simple method in it to download a string from a URL. On other platforms, I would use:

 public class TextDownloader
 {
      public string GetString(string url)
      {
           WebClient web = new WebClient();
           string s = web.DownloadString("http://www.google.com");
           return s;
      }
 }

It would work well: simple, minimal code. However, the WebClient.DownloadString method is not available on Windows Phone 7, nor many WebRequest options. Are there any alternative ways to download data non asynchronously in Windows Phone? I would rather not have to create multiple events for download and error, just have a simple method return a value or throw an exception.

Answer 1

Indeed, you cannot use the synchronous model for downloads with WebClient out-of-the-box. This is by design, and given the nature of Windows Phone applications, you should follow this methodology.

The solution to your problem - callbacks . You can easily refactor your function to something like this:

public void GetString(string url, Action<string> onCompletion = null)
{
    WebClient client = new WebClient();
    client.DownloadStringCompleted += (s, e) =>
        {
            if (onCompletion != null)
                onCompletion(e.Result);
        };
    client.DownloadStringAsync(new Uri(url));
}

This makes it relatively easy to use and trigger an action when it is completed. There is another way to do this as well, and that is - async/await . You will need to install the Bcl.Async package through NuGet:

Install-Package Microsoft.Bcl.Async -Pre 

It would let you do this:

public async Task<string> DownloadString(string url)
{
    WebClient client = new WebClient();
    return await client.DownloadStringTaskAsync(url);
}

Still, though, it will be bound to the asynchronous model, just wrapped in a different manner and the thread will be waiting to get the string before returning it back to the caller.