WebClient - wait until file has downloaded

Question

I\'m developing a function to return a collection, generated from an xml file.

Initially, I was using a local xml file for testing, but now I\'m ready to have the a

Answer 1

You move the foreach() loop to the completed event.

And that indeed means you cannot return anything from the original method. Make it a void.

This is how async I/O works, better get used to it. You will need to rethink your design.

Answer 2

You should start to take a look at async programming. One (old school) way would be to implement a public event and subscribe to that event in the calling class.

However, using callbacks is more elegant. I whipped up a simple (useless, but still conceptually valid) example that you can build upon:

public static void Main(string[] args)
{
  List<string> list = new List<string>();

  GetData(data =>
  {
    foreach (var item in data)
    {
      list.Add(item);
      Console.WriteLine(item);
    }
    Console.WriteLine("Done");
  });
  Console.ReadLine();
}

public static void GetData(Action<IEnumerable<string>> callback)
{
  WebClient webClient = new WebClient();
  webClient.DownloadStringCompleted += (s, e) =>
    {
      List<string> data = new List<string>();
      for (int i = 0; i < 5; i++)
      {
        data.Add(e.Result);
      }
      callback(e.Error == null ? data : Enumerable.Empty<string>());
    };

  webClient.DownloadStringAsync(new Uri("http://www.google.com"));
}

If you want to jump onto the C# async bandwagon (link for WP7 implementation), you can implement it using the new async and await keywords:

public static async void DoSomeThing()
{
  List<string> list = new List<string>();
  list = await GetDataAsync();

  foreach (var item in list)
  {
    Console.WriteLine(item);
  }
}

public static async Task<List<string>> GetDataAsync()
{
  WebClient webClient = new WebClient();
  string result = await webClient.DownloadStringTaskAsync(new Uri("http://www.google.com"));

  List<string> data = new List<string>();
  for (int i = 0; i < 5; i++)
  {
    data.Add(result);
  }
  return data;
}