How to use async/await to achieve asynchronous pag

Question

We can now use the async/await key words in ASP.NET MVC 4.

public async Task<ActionResult> TestAsync()
{
    WebClient client = new WebClient();
    return Content(await client.DownloadStringTaskAsync("http://www.google.com"));
}

But how to use it in ASP.NET WebForms?

Answer 1

According to http://www.hanselman.com/blog/TheMagicOfUsingAsynchronousMethodsInASPNET45PlusAnImportantGotcha.aspx the only reliable way to use async in web forms is to call Page.RegisterAsyncTask.

The code to support simple things like async Page_Load is extremely complicated and not well-tested for anything beyond basic scenarios.

Using async with voids is not stable or reliable. However, all you have to do is call Page.RegisterAyncTask - it's not any trouble and you'll be in a better more flexible place

.

public void Page_Load(object sender, EventArgs e)
{
    RegisterAsyncTask(new PageAsyncTask(LoadSomeData));
}

public async Task LoadSomeData()
{
var clientcontacts = Client.DownloadStringTaskAsync("api/contacts");
var clienttemperature = Client.DownloadStringTaskAsync("api/temperature");

var contacts = Newtonsoft.Json.JsonConvert.DeserializeObject>(await clientcontacts);
var temperature = Newtonsoft.Json.JsonConvert.DeserializeObject(await clienttemperature);

listcontacts.DataSource = contacts;
listcontacts.DataBind();
Temparature.Text = temperature; 
}

Answer 2

One easy way is to just make your event handlers async. First, add the Async="true" parameter to the @Page directive, and then you should be able to write async event handlers as such:

protected async void Page_Load(object sender, EventArgs e)
{
  var client = new WebClient();
  var content = await client.DownloadStringTaskAsync("http://www.google.com");
  Response.Write(content);
}

I say "should be able to" because I haven't actually tried this out myself. But it should work.

Update: This does not work for Page_Load (see this MSDN forum thread), but should work for other events such as button clicks.

Update: This does work for Page_Load in ASP.NET 4.5. Also, they added checks if you improperly use an async event handler. See this video for details.