If you missed Anders Hejlsberg talk on the Future directions for C# and Visual Basic, go check it out now, I’ll wait.
I hope you'll agree that asynchronous programming just got a whole lot easier to do. The point that Anders made that really cemented in my mind how the new async and await C# keywords work in practice was this:
public async Task<XElement> GetXmlAsync(string url)
{
var client = new HttpClient();
var response = await client.GetAsync(url);
var text = response.Content.ReadAsString();
return XElement.Parse(text);
}
which is effectively translated to the following
public Task<XElement> GetXmlAsync(string url)
{
var tcs = TaskCompletionSource<XElement>();
var client = new HttpClient();
client.GetAsync(url).ContinueWith(task =>
{
var response = task.Result;
var text = response.Content.ReadAsString();
tcs.SetResult(XElement.Parse(text));
});
return tcs.Task;
}
Something about this feels particularly familiar. If you’ve written any type of web application that utilizes AJAX, writing callback functions in JavaScript (or more likely, using a JavaScript library) happens all over the place. You know that when you see this
function updatePerson() {
var returnedMessage;
$.ajax({
type: "POST",
url: "people/update/2",
data: { firstName : 'John', secondName : 'Smith' },
success: function(msg){
returnedMessage = msg;
}
});
return returnedMessage;
}
that returnedMessage will be undefined until the request made via AJAX returns and returnedMessage is set to the value of msg returned from the server. This is a common mistake that people make when first using JavaScript to make AJAX requests and may have been some of the impetus behind the introduction of $.Deferred() in jQuery 1.5. If you wanted the above to work as intended and perhaps be a little clearer, you could rewrite it as
function updatePerson() {
return $.ajax({
type: "POST",
url: "/people/update/2",
data: { firstName : 'John', secondName : 'Smith' }
}).promise();
}
updatePerson().done(function(msg) {
// do something with msg
});
This returns a promise that provides a view onto the current state of the task and allows the addition of further callbacks. Here’s a demo from jsfiddle: