After two days of intensive study , Read a lot of webpages , Click on a lot of key points blogs, Finally, I have a basic understanding of async/await How to use it together , To sum up

1. async/await It should be just a grammatical dessert , Let programmers who want to run programs asynchronously focus on code logic , And don't let the original Begin... End... Type or IAsync ...Async The old asynchronous belt is getting farther and farther away . Think of just trying to write Tcp Listener When , It's a nightmare . The asynchronous response must be placed in another callback method or event , The asynchronous reception from the asynchronous response must be put into another callback method or event again , It's even scarier , How to cycle them .

2. async/await Method of identification ,“ It doesn't create extra threads on its own initiative ”—— This sentence is easy to misunderstand .《Async and Await The principle of asynchronous programming 》 This article is quite detailed , asynchronous , Must be multithreaded , Not to mention from UI Thread started . It's just that multithreading is Framework The transparency is off , Users don't have to think about how to create threads themselves , How to recycle , How to catch anomalies , You have to make the code look better .

3. await I fell into a misunderstanding at the beginning , Think of using var result = await FunRun(...) This kind of writing ,FunRun() Methods will run asynchronously without blocking UI Threads , This is wrong , I've overlooked a very serious problem . Let's take a look at the original error code :

 private async Task<TimeSpan> Download(Uri address, string fileName)
{
this.uri = address;
this.fileName = fileName;
DateTime start = DateTime.Now;
//
HttpWebRequest request;
HttpWebResponse response = null;
try
{
request = (HttpWebRequest)WebRequest.Create(address);
response = (HttpWebResponse)request.GetResponse();
using (Stream httpStream = response.GetResponseStream())
{
using (FileStream writer = new FileStream(fileName, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.Read))
{
byte[] buffer = new byte[];
int readLength = httpStream.Read(buffer, , buffer.Length);
//
while (readLength > )
{
writer.Write(buffer, , readLength);
readLength = httpStream.Read(buffer, , buffer.Length);
}
}
}
}
catch(Exception)
{
if (response != null) { response.Close(); }
} return DateTime.Now - start;
}

The code completes the function of downloading a resource to a specified location , There is no mistake in logic itself , It seems to be a standard with async Labelling and Task<T> Method of return type , So I used it slowly .

 private async void button1_Click(object sender, EventArgs e)
{
// Something that skips part of the logical judgment
...... TestApp.HttpDownloader.HttpDownloaderEngine hdEngine = new HttpDownloaderEngine();
var time = await hdEngine.Download(address, saveFileTextBox.Text);
MessageBox.Show(string.Format(" Download complete , Total time consuming {0} second ", time.Seconds.ToString()));
}

Call the wrong way to write

Button1 Of Click There is no problem with the incident itself , I also carefully added async Mark ( Of course , I'm not allowed to go there ), Everything's fine. I'll run it right away , It turns out to be confusing , It did download successfully Mp3 To my computer , No matter how long it takes , my UI It's still synchronized , Cannot respond until execution is complete . Where is the problem ?

People need to rest , Study -> rest -> Digestion , After combing and reviewing the data again , I found out the second 3 What I said at the beginning ,Bug lie in HttpDownloadEngine Of Download The method I marked async,button_click mistook Download Method can do asynchronous work ,button_click That's right , Wrong Download The method is not , There is no “ Code executed asynchronously ”! in other words ,Download There is no one in the method await , This makes it possible to mark async Of Download Methods are still synchronized ,button_click A synchronization method is called and executed synchronously .

How to modify , stay Download The key point in the method is execution await Task<T> ....Async , Such as  readLength = await responseStream.ReadAsync(buffer, 0, buffer.Length);, The key point is where asynchronous execution is really needed .

4. I want to thank you Microsoft, I'm not a God , I'm a fan , It was Microsoft that drew on and created .Net And unparalleled VS, Let me and people like me appreciate more of the charm of the computer , The charm , From creating and sharing to others .

Post a picture of the result of the download

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