repeat

stay .Net5 Under the platform , Create a console program , Pay attention to the console program Main() The method is as follows ：

static async Task Main(string[] args)

The body of the method is very simple , Use Task.Run Create an immediate Task, Keep outputting threads inside id, Until the program is manually closed , The code is as follows ：

code snippet 1

Click to view the code 
static async Task Main(string[] args)

{

    Console.WriteLine(" Main thread thread id：" + Thread.CurrentThread.ManagedThreadId);

    await Task.Run(async () =>

    {

        while (true)

        {

            Console.WriteLine("Fuck World!  Threads id：" + Thread.CurrentThread.ManagedThreadId);

            await Task.Delay(2000);

            Console.WriteLine(" Threads id：" + Thread.CurrentThread.ManagedThreadId);

        }

    });

}

This code runs as scheduled , And there is no need to use at the end of the program Console.ReadLine() The control program does not stop .

But if we use Task.Factory.StartNew() Replace Task.Run() Words , The program will flash by , Quit immediately .

If you use New Task() Create words , As shown in the following code ：

code snippet 2

Click to view the code 
var t = new Task(async () =>

 {

     while (true)

     {

         Console.WriteLine("Fuck World! Threads ：" + Thread.CurrentThread.ManagedThreadId);

         await Task.Delay(2000);

         Console.WriteLine(" Threads id：" + Thread.CurrentThread.ManagedThreadId);

     }

 });

t.Start();

await t;

The procedure remains unchanged In a flash , Quit immediately .

analysis

First of all, analyze Task.Run() and Task.Factory.StartNew().

We will async Of the tag λ After the expression is passed in as a parameter , The compiler will λ The expression is mapped to Func<Task> perhaps Func<Task<TResult>> entrust , In this example, because there is no return value , So the mapping is Func<Task>.

If we use F12 Investigate Task.Run() and Task.Factory.StartNew() Input parameter is Func<TResult> In the case of the return value type , You will find that the return type of both of them is Task<TResult>. But in the example , You'll find that , The return value is different .Task.Run(async () ...) The return type of is Task, and Task.Factory.StartNew(async () ...) The return type of is Task<Task>.

therefore , We are await Task.Factory.StartNew(async () ...) When , In fact await Task<Task>, As a result, , It's still a Task. In that case , Want to reach and await Task.Run(async () ...) The effect is very simple , Just add one more await, namely await await Task.Factory.StartNew(async () ...). Readers can try it on their own .

The difference between the behavior of these two methods , The reason can be found in the source code ：

Task.Run The interior of Unwrap, hold Task<Task> The outer Task Tear it down .UnWrap() There is a way , Can be called directly , namely Task.Factory.StartNew(async () ...).Unwrap(), The result of the call is Task. therefore await await Task.Factory.StartNew(async () ...) And await Task.Factory.StartNew(async () ...).Unwrap() The results are consistent . At this point ,Unwrap() The role of await It does the same thing .

That is to say ：await Task.Run(async () ...) == await await Task.Factory.StartNew(async () ...) == await Task.Factory.StartNew(async () ...).Unwrap().

Next, take a look New Task() In the form of . stay code snippet 2 in , Although the call await t, But the code didn't run as expected , But in a flash , Program exit . Actually , Although the parameters passed in are consistent with the previous , however The compiler does not map parameters to Func<Task>, Instead, it maps to Action(), That is, there is no return value .t.Start() Result , Just let that Action() Start execution , And then ,Task completion of enforcement ,await t It will be completed in an instant , There is no result —— because Action() There is no return value . In this code ,Action() Actually, it runs in a background thread , If used on the main thread Thread.Sleep(10000) after , You will find that the console has been outputting content .

If you want to New The way to create Task The example of achieves the same output effect , Just make a small change , As shown below ：

code snippet 3

Click to view the code 
 var t = new Task<Task>(async () =>

 {

     while (true)

     {

         Console.WriteLine("Fuck World! Threads ：" + Thread.CurrentThread.ManagedThreadId);

         await Task.Delay(2000);

         Console.WriteLine(" Threads id：" + Thread.CurrentThread.ManagedThreadId);

     }

 });

 t.Start();

 await await t;

take New Task(async ()...) Change it to Nwe Task<Task>(async ()...) That's all right. , such λ expression async ()... It will be mapped to Func<Task>, It meets our asynchronous needs .

Reference resources

Task Source code ：https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs

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