C#TasksAsyncAwait
最近一段时间在Youtube上面看了不少关于计算机方面的编程视频，其中UP主AngelSix的C#视频个人感觉讲得还可以，C# Tasks Async Await这个视频讲解了C#中的任务以及结合Async/Await的用法。Github仓库对应的代码地址为：TasksInConsole
不过个人感觉微软的官方文档讲解得更加详细，官方网址为：使用Async和Await的任务异步编程(TAP)模型 C#
这节视频的相关代码如下：
using System;using System.Net;
using System.Threading;
using System.Threading.Tasks;
namespace TasksInConsole
{
class Program
{
#region Private Members
/// <summary>
/// The event finished callback for the Thread event example
/// </summary>
private static event Action EventFinished = () => { };
/// <summary>
/// Whether to run the thread examples
/// </summary>
private static bool RunThreadExamples = false;
#endregion
static void Main(string[] args)
{
// Log it
Log("Hello World!");
//
// Author: Luke Malpass
// License: MIT
// Support Me: https://www.patreon.com/angelsix
// Source Code: http://www.github.com/angelsix/youtube/Tasks
// Website: http://www.angelsix.com
// Contact: contact@angelsix.com
//
//
// What is Asynchronous
// ======================
//
// Asynchronous is if you start something, and don't wait while its happening.
// It literally means to not occur at the same time.
//
// This means not that our code returns early, but rather it doesn't sit there
// blocking the code while it waits (doesn't block the thread)
//
//
// Issues with Threads
// =====================
//
// Threads are asynchronous, as they naturally do something while the calling thread
// that made it doesn't wait for it.
//
#region Threads are asynchronous
if (RunThreadExamples)
{
// Log it
Log("Before first thread");
// Start new thread
new Thread(() =>
{
// Sleep a little
Thread.Sleep(500);
// Log it
Log("Inside first thread");
}).Start();
// Log it
Log("After first thread");
// Wait for work to finish
Thread.Sleep(1000);
Console.WriteLine("---------------------------------------------");
}
#endregion
// What's the issue with Threads?
//
// 1. Expensive to make
// 2. Not natural to be able to resume after a thread has finished to do something
// related to the thread that created it
//
// Issue 1 was solved with a ThreadPool. However issue 2 is still an issue for threads,
// and is one reason why Tasks were made.In order to resume work after some
// asynchronous operation has occurred we could with a Thread:
//
// 1. Block your code waiting for it (no better than just doing it on same thread)
//
#region Blocking Wait
if (RunThreadExamples)
{
// Log it
Log("Before blocking thread");
// Create new thread
var blockingThread = new Thread(() =>
{
// Sleep a little
Thread.Sleep(500);
// Log it
Log("Inside blocking thread");
});
// Start thread
blockingThread.Start();
// Block and wait
blockingThread.Join();
// Log
Log("After blocking thread");
Console.WriteLine("---------------------------------------------");
}
#endregion
// 2. Constantly poll for completion, waiting for a bool flag to say done (inefficient, slow)
#region Polling Wait
if (RunThreadExamples)
{
// Log it
Log("Before polling thread");
// Create poll flag
var pollComplete = false;
// Create thread
var pollingThread = new Thread(() =>
{
// Log it
Log("Inside polling thread");
// Sleep a little
Thread.Sleep(500);
// Set flag complete
pollComplete = true;
});
// Start thread
pollingThread.Start();
// Poll for completion
while (!pollComplete)
{
// Log it
Log("Polling....");
// Sleep a little
Thread.Sleep(100);
}
// Log it
Log("After polling thread");
Console.WriteLine("---------------------------------------------");
}
#endregion
// 3. Event-based callbacks (lose the calling thread on callback, and causes nesting)
#region Event-based Wait
if (RunThreadExamples)
{
// Log it
Log("Before event thread");
// Create thread
var eventThread = new Thread(() =>
{
// Log it
Log("Inside event thread");
// Sleep a little
Thread.Sleep(500);
// Fire completed event
EventFinished();
});
// Hook into callback event
EventFinished += () =>
{
// Log it
Log("Event thread callback on complete");
};
// Start thread
eventThread.Start();
// Log it
Log("After event thread");
// Wait for work to finish
Thread.Sleep(1000);
Console.WriteLine("---------------------------------------------");
}
#endregion
#region Event-based Wait Method
if (RunThreadExamples)
{
// Log it
Log("Before event method thread");
// Call event callback style method
EventThreadCallbackMethod(() =>
{
// Log it
Log("Event thread callback on complete");
});
// Log it
Log("After event method thread");
// Wait for work to finish
Thread.Sleep(1000);
Console.WriteLine("---------------------------------------------");
}
#endregion
//
// However that makes every time we want to do something asynchronous a lot of code
// and not easy to follow.
//
//
// What is a Task
// ================
//
// A Task encapsulates the promise of an operation completing in the future
//
//
// Tasks, Async and Await
// ========================
//
// Async in C# is mainly 2 words. async and await
//
// The point is to allow easy and clean asynchronous code to be written without complex or messy code.
//
#region Sync vs Async Method
// Log it
Log("Before sync thread");
// Website to fetch
var website = "http://www.google.co.uk";
// Download the string
WebDownloadString(website);
// Log it
Log("After sync thread");
Console.WriteLine("---------------------------------------------");
// Log it
Log("Before async thread");
// Download the string asynchronously
var downloadTask = WebDownloadStringAsync(website);
// Log it
Log("After async thread");
// Wait for task to complete
downloadTask.Wait();
Console.WriteLine("---------------------------------------------");
var task = Task.Run(async () =>
{
// Log it
Log("Before async await thread");
// Download the string asynchronously
await WebDownloadStringAsync(website);
// Log it
Log("After async await thread");
Console.WriteLine("---------------------------------------------");
});
// Wait the main task
task.Wait();
#endregion
// Async and await are always used together. A method or lambda tagged with
// async can then await any Task
//
// When you await something, the thread which called the await is free to then return to
// what it was doing, while in parallel the task inside the await is now run on another thread.
//
// Once the task is done, it returns either to the original calling thread, or carries on,
// on another thread to do the work that codes after the await.
//
//
//
// Async Analogy
// ===============
//
// Imagine you go to Starbucks and the entire shop is run by one person.
// His name is Mr UI Thread. You walk in and ask Mr Thread for a Vanilla Latte.
// He obliges and starts to make your coffee.
//
// He puts the milk into the container and turns on the hot steam, and proceeds
// to stand there and wait for the milk to reach 70 degrees.
//
// During this time you remember you wanted a muffin as well, so you shout over
// to Mr Thread and ask for a muffin... but he ignores you. He is blocked
// waiting for the milk to boil.
//
// Several minutes goes by and 3 more customers have come in and are waiting
// to be served. Finally the milk is finished and he completes the Latte.
// Returning to you. You are a little annoyed at being ignored for minutes
// and decide to leave your muffin.
//
// Then he continues to serve one customer at a time, doing one job at a time.
// Not a good situation.
//
// This is what happens with a single threaded application.
//
// Now in order to improve business Mr Thread employs 2 new members of staff
// called Mrs and Mrs Worker Thread. The pair work well independently and
// as Mr Thread takes orders from the customers, he asks Mrs Worker Thread
// to complete the order, and then without waiting for Mrs Worker Thread to
// finish the drink, proceeds to serve the next customer.
//
// Once Mrs Worker Thread has finished a drink, instead of having to take
// the drinks to the customers she asks Mr Worker Thread to serve the drinks
// and then without waiting she proceeds to start the next order.
//
// The business is now a well-oiled, multi-threaded business.
//
//
// The Synchronous part in Tasks
// ===============================
//
//
#region The Synchronous Part of Tasks
// Run some work to show the synchronous parts of the call
Task.Run(async () =>
{
// Log it
Log($"Before DoWork thread");
// Do work
// This will return a Task and run the lines of code inside the method
// up until the point at which the first await is hit
var doWorkTask = DoWorkAsync("me");
// Await the task
// This will then spin off to a new thread and come with the result
await doWorkTask;
// Log it
Log($"After DoWork thread");
}).Wait();
Console.WriteLine("---------------------------------------------");
#endregion
//
// Async Return Types
// ====================
//
// You can only return void, Task or Task<T> for a method marked as async, as the method is
// not complete when it returns, so no other result is valid.
//
#region Method 1 Getting Result of Async From Sync
// Get the task
var doWorkResultTask = DoWorkAndGetResultAsync("Return this");
// Wait for it
doWorkResultTask.Wait();
// Get the result
var doWorkResult = doWorkResultTask.Result;
Console.WriteLine("---------------------------------------------");
#endregion
#region Method 2 Getting Result of Async From Sync
Task.Run(async () =>
{
var doWorkResult2 = await DoWorkAndGetResultAsync("Return this 2");
}).Wait();
Console.WriteLine("---------------------------------------------");
#endregion
//
// Async Keyword
// ===============
//
// The async keyword is not actually added to the method declaration signature,
// the only effect is has is to change the compiled code.
//
// That's why interfaces cannot declare async, as it isn't a declarative statement,
// its a compilation statement to change the flow of the function.
//
//
// Consuming Async Methods
// =========================
//
// The best way to consume or call a method that returns a task is to be async yourself
// in the caller method, to ultimately awaiting it.
//
// By that definition async methods are naturally contagious.
//
#region Consuming Wait
// Store the taks
var workResultTask = DoWorkAndGetResultAsync("Consume Wait");
// Wait for it
workResultTask.Wait();
// Get the result
var workResult = workResultTask.Result;
Console.WriteLine("---------------------------------------------");
#endregion
#region Consuming via Task
// Declare the result
var workResultViaTask = default(string);
// Store the taks
Task.Run(async () =>
{
// Get result
workResultViaTask = await DoWorkAndGetResultAsync("Consume via Task");
}).Wait();
Console.WriteLine("---------------------------------------------");
#endregion
//
// What happens during an Async call
// ===================================
//
// Code inside a function that returns a Task runs its code on the callers thread
// up until the first line that calls await. At this point in time:
//
// 1. The current thread executing your code is released (making your code asynchronous).
// This means from a normal point of view, your function has returned at this point
// (it has return the Task object).
//
// 2. Once the task you have awaited completes, your method should appear to continue
// from where it left off, as if it never returned from the method, so resume on
// the line below the await.
//
// To achieve this, C# at the point of reaching the await call:
//
// 1. Stores all local variables in the scope of the method
// 2. Stores the parameters of your method
// 3. The "this" variable to store all class-level variables
// 4. Stores all contexts(Execution, Security, Call)
//
// And on resuming to the line after the await, restores all of these values as if nothing
// had changed. All of this information is stored on the garbage collection heap.
//
//
// What is happening with threads during an async call
// =====================================================
//
// As you call a method that returns a `Task` and uses `async`, inside the method all code,
// up until the first `await` statement, is run like a normal function on the thread that
// called it.
//
// Once it hits the `await` the function returns the `Task` to the caller, and does its work
// thats inside the `await` call on a new thread (or existing).
//
// Once its done, and effectively "after" the `await` line, execution returns to a certain
// thread.
//
// That thread is determined by first checking if the thread has an synchronization context
// and if it does it asks that what thread to return to. For UI threads this will return work
// to the UI thread itself.
//
// Console application has no synchronization context
var syncContext = SynchronizationContext.Current;
//
// For normal threads that have no synchronization context, the code after the `await`
// typically, but not always, continues on the same thread that the inner work was being done
// on, but has no requirement to resume on any specific thread.
//
// Typically if you use `ContinueWith` instead of `await`, the code inside `ContinueWith` runs
// on a different thread than the inner task was running on, and using `await` typically
// continues on the same thread.
//
// Show ContinueWith typically changing thread ID's
DoWorkAsync("ContinueWith").ContinueWith(t =>
{
Log("ContinueWith Complete");
}).Wait();
Console.WriteLine("---------------------------------------------");
//
// This also means after every await the next line is typically on a new thread if there is no
// synchronization context.
//
// **********************************************************************************************
//
// An exception is if you use `ConfigureAwait(false)` then the SynchronizationContext is
// totally ignored and the resuming thread is treated as if there were no context.
//
// Resuming on the original thread via the synchronization context is an expensive thing
// (takes time) and so if you choose to not care about resuming on the same thread and want
// to save time you can use `ConfigureAwait` to remove that overhead
//
// **********************************************************************************************
//
//
// Exceptions in Async calls
// ===========================
//
// Any exceptions thrown that are not caught by the method itself are thrown into the Task
// objects value `IsFaulted` and the `Exception` property.
//
// If you do not await the Task, the exception will not throw on your calling thread.
//
#region Throw on Calling Thread, Without Awaiting
Log("Before ThrowAwait");
var crashedTask = ThrowAwait(true);
// Did it crash?
var isFaulted = crashedTask.IsFaulted;
// The exception
Log(crashedTask.Exception.Message);
Log("After ThrowAwait");
#endregion
//
// If you await, the exception will rethrow onto the caller thread that awaited it.
//
// The exception to the rule is a method with async void. As it cannot be awaited, any
// exceptions that occur in an async void method are re-thrown like this:
//
// 1. If there is a synchronization context the exception is Post back to the caller thread.
// 2. If not, it is thrown on the thread pool
//
#region Throw on Calling Thread, Without Awaiting
Log("Before ThrowVoid");
ThrowAwaitVoid(true);
Log("After ThrowVoid");
#endregion
Console.ReadLine();
}
#region Helper Methods
/// <summary>
/// Output a message with the current thread ID appended
/// </summary>
/// <param name="message"></param>
private static void Log(string message)
{
// Write line
Console.WriteLine($"{message} [{Thread.CurrentThread.ManagedThreadId}]");
}
#endregion
#region Thread Methods
/// <summary>
/// Shows an event-based thread callback via a method
/// </summary>
/// <param name="completed">The callback to call once the work is complete</param>
private static void EventThreadCallbackMethod(Action completed)
{
// Start a new thread
new Thread(() =>
{
// Log it
Log("Inside event thread method");
// Sleep
Thread.Sleep(500);
// Fire completed event
completed();
}).Start();
}
#endregion
#region Task Example Methods
/// <summary>
/// Downloads a string from a website URL sychronously
/// </summary>
/// <param name="url">The URL to download</param>
private static void WebDownloadString(string url)
{
// Synchronous pattern
var webClient = new WebClient();
var result = webClient.DownloadString(url);
// Log
Log($"Downloaded {url}. {result.Substring(0, 10)}");
}
/// <summary>
/// Downloads a string from a website URL asychronously
/// </summary>
/// <param name="url">The URL to download</param>
private static async Task WebDownloadStringAsync(string url)
{
// Asynchronous pattern
var webClient = new WebClient();
var result = await webClient.DownloadStringTaskAsync(new Uri(url));
// Log
Log($"Downloaded {url}. {result.Substring(0, 10)}");
}
/// <summary>
/// Does some work asynchronously for somebody
/// </summary>
/// <param name="forWho">Who we are doing the work for</param>
/// <returns></returns>
private static async Task DoWorkAsync(string forWho)
{
// Log it
Log($"Doing work for {forWho}");
// Start a new task (so it runs on a different thread)
await Task.Run(async () =>
{
// Log it
Log($"Doing work on inner thread for {forWho}");
// Wait
await Task.Delay(500);
// Log it
Log($"Done work on inner thread for {forWho}");
});
// Log it
Log($"Done work for {forWho}");
}
/// <summary>
/// Does some work asynchronously for somebody, and return a result
/// </summary>
/// <param name="forWho">Who we are doing the work for</param>
/// <returns></returns>
private static async Task<string> DoWorkAndGetResultAsync(string forWho)
{
// Log it
Log($"Doing work for {forWho}");
// Start a new task (so it runs on a different thread)
await Task.Run(async () =>
{
// Log it
Log($"Doing work on inner thread for {forWho}");
// Wait
await Task.Delay(500);
// Log it
Log($"Done work on inner thread for {forWho}");
});
// Log it
Log($"Done work for {forWho}");
// Return what we received
return forWho;
}
/// <summary>
/// Throws an exception inside a task
/// </summary>
/// <param name="before">Throws the exception before an await</param>
/// <returns></returns>
private static async Task ThrowAwait(bool before)
{
if (before)
throw new ArgumentException("Oopps");
await Task.Delay(1);
throw new ArgumentException("Oopps");
}
/// <summary>
/// Throws an exception inside a void async before awaiting
/// </summary>
/// <param name="before">Throws the exception before an await</param>
/// <returns></returns>
private static async void ThrowAwaitVoid(bool before)
{
if (before)
throw new ArgumentException("Oopps");
await Task.Delay(1);
throw new ArgumentException("Oopps");
}
#endregion
}
}
整个视频的源代码可以从他的Github仓库下载：
git clone https://github.com/angelsix/youtube.git本文同步分享在 博客“雪域迷影”（CSDN）。
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