Async and Await

Async. A river rushes along with no interruptions. Now imagine another river some distance away. It too flows with no interruptions. A bird's song can be heard.

With async and await in C# we call functions in an asynchronous way. Like the rivers these tasks can run with no interference. Many things happen at once.

Task:A Task returns no value (it is void). A Task<int> returns an element of type int. This is a generic type.

Note:An async method will be run synchronously if it does not contain the await keyword.

First program. We use the async and await keywords to asynchronously run a method. The program begins a long-running method (HandleFileAsync).

First:We create a Task instance by calling the HandleFileAsync method. The task starts, and we call Wait() for it to finish.

Messages:The method displays a status message after it starts. When it ends, the results are displayed.

HandleFileAsync:In HandleFileAsync, we use the StreamReader type and await the ReadToEndAsync method. We perform some computations.

Based on: .NET (2018)

C# program that uses async, await, Task

using System;
using System.IO;
using System.Threading.Tasks;

class Program
{
    public static void Main()
    {
        // Start the HandleFile method.
        Task<int> task = HandleFileAsync();

        // Control returns here before HandleFileAsync returns.
        // ... Prompt the user.
        Console.WriteLine("Please wait patiently " +
            "while I do something important.");

        // Do something at the same time as the file is being read.
        string line = Console.ReadLine();
        Console.WriteLine("You entered (asynchronous logic): " + line);

        // Wait for the HandleFile task to complete.
        // ... Display its results.
        task.Wait();
        var x = task.Result;
        Console.WriteLine("Count: " + x);

        Console.WriteLine("[DONE]");
        Console.ReadLine();
    }

    static async Task<int> HandleFileAsync()
    {
        string file = @"C:\programs\enable1.txt";
        Console.WriteLine("HandleFile enter");
        int count = 0;

        // Read in the specified file.
        // ... Use async StreamReader method.
        using (StreamReader reader = new StreamReader(file))
        {
            string v = await reader.ReadToEndAsync();

            // ... Process the file data somehow.
            count += v.Length;

            // ... A slow-running computation.
            //     Dummy code.
            for (int i = 0; i < 10000; i++)
            {
                int x = v.GetHashCode();
                if (x == 0)
                {
                    count--;
                }
            }
        }
        Console.WriteLine("HandleFile exit");
        return count;
    }
}

Output: initial

HandleFile enter
Please wait patiently while I do something important.

Output: part 2

HandleFile enter
Please wait patiently while I do something important.
test
You entered (asynchronous logic): test

Output: final

HandleFile enter
Please wait patiently while I do something important.
test
You entered (asynchronous logic): test
HandleFile exit
Count: 1916146
[DONE]

Above, the slow computation done in HandlFileAsync is for demonstration. If you change the path to a large text file that exists on your computer, the program should work.

Note:We can do something (such as write a message) after the async method starts. This is not possible with synchronous methods.

Important:We must be careful to call Wait() on the task we want to wait for. Sometimes the wrong Task can be waited on.

Simple example. This program runs a computation asynchronously on every line entered in the console. It keeps accepting lines even when computations are running.

Action:A lambda expression is specified as the argument to Task.Run. This is an action delegate.

Action

Allocate:This method does a slow-running computation. But when run asynchronously, it does not cause the program to freeze.

Result:Many user inputs can be handled while the computation is running. Each Allocate() call finishes at its own pace.

C# program that uses async computation

using System;
using System.Threading.Tasks;

class Program
{
    static void Main()
    {
        while (true)
        {
            // Start computation.
            Example();
            // Handle user input.
            string result = Console.ReadLine();
            Console.WriteLine("You typed: " + result);
        }
    }

    static async void Example()
    {
        // This method runs asynchronously.
        int t = await Task.Run(() => Allocate());
        Console.WriteLine("Compute: " + t);
    }

    static int Allocate()
    {
        // Compute total count of digits in strings.
        int size = 0;
        for (int z = 0; z < 100; z++)
        {
            for (int i = 0; i < 1000000; i++)
            {
                string value = i.ToString();
                size += value.Length;
            }
        }
        return size;
    }
}

Output

hello
You typed: hello
good
You typed: good
day
You typed: day
Compute: 588889000
friend
You typed: friend
Compute: 588889000
Compute: 588889000
Compute: 588889000
Compute: 588889000

Main method. The async keyword cannot be used on the Main method. So we will need to add a second method before using an await call.

C# program that causes compile-time error

using System;
using System.Threading.Tasks;

class Program
{
    static async void Main()
    {
    }
}

Output

error CS4009: 'Program.Main()': an entry point cannot be marked
    with the 'async' modifier

A pattern. Async and await are a code pattern—they allow methods to asynchronously run. They are a form of syntactic sugar. They make code that uses threads easier to read.

Complexity. With async and await, the compiler helps with asynchronous code. We return a Task or void from an async method. Visual Studio reports warnings or errors on incorrect methods.

Types (StreamReader, HttpClient) contain "Async" methods. These should be called with the await keyword. And the await keyword must be used within an async method.StreamReaderHttpClient

Task.Start:The first async method call can occur with the Task Start method. This is an instance method.

Also:Event handlers can be used with async methods. This is not currently shown here.

ReadToEndAsync. When can ReadToEndAsync lead to a performance gain in a program? When we have high CPU usage with a file load, we can speed up completion time with async and await.StreamReader ReadToEndAsync

Asynchronous. This term does not mean multithreaded code. By default, code written with async and await is single-threaded. But threaded code works well here.Threads

Task.Run. With the Task.Run method, we can make code that uses async and await multithreaded. Asynchronous code is code that returns upon completion.

And:Other code can execute (even on the same thread) after an asynchronous task has started.

Some corrections. Thanks to Donnie Karns for pointing out that async code statements are not (by default) run on another thread. Thanks to Jim Boyer for correcting a "nested" task problem.

And:Thanks to Andrew Dennison for reviewing the Allocate() method code which had an unnecessary statement.

Quote:The async and await keywords don't cause additional threads to be created. Async methods don't require multithreading because an async method doesn't run on its own thread.