C# Async, Await

Threading is complex. The async and await keywords, added in .NET 4.5, help reduce the syntax complexity of threading. We use async on a method declaration. We use await to wait for an async method to return.

Tip: An async method can return only void or a Task.
A Task returns no value.
A Task<int> returns an element of type int.

Void

Tip 2: The Main method cannot be async.
It cannot use the await keyword.
It must start an async method with the Task class.

Tip 3: An async method will be run synchronously if it does not contain the await keyword.

Based on:

.NET 4.5

Example

This program uses the async and await keywords to asynchronously run a method. The program begins a long-running method (HandleFileAsync). It displays a status message after this method starts.

And: When the method ends, the results of the computation are written to the screen.

The only difference between this program and a single-threaded one is that we can do something (such as write a message) on the original thread after the async method starts. It avoids blocking the original thread.

To start, we create a Task instance with the ProcessDataAsync method as the argument. We Start this task, and Wait for it to finish. In ProcessDataAsync, we call the HandleFileAsync method. We write a status message to the screen.

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

Program that uses async, await, Task: C#

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

class Program
{
    static void Main()
    {
 // Create task and start it.
 // ... Wait for it to complete.
 Task task = new Task(ProcessDataAsync);
 task.Start();
 task.Wait();
 Console.ReadLine();
    }

    static async void ProcessDataAsync()
    {
 // Start the HandleFile method.
 Task<int> task = HandleFileAsync("C:\\enable1.txt");

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

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

    static async Task<int> HandleFileAsync(string file)
    {
 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: final

HandleFile enter
Please wait patiently while I do something important.
HandleFile exit
Count: 1916146

In this example, the slow computation done in HandleFileAsync is only for demonstration. It does nothing useful. If you change the string "C:\enable1.txt" to a large text file that exists on your computer, the program should work.

Discussion

Async and await are a code pattern—they allow methods to run on separate threads. They are a form of syntactic sugar. They make code that uses threads easier to read. And this in turn makes that code less prone to flaws.

However: There are many complexities in using async and await. We must return the a Task or void type from an async method.

And: Async methods that are incorrect will cause Visual Studio to report warnings or errors.

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. The first async method call can occur with the Task Start method.
StreamReader HttpClient

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

Summary

A program can create threads in many ways. The Thread type is only one option—async and await provides a new option. Other alternatives, such as BackgroundWorker, ThreadPool, and parallel LINQ expressions, remain available.