How different async programming is from Threads?

Question

I\'ve been reading some async articles here: http://www.asp.net/web-forms/tutorials/aspnet-45/using-asynchronous-methods-in-aspnet-45 and the author says :

Answer 1

First time when I saw async and await, I thougth they were C# Syntactic sugar for Asynchronous Programming Model. I was wrong, async and await are more than that. It is a brand new asynchronous pattern Task-based Asynchronous Pattern, http://www.microsoft.com/en-us/download/details.aspx?id=19957 is a good article to get start. Most of the FCL classes which inplement TAP are call APM methods (BegingXXX() and EndXXX()). Here are two code snaps for TAP and AMP:

TAP sample:

    static void Main(string[] args)
    {
        GetResponse();
        Console.ReadLine();
    }

    private static async Task GetResponse()
    {
        var webRequest = WebRequest.Create("http://www.google.com");
        Task response = webRequest.GetResponseAsync();
        Console.WriteLine(new StreamReader(response.Result.GetResponseStream()).ReadToEnd());
        return response.Result;
    }

APM sample:

    static void Main(string[] args)
    {
        var webRequest = WebRequest.Create("http://www.google.com");
        webRequest.BeginGetResponse(EndResponse, webRequest);
        Console.ReadLine();
    }

    static void EndResponse(IAsyncResult result)
    {
        var webRequest = (WebRequest) result.AsyncState;
        var response = webRequest.EndGetResponse(result);
        Console.WriteLine(new StreamReader(response.GetResponseStream()).ReadToEnd());
    }

Finally these two will be the same, because GetResponseAsync() call BeginGetResponse() and EndGetResponse() inside. When we reflector the source code of GetResponseAsync(), we will get code like this:

task = Task.Factory.FromAsync(
       new Func(this.BeginGetResponse), 
       new Func(this.EndGetResponse), null);

For APM, in the BeginXXX(), there is an argument for a callback method which will invoked when the task (typically is an IO heavy operation) was completed. Creating a new thread and asynchronous, both of them will immediately return in main thread, both of them are unblocked. On performance side, creating new thread will cost more resource when process I/O-bound operations such us read file, database operation and network read. There are two disadvantages in creating new thread,

1. like in your mentioned article, there are memory cost and CLR are
   limitation on thread pool.
2. Context switch will happen. On the other hander, asynchronous will not create any thread manually and it will not have context switch when the the IO-bound operations return.

Here is an picture which can help to understand the differences:

This diagram is from a MSDN article "Asynchronous Pages in ASP.NET 2.0", which explain very detail about how the old asynchronous working in ASP.NET 2.0.

About Asynchronous Programming Model, please get more detail from Jeffrey Richter's article "Implementing the CLR Asynchronous Programming Model", also there are more detail on his book "CLR via Csharp 3rd Edition" in chapter 27.