using System.Net.Http;
using System.Threading.Tasks;

namespace LanguageFeatures.Models {
    public class MyAsyncMethods {
        public static Task<long?> GetPageLength() {
            HttpClient clint = new HttpClient();

            var httpTask = client.GetAsync("http://apress.com");

            // we chould do other things here while we are waiting
            // for HTTP request to complete

            // Task continuation
            return httpTask.ContinueWith((Task<HttpResponsMessage> antecedent) => {
                return antecedent.Result.Content.Headers.ContentLength;
            })
        }
    }
}

Where:

Objective of method GetPageLength: use a System.Net.Http.HttpClient object to request the contents of the Apress home page and returns its lengths.
Task continuation: key of this async method
.NET represent work that can be done asynchronously as Task.
- Task object are strongly typed based on result produced by background work.
- client.GetAsync("..") returns a Task<HttpResponseMessage> (i.e. type of httpTask), which indicates that the request will be performed in background and result of request will be of type HttpResponseMessage.
- Detail of MVC async methods are in Chap19
Task continuation is most mind-boggling part, it's .NET mechanism to specify what happen when background task is completed.
- Step 1: httpTask is used to get HttpResponseMessage object. The returned object has .ContinueWith method
- Step 2: .ContinueWith method use lambda expression to return length of the content as a property.
- Double return: 1st return specify that Task<HttpResponseMessage> object is returned; 2nd return specify when task is complete, the length of ContentLength header will be returned.
No need to fully understand this example. It's illustration of complex async operation in previous .NET Framework

4.9.1 Applying the async and await Keywords

C# two keywords async & await are used to simplify async methods.

Listing 4-33. Using the Async and Await Keywords in the MyAsyncMethods.cs File

using System.Net.Http;
using System.Threading.Tasks;

namespace LanguageFeatures.Models {
  public class MyAsyncMethods {
    public async static Task<long?> GetPageLength() {
      HttpClient client = new HttpClient();
      var httpMessage = await client.GetAsync("http://apress.com");

      // Do other things while waiting for HTTP request to complete

      return httpMessage.Content.Headers.ContentLength;
    }
  }
}

Where:

await is used when calling async methods (e.g. .GetAsync(...)). It
- Step 1: tells C# compiler that we want to wait for the result of Task that returned by GetAsync method
- Step 2: continue executing other statements in same method.
async needed to be assigned on method signature when awit keyword appear in method.
Benefit of await & async: we can use return keyword in normal way to produce a result from other methods.