I was pleased to hear the
announcement at PDC2010 by Anders Hejlsberg that C# and VB will likely be following the lead of F# by including asynchronous programming support. As Don Syme states in
his post on this topic "the proposed design of C# and VB asynchronous programming is pleasant and simple, and heavily inspired by the corresponding feature of F#". In this post I will provide a simple example in F#, C#, and VB. The provided example is based on an example from a post by Don entitled "
Async and Parallel Design Patterns in F#: Parallelizing CPU and I/O Computations". This example is also very similar to those provided in a
series of posts on this topic by Tomas Petricek. I strongly recommend reading Don's post as well as Tomas's series.
F#:
open System
open System.IO
open System.Net
let getHtml url printStrategy =
async { let request = WebRequest.Create(Uri url)
use! response = request.AsyncGetResponse()
use stream = response.GetResponseStream()
use reader = new StreamReader(stream)
let contents = reader.ReadToEnd()
do printStrategy url contents
return contents }
let sites = ["http://www.bing.com";
"http://www.google.com";
"http://www.yahoo.com";
"http://msdn.microsoft.com/en-us/fsharp/default.aspx"]
let printStrategy url (contents:string) =
printfn "%s - HTML Length %d" url contents.Length
let sitesHtml = Async.Parallel [for site in sites -> getHtml site printStrategy]
|> Async.RunSynchronously
do printfn "\r\nProcess Complete\r\nPress any key to continue"
do Console.ReadLine() |> ignore
C#:
using System;
using System.IO;
using System.Net;
using System.Threading.Tasks;
using System.Linq;
namespace CSharpAsync
{
class Program
{
static async Task<string> GetHtml(string url,
Action<string, string> printStrategy)
{
var request = WebRequest.Create(new Uri(url));
using (var response = await request.GetResponseAsync())
{
using (var stream = response.GetResponseStream())
{
using (var reader = new StreamReader(stream))
{
var contents = reader.ReadToEnd();
printStrategy.Invoke(url, contents);
return contents;
}
}
}
}
static async Task ProcessSites(string[] sites,
Action<string, string> printStrategy)
{
var sitesHtml =
await TaskEx.WhenAll(
sites.Select(site => GetHtml(site, printStrategy)));
Console.WriteLine("\r\nProcess Complete\r\nPress any key to continue");
}
static void Main(string[] args)
{
var sites = new[] { "http://www.bing.com",
"http://www.google.com",
"http://www.yahoo.com",
"http://msdn.microsoft.com/en-us/fsharp/default.aspx" };
Action<string, string> printStrategy =
(url, contents) =>
Console.WriteLine("{0} - HTML Length {1}", url, contents.Length);
ProcessSites(sites, printStrategy).Wait();
Console.ReadLine();
}
}
}
VB:
Imports System
Imports System.IO
Imports System.Net
Imports System.Threading.Tasks
Imports System.Linq
Module Module1
Async Function GetHtml(ByVal url As String, ByVal printStrategy As Action(Of String, String)) As Task(Of String)
Dim request = WebRequest.Create(New Uri(url))
Using response = Await request.GetResponseAsync()
Using stream = response.GetResponseStream()
Using reader = New StreamReader(stream)
Dim contents = reader.ReadToEnd()
printStrategy.Invoke(url, contents)
Return contents
End Using
End Using
End Using
End Function
Async Function ProcessSites(ByVal sites As String(), ByVal printStrategy As Action(Of String, String)) As Task
Dim sitesHtml = _
Await TaskEx.WhenAll(sites.Select(Function(site) GetHtml(site, printStrategy)))
Console.WriteLine("{0}Process Complete{0}Press any key to continue", _
Environment.NewLine)
End Function
Sub Main()
Dim sites = New String() {"http://www.bing.com", _
"http://www.google.com", _
"http://www.yahoo.com", _
"http://msdn.microsoft.com/en-us/fsharp/default.aspx"}
Dim printStrategy As Action(Of String, String) = _
Sub(url, contents) Console.WriteLine("{0} - HTML Length {1}", url, contents.Length)
ProcessSites(sites, printStrategy).Wait()
Console.ReadLine()
End Sub
End Module
