I am trying to download multiple webpages using the WebClient class. When I try to download a website's html, a TargetInvocationException is thrown, and I do not know why it happens. Here is my code:
public HashSet<string> DownloadWebpages(HashSet<string> urls)
{
HashSet<string> HTML = new HashSet<string>();
for (int i = 0; i < urls.Count; i++)
{
WebClient client = new WebClient();
client.DownloadStringCompleted += (s, e) =>
{
try
{
lock (HTML)
{
HTML.Add(e.Result); //The exception happens on this line
}
}
catch { }
};
client.DownloadStringAsync(new Uri(urls.ElementAt(i)));
}
return HTML;
}
Is there any way to fix this. All I'm trying to do is download multiple webpages using async, trying to make it has fast as possible.
The TargetInvocationException is thrown, because the webclient is not able to download the website. Here is a test,
string html = new WebClient().DownloadString("https://www.siteth#tw!llcause an error!/randompage/");
This will cause an exception. So if you tried to download the same webpage with your code, it will cause an TargetInvocationException
WebClient is an obsolete class replaced since 2012 by HttpClient. It was never built with HTTP APIs or thread safety in mind. It's easier to do what you want with HttpClient's GetStringAsync:
public async Task<HashSet<string>> DownloadWebpages(IEnumerable<string> urls)
{
HashSet<string> HTML = new HashSet<string>();
var client=new HttpClient();
foreach (var url in urls)
{
var source=await client.GetStringAsync(url);
HTML.Add(source);
}
return HTML;
}
Since .NET 6 you can even retrieve the URLs concurrently with Parallel.ForEachAsync. In this case you'd need a thread-safe collection to store them, eg a ConcurrentDictionary :
HttpClient _client=new HttpClient();
public async Task<ConcurrentDictionary<string,string>> DownloadWebpages(IEnumerable<string> urls)
{
var HTML = new ConcurrentDictionary<string,string>();
await Parallel.ForeachAsync(urls,async url=>{
{
var source=await client.GetStringAsync(url);
HTML.Add(url,source);
});
return HTML;
}
HttpClient is thread-safe and meant to be reused.
If you absolutely have to use WebClient (why???) you can use DownloadStringTaskAsync. You won't be able to make concurrent calls though because WebClient isn't thread-safe.
public async Task<HashSet<string>> DownloadWebpages(IEnumerable<string> urls)
{
HashSet<string> HTML = new HashSet<string>();
var client=new WebClient();
foreach (var url in urls)
{
var source=await client.DownloadStringTaskAsync(url);
HTML.Add(source);
}
return HTML;
}
Related
The problem I have it is:
I tried to download 1000+ images -> it works, but it takes a very long time to load the image downloaded completely, and the program continues and downloads the next image etc... Until let's admit 100 but the 8th image is still not finished downloading.
So I would like to understand why I encounter such a problem here and / or how to fix this problem.
Hope to see an issue
private string DownloadSourceCode(string url)
{
string sourceCode = "";
try
{
using (WebClient WC = new WebClient())
{
WC.Encoding = Encoding.UTF8;
WC.Headers.Add("Accept", "image / webp, */*");
WC.Headers.Add("Accept-Language", "fr, fr - FR");
WC.Headers.Add("Cache-Control", "max-age=1");
WC.Headers.Add("DNT", "1");
WC.Headers.Add("Origin", url);
WC.Headers.Add("TE", "Trailers");
WC.Headers.Add("user-agent", Fichier.LoadUserAgent());
sourceCode = WC.DownloadString(url);
}
}
catch (WebException e)
{
if (e.Status == WebExceptionStatus.ProtocolError)
{
string status = string.Format("{0}", ((HttpWebResponse)e.Response).StatusCode);
LabelID.TextInvoke(string.Format("{0} {1} {2} ", status,
((HttpWebResponse)e.Response).StatusDescription,
((HttpWebResponse)e.Response).Server));
}
}
catch (NotSupportedException a)
{
MessageBox.Show(a.Message);
}
return sourceCode;
}
private void DownloadImage(string URL, string filePath)
{
try
{
using (WebClient WC = new WebClient())
{
WC.Encoding = Encoding.UTF8;
WC.Headers.Add("Accept", "image / webp, */*");
WC.Headers.Add("Accept-Language", "fr, fr - FR");
WC.Headers.Add("Cache-Control", "max-age=1");
WC.Headers.Add("DNT", "1");
WC.Headers.Add("Origin", "https://myprivatesite.fr//" + STARTNBR.ToString());
WC.Headers.Add("user-agent", Fichier.LoadUserAgent());
WC.DownloadFile(URL, filePath);
NBRIMAGESDWLD++;
}
STARTNBR = CheckBoxBack.Checked ? --STARTNBR : ++STARTNBR;
}
catch (IOException)
{
LabelID.TextInvoke("Accès non autorisé au fichier");
}
catch (WebException e)
{
if (e.Status == WebExceptionStatus.ProtocolError)
{
LabelID.TextInvoke(string.Format("{0} / {1} / {2} ", ((HttpWebResponse)e.Response).StatusCode,
((HttpWebResponse)e.Response).StatusDescription,
((HttpWebResponse)e.Response).Server));
}
}
catch (NotSupportedException a)
{
MessageBox.Show(a.Message);
}
}
private void DownloadImages()
{
const string URL = "https://myprivatesite.fr/";
string imageIDURL = string.Concat(URL, STARTNBR);
string sourceCode = DownloadSourceCode(imageIDURL);
if (sourceCode != string.Empty)
{
string imageNameURL = Fichier.GetURLImage(sourceCode);
if (imageNameURL != string.Empty)
{
string imagePath = PATHIMAGES + STARTNBR + ".png";
LabelID.TextInvoke(STARTNBR.ToString());
LabelImageURL.TextInvoke(imageNameURL + "\r");
DownloadImage(imageNameURL, imagePath);
Extension.SaveOptions(STARTNBR, CheckBoxBack.Checked);
}
}
STARTNBR = CheckBoxBack.Checked ? --STARTNBR : ++STARTNBR;
}
// END FUNCTIONS
private void BoutonStartPause_Click(object sender, EventArgs e)
{
if (Fichier.RGBIMAGES != null)
{
if (boutonStartPause.Text == "Start")
{
boutonStartPause.ForeColor = Color.DarkRed;
boutonStartPause.Text = "Pause";
if (myTimer == null)
myTimer = new System.Threading.Timer(_ => new Task(DownloadImages).Start(), null, 0, Trackbar.Value);
}
else if (boutonStartPause.Text == "Pause")
EndTimer();
Extension.SaveOptions(STARTNBR, CheckBoxBack.Checked);
}
}
So I would like to understand why I encounter such a problem here and / or how to fix this problem.
There are probably two reasons I can think of.
Connection/Port Exhaustion
Thread Pool Exhaustion
Connection/Port Exhaustion
This happens when you're attempting to create too many connections at once, or when the connections you made previously have not yet been released. When you use a WebClient the resources it uses sometimes don't get released immediately. This causes a delay between when that object is disposed and the actual time that the next WebClient attempting to use the same port/connection actually gets access to that port.
An example of something that would most likely cause Connection/Port Exhaustion
int i = 1_000;
while(i --> 0)
{
using var Client = new WebClient();
// do some webclient stuff
}
When you create a lot of web clients, which is sometimes necessary due to the inherent lack of concurrency in WebClient. There's a possibility that by the time the next WebClient is instantiated, the port that the last one was using may not be available yet, causing either a delay(while it waits for the port) or worse the next WebClient opening another port/connection. This can cause a never ending list of connections to open causing things to grind to a halt!
Thread Pool Exhaustion
This is caused by trying to create too many Task or Thread objects at once that block their own execution(via Thread.Sleep or a long running operation).
Normally this isn't an issue since the built in TaskScheduler does a really good job of keeping track of a lot of tasks and makes sure that they all get turns to execute their code.
Where this becomes a problem is the TaskScheduler has no context for which tasks are important, or which tasks are going to need more time than others to complete. So therefor when many tasks are processing long running operations, blocking, or throwing exceptions, the TaskScheduler has to wait for those tasks to finish before it can start new ones. If you are particularly unlucky the TaskScheduler can start a bunch of tasks that are all blocking and no tasks can start, even if all the other tasks waiting are small and would complete instantly.
You should generally use as few tasks as possible to increase reliability and avoid thread pool exhaustion.
What you can do
You have a few options to help improve the reliability and performance of this code.
Consider using HttpClient instead. I understand you may be required to use WebClient so I have provided answers using WebClient exclusively.
Consider Requesting multiple downloads/strings within the same task to avoid Thread Pool Exhaustion
Consider using a WebClient helper class that limits the available webclients that can be active at once, and has the ability to keep webclients open if you're going to be accessing the same website multiple times.
WebClient Helper Class
I created a very simple helper class to get you started. This will allow you to create WebClient requests asynchronously without having to worry about creating too many clients at once. The default limit is the number of Cores in the client's processor(this was chosen arbitrarily).
public class ConcurrentWebClient
{
// limits the number of maximum clients able to be opened at once
public static int MaxConcurrentDownloads => Environment.ProcessorCount;
// holds any clients that should be kept open
private static readonly ConcurrentDictionary<string, WebClient> Clients;
// prevents more than the alloted webclients to be open at once
public static readonly SemaphoreSlim Locker;
// allows cancellation of clients
private static CancellationTokenSource TokenSource = new();
static ConcurrentWebClient()
{
Clients = new ConcurrentDictionary<string, WebClient>();
Locker ??= new SemaphoreSlim(MaxConcurrentDownloads, MaxConcurrentDownloads);
}
// creates new clients, or if a name is provided retrieves it from the dictionary so we don't need to create more than we need
private async Task<WebClient> CreateClient(string Name, bool persistent, CancellationToken token)
{
// try to retrieve it from the dictionary before creating a new one
if (Clients.ContainsKey(Name))
{
return Clients[Name];
}
WebClient newClient = new();
if (persistent)
{
// try to add the client to the dict so we can reference it later
while (Clients.TryAdd(Name, newClient) is false)
{
token.ThrowIfCancellationRequested();
// allow other tasks to do work while we wait to add the new client
await Task.Delay(1, token);
}
}
return newClient;
}
// allows sending basic dynamic requests without having to create webclients outside of this class
public async Task<T> NewRequest<T>(Func<WebClient, T> Expression, int? MaxTimeout = null, string Id = null)
{
// make sure we dont have more than the maximum clients open at one time
// 100s was chosen becuase WebClient has a default timeout of 100s
await Locker.WaitAsync(MaxTimeout ?? 100_000, TokenSource.Token);
bool persistent = true;
if (Id is null)
{
persistent = false;
Id = string.Empty;
}
try
{
WebClient client = await CreateClient(Id, persistent, TokenSource.Token);
// run the expression to get the result
T result = await Task.Run<T>(() => Expression(client), TokenSource.Token);
if (persistent is false)
{
// just in case the user disposes of the client or sets it to ull in the expression we should not assume it's not null at this point
client?.Dispose();
}
return result;
}
finally
{
// make sure even if we encounter an error we still
// release the lock
Locker.Release();
}
}
// allows assigning the headers without having to do it for every webclient manually
public static void AssignDefaultHeaders(WebClient client)
{
client.Encoding = System.Text.Encoding.UTF8;
client.Headers.Add("Accept", "image / webp, */*");
client.Headers.Add("Accept-Language", "fr, fr - FR");
client.Headers.Add("Cache-Control", "max-age=1");
client.Headers.Add("DNT", "1");
// i have no clue what Fichier is so this was not tested
client.Headers.Add("user-agent", Fichier.LoadUserAgent());
}
// cancels a webclient by name, whether its being used or not
public async Task Cancel(string Name)
{
// look to see if we can find the client
if (Clients.ContainsKey(Name))
{
// get a token incase we have to emergency cance
CancellationToken token = TokenSource.Token;
// try to get the client from the dictionary
WebClient foundClient = null;
while (Clients.TryGetValue(Name, out foundClient) is false)
{
token.ThrowIfCancellationRequested();
// allow other tasks to perform work while we wait to get the value from the dictionary
await Task.Delay(1, token);
}
// if we found the client we should cancel and dispose of it so it's resources gets freed
if (foundClient != null)
{
foundClient?.CancelAsync();
foundClient?.Dispose();
}
}
}
// the emergency stop button
public void ForceCancelAll()
{
// this will throw lots of OperationCancelledException, be prepared to catch them, they're fast.
TokenSource?.Cancel();
TokenSource?.Dispose();
TokenSource = new();
foreach (var item in Clients)
{
item.Value?.CancelAsync();
item.Value?.Dispose();
}
Clients.Clear();
}
}
Request Multiple Things at Once
Here all I did was switch to using the helper class, and made it so you can request multiple things using the same connection
public async Task<string[]> DownloadSourceCode(string[] urls)
{
var downloader = new ConcurrentWebClient();
return await downloader.NewRequest<string[]>((WebClient client) =>
{
ConcurrentWebClient.AssignDefaultHeaders(client);
client.Headers.Add("TE", "Trailers");
string[] result = new string[urls.Length];
for (int i = 0; i < urls.Length; i++)
{
string url = urls[i];
client.Headers.Remove("Origin");
client.Headers.Add("Origin", url);
result[i] = client.DownloadString(url);
}
return result;
});
}
private async Task<bool> DownloadImage(string[] URLs, string[] filePaths)
{
var downloader = new ConcurrentWebClient();
bool downloadsSucessful = await downloader.NewRequest<bool>((WebClient client) =>
{
ConcurrentWebClient.AssignDefaultHeaders(client);
int len = Math.Min(URLs.Length, filePaths.Length);
for (int i = 0; i < len; i++)
{
// side-note, this is assuming the websites you're visiting aren't mutating the headers
client.Headers.Remove("Origin");
client.Headers.Add("Origin", "https://myprivatesite.fr//" + STARTNBR.ToString());
client.DownloadFile(URLs[i], filePaths[i]);
NBRIMAGESDWLD++;
STARTNBR = CheckBoxBack.Checked ? --STARTNBR : ++STARTNBR;
}
return true;
});
return downloadsSucessful;
}
I have a challenge, I need to call many http request and handle each of them.
How to do it, I don't want to wait for get response from one of them and then call next, how to assign a method for process response (like callback).
How can define callback and assign to each of them ?
What you need is an Asynchronous programming model where you create async tasks and later use await keyword for the response.
So essentially you are not waiting for the first async call to finish, you'd just fire as many async tasks as you wish and wait to get a response only when you need the response to move ahead with your program logic.
Have a look at below for more details:
https://msdn.microsoft.com/en-us/library/hh696703.aspx
1) you can call that normaly(noneasync):
public string TestNoneAsync()
{
var webClient = new WebClient();
return webClient.DownloadString("http://www.google.com");
}
2) you can use APM (async):
private void SpecAPI()
{
var req = (HttpWebRequest)WebRequest.Create("http://www.google.com");
//req.Method = "HEAD";
req.BeginGetResponse(
asyncResult =>
{
var resp = (HttpWebResponse)req.EndGetResponse(asyncResult);
var headersText = formatHeaders(resp.Headers);
Console.WriteLine(headersText);
}, null);
}
private string formatHeaders(WebHeaderCollection headers)
{
var headerString = headers.Keys.Cast<string>()
.Select(header => string.Format("{0}:{1}", header, headers[header]));
return string.Join(Environment.NewLine, headerString.ToArray());
}
3) you can create a callback and asign it,EAP.(async .net 2):
public void EAPAsync()
{
var webClient = new WebClient();
webClient.DownloadStringAsync(new Uri("http://www.google.com"));
webClient.DownloadStringCompleted += webClientDownloadStringCompleted;
}
void webClientDownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
// use e.Result
Console.WriteLine("download completed callback.");
}
4) you can use newer way TAP, cleare way (c# 5). it's recommended:
public async Task<string> DownloadAsync(string url)
{
var webClient = new WebClient();
return await webClient.DownloadStringTaskAsync(url);
}
public void DownloadAsyncHandler()
{
//DownloadAsync("http://www.google.com");
}
threading in this solution is't good approch.(many threads that pending to call http request!)
I've been struggling with async / await for a week now. Some background: the code below is part of an MVC4 website project. The website has a large number of API calls happening. The goal is to get those API calls happening in parallel instead of synchronous to improve site responsiveness. Right now all the API calls block each other. So if one page requires 4 calls...longer load time.
I've built out individual methods for both the synchronous and async versions of all the API calls. The problem I have is the await never responds. I think it's related to this question. However, I'm really not sure how to solve it. I've tried the ConfigureAwait(false), and that hasn't helped me.
Here's the code:
The initial call in the controller it looks like so:
BaseData bdata = API.GetBaseData().Result;
I'd love to use await here, but that's not an option without an AsyncController, which we can't use due to needing request / response access. The other methods are in the API class:
internal static async Task<BaseData> GetBaseData() {
var catTask = GetParentCategoriesAsync();
var yearTask = GetYearsAsync();
await Task.WhenAll(new Task[] { catTask, yearTask });
var bdata = new BaseData {
years = await yearTask,
cats = await catTask
};
return bdata;
}
internal static async Task<List<APICategory>> GetParentCategoriesAsync() {
try {
WebClient wc = new WebClient();
wc.Proxy = null;
string url = getAPIPath();
url += "GetFullParentCategories";
url += "?dataType=JSON";
Uri targeturi = new Uri(url);
List<APICategory> cats = new List<APICategory>();
var cat_json = await wc.DownloadStringTaskAsync(targeturi);
cats = JsonConvert.DeserializeObject<List<APICategory>>(cat_json);
return cats;
} catch (Exception) {
return new List<APICategory>();
}
}
internal static async Task<List<double>> GetYearsAsync() {
WebClient wc = new WebClient();
wc.Proxy = null;
Uri targeturi = new Uri(getAPIPath() + "getyear?dataType=JSON");
var year_json = await wc.DownloadStringTaskAsync(targeturi);
List<double> years = JsonConvert.DeserializeObject<List<double>>(year_json);
return years;
}
When these methods are called, I can put breakpoints in GetYearsAsync() and GetParentCategoriesAsync(). Everything fires until the await wc.DownloadStringTaskAsync(targeturi) command. That's where stops.
I've added ConfigureAwait(continueOnCapturedContext: false) to all the tasks, but that hasn't helped. I'm assuming the problem is that the threads are not in the same context. However, I'm not certain. I am certain, however, that I'm doing something wrong. I'm just not sure what. It's either that or I'm just trying to do something that can't be done with .NET MVC4. Any thoughts would be supremely appreciated.
The problem is actually due to WebClient, which will always sync back to the request context (which is blocked due to the Result call).
You can use HttpClient instead, combined with ConfigureAwait(false):
internal static async Task<BaseData> GetBaseDataAsync() {
var catTask = GetParentCategoriesAsync();
var yearTask = GetYearsAsync();
await Task.WhenAll(catTask, yearTask).ConfigureAwait(false);
var bdata = new BaseData {
years = await yearTask,
cats = await catTask
};
return bdata;
}
internal static async Task<List<APICategory>> GetParentCategoriesAsync() {
try {
var client = new HttpClient();
string url = getAPIPath();
url += "GetFullParentCategories";
url += "?dataType=JSON";
Uri targeturi = new Uri(url);
List<APICategory> cats = new List<APICategory>();
var cat_json = await client.GetStringAsync(targeturi).ConfigureAwait(false);
cats = JsonConvert.DeserializeObject<List<APICategory>>(cat_json);
return cats;
} catch (Exception) {
return new List<APICategory>();
}
}
internal static async Task<List<double>> GetYearsAsync() {
var client = new HttpClient();
Uri targeturi = new Uri(getAPIPath() + "getyear?dataType=JSON");
var year_json = await client.GetStringAsync(targeturi).ConfigureAwait(false);
List<double> years = JsonConvert.DeserializeObject<List<double>>(year_json);
return years;
}
That should enable you to call it as such:
BaseData bdata = API.GetBaseDataAsync().Result;
However, I strongly recommend that you call it as such:
BaseData bdata = await API.GetBaseDataAsync();
You'll find that the code both before and after the await can access the request and response context just fine.
I ended up following a combination of Servy's and Stephen Cleary's advice. Based on Stephen's response, I realized I could access the request/response as long as I did it before I made any asynchronous calls in the controller.
If I stored the HttpContext in a variable, I could pass that context around to any model/service method that needed access to it. This allowed me to just go async all the way up like Servy suggested. After that, I had no issues with anything I wanted with the async patterns.
Even though I am creating new instances of WebClient, and following standard procedures to ensure that the WebClient is removed by GC, which shouldn't even matter: the webclient retrieves content from my server that it previously retrieved, and only restarting the app will it allow new content from my server to be retrieved. The content is a simple text file string, no fancy caching since it works on WinRT just fine.
This is a mystery, as I am trying to make a ChatBox; I need to refresh to gain new content yet the WebClient returns content it retrieved the first time.
My Code:
public async Task<string> RetrieveDocURL(Uri uri)
{
return await DownloadStringTask(new WebClient(), uri);
}
/*
public async Task<byte[]> RetrieveImageURL(string url)
{
return await _webClient.GetByteArrayAsync(url);
}
* */
private Task<string> DownloadStringTask(WebClient client, Uri uri)
{
var tcs = new TaskCompletionSource<string>();
client.DownloadStringCompleted += (o, e) =>
{
if (e.Error != null)
tcs.SetException(e.Error);
else
tcs.SetResult(e.Result);
};
client.DownloadStringAsync(uri);
return tcs.Task;
}
The WebClient's caching strategy is really aggressive. If you're querying the same URL each time, you should consider adding a random parameter at the end. Something like:
"http://www.yourserver.com/yourService/?nocache=" + DateTime.UtcNow.Ticks
My application requires that I download a large amount of webpages into memory for further parsing and processing. What is the fastest way to do it? My current method (shown below) seems to be too slow and occasionally results in timeouts.
for (int i = 1; i<=pages; i++)
{
string page_specific_link = baseurl + "&page=" + i.ToString();
try
{
WebClient client = new WebClient();
var pagesource = client.DownloadString(page_specific_link);
client.Dispose();
sourcelist.Add(pagesource);
}
catch (Exception)
{
}
}
The way you approach this problem is going to depend very much on how many pages you want to download, and how many sites you're referencing.
I'll use a good round number like 1,000. If you want to download that many pages from a single site, it's going to take a lot longer than if you want to download 1,000 pages that are spread out across dozens or hundreds of sites. The reason is that if you hit a single site with a whole bunch of concurrent requests, you'll probably end up getting blocked.
So you have to implement a type of "politeness policy," that issues a delay between multiple requests on a single site. The length of that delay depends on a number of things. If the site's robots.txt file has a crawl-delay entry, you should respect that. If they don't want you accessing more than one page per minute, then that's as fast as you should crawl. If there's no crawl-delay, you should base your delay on how long it takes a site to respond. For example, if you can download a page from the site in 500 milliseconds, you set your delay to X. If it takes a full second, set your delay to 2X. You can probably cap your delay to 60 seconds (unless crawl-delay is longer), and I would recommend that you set a minimum delay of 5 to 10 seconds.
I wouldn't recommend using Parallel.ForEach for this. My testing has shown that it doesn't do a good job. Sometimes it over-taxes the connection and often it doesn't allow enough concurrent connections. I would instead create a queue of WebClient instances and then write something like:
// Create queue of WebClient instances
BlockingCollection<WebClient> ClientQueue = new BlockingCollection<WebClient>();
// Initialize queue with some number of WebClient instances
// now process urls
foreach (var url in urls_to_download)
{
var worker = ClientQueue.Take();
worker.DownloadStringAsync(url, ...);
}
When you initialize the WebClient instances that go into the queue, set their OnDownloadStringCompleted event handlers to point to a completed event handler. That handler should save the string to a file (or perhaps you should just use DownloadFileAsync), and then the client, adds itself back to the ClientQueue.
In my testing, I've been able to support 10 to 15 concurrent connections with this method. Any more than that and I run into problems with DNS resolution (`DownloadStringAsync' doesn't do the DNS resolution asynchronously). You can get more connections, but doing so is a lot of work.
That's the approach I've taken in the past, and it's worked very well for downloading thousands of pages quickly. It's definitely not the approach I took with my high performance Web crawler, though.
I should also note that there is a huge difference in resource usage between these two blocks of code:
WebClient MyWebClient = new WebClient();
foreach (var url in urls_to_download)
{
MyWebClient.DownloadString(url);
}
---------------
foreach (var url in urls_to_download)
{
WebClient MyWebClient = new WebClient();
MyWebClient.DownloadString(url);
}
The first allocates a single WebClient instance that is used for all requests. The second allocates one WebClient for each request. The difference is huge. WebClient uses a lot of system resources, and allocating thousands of them in a relatively short time is going to impact performance. Believe me ... I've run into this. You're better off allocating just 10 or 20 WebClients (as many as you need for concurrent processing), rather than allocating one per request.
Why not just use a web crawling framework. It can handle all the stuff for you like (multithreading, httprequests, parsing links, scheduling, politeness, etc..).
Abot (https://code.google.com/p/abot/) handles all that stuff for you and is written in c#.
In addition to #Davids perfectly valid answer, I want to add a slightly cleaner "version" of his approach.
var pages = new List<string> { "http://bing.com", "http://stackoverflow.com" };
var sources = new BlockingCollection<string>();
Parallel.ForEach(pages, x =>
{
using(var client = new WebClient())
{
var pagesource = client.DownloadString(x);
sources.Add(pagesource);
}
});
Yet another approach, that uses async:
static IEnumerable<string> GetSources(List<string> pages)
{
var sources = new BlockingCollection<string>();
var latch = new CountdownEvent(pages.Count);
foreach (var p in pages)
{
using (var wc = new WebClient())
{
wc.DownloadStringCompleted += (x, e) =>
{
sources.Add(e.Result);
latch.Signal();
};
wc.DownloadStringAsync(new Uri(p));
}
}
latch.Wait();
return sources;
}
You should use parallel programming for this purpose.
There are a lot of ways to achieve what u want; the easiest would be something like this:
var pageList = new List<string>();
for (int i = 1; i <= pages; i++)
{
pageList.Add(baseurl + "&page=" + i.ToString());
}
// pageList is a list of urls
Parallel.ForEach<string>(pageList, (page) =>
{
try
{
WebClient client = new WebClient();
var pagesource = client.DownloadString(page);
client.Dispose();
lock (sourcelist)
sourcelist.Add(pagesource);
}
catch (Exception) {}
});
I Had a similar Case ,and that's how i solved
using System;
using System.Threading;
using System.Collections.Generic;
using System.Net;
using System.IO;
namespace WebClientApp
{
class MainClassApp
{
private static int requests = 0;
private static object requests_lock = new object();
public static void Main() {
List<string> urls = new List<string> { "http://www.google.com", "http://www.slashdot.org"};
foreach(var url in urls) {
ThreadPool.QueueUserWorkItem(GetUrl, url);
}
int cur_req = 0;
while(cur_req<urls.Count) {
lock(requests_lock) {
cur_req = requests;
}
Thread.Sleep(1000);
}
Console.WriteLine("Done");
}
private static void GetUrl(Object the_url) {
string url = (string)the_url;
WebClient client = new WebClient();
Stream data = client.OpenRead (url);
StreamReader reader = new StreamReader(data);
string html = reader.ReadToEnd ();
/// Do something with html
Console.WriteLine(html);
lock(requests_lock) {
//Maybe you could add here the HTML to SourceList
requests++;
}
}
}
You should think using Paralel's because the slow speed is because you're software is waiting for I/O and why not while a thread i waiting for I/O another one get started.
While the other answers are perfectly valid, all of them (at the time of this writing) are neglecting something very important: calls to the web are IO bound, having a thread wait on an operation like this is going to strain system resources and have an impact on your system resources.
What you really want to do is take advantage of the async methods on the WebClient class (as some have pointed out) as well as the Task Parallel Library's ability to handle the Event-Based Asynchronous Pattern.
First, you would get the urls that you want to download:
IEnumerable<Uri> urls = pages.Select(i => new Uri(baseurl +
"&page=" + i.ToString(CultureInfo.InvariantCulture)));
Then, you would create a new WebClient instance for each url, using the TaskCompletionSource<T> class to handle the calls asynchronously (this won't burn a thread):
IEnumerable<Task<Tuple<Uri, string>> tasks = urls.Select(url => {
// Create the task completion source.
var tcs = new TaskCompletionSource<Tuple<Uri, string>>();
// The web client.
var wc = new WebClient();
// Attach to the DownloadStringCompleted event.
client.DownloadStringCompleted += (s, e) => {
// Dispose of the client when done.
using (wc)
{
// If there is an error, set it.
if (e.Error != null)
{
tcs.SetException(e.Error);
}
// Otherwise, set cancelled if cancelled.
else if (e.Cancelled)
{
tcs.SetCanceled();
}
else
{
// Set the result.
tcs.SetResult(new Tuple<string, string>(url, e.Result));
}
}
};
// Start the process asynchronously, don't burn a thread.
wc.DownloadStringAsync(url);
// Return the task.
return tcs.Task;
});
Now you have an IEnumerable<T> which you can convert to an array and wait on all of the results using Task.WaitAll:
// Materialize the tasks.
Task<Tuple<Uri, string>> materializedTasks = tasks.ToArray();
// Wait for all to complete.
Task.WaitAll(materializedTasks);
Then, you can just use Result property on the Task<T> instances to get the pair of the url and the content:
// Cycle through each of the results.
foreach (Tuple<Uri, string> pair in materializedTasks.Select(t => t.Result))
{
// pair.Item1 will contain the Uri.
// pair.Item2 will contain the content.
}
Note that the above code has the caveat of not having an error handling.
If you wanted to get even more throughput, instead of waiting for the entire list to be finished, you could process the content of a single page after it's done downloading; Task<T> is meant to be used like a pipeline, when you've completed your unit of work, have it continue to the next one instead of waiting for all of the items to be done (if they can be done in an asynchronous manner).
I am using an active Threads count and a arbitrary limit:
private static volatile int activeThreads = 0;
public static void RecordData()
{
var nbThreads = 10;
var source = db.ListOfUrls; // Thousands urls
var iterations = source.Length / groupSize;
for (int i = 0; i < iterations; i++)
{
var subList = source.Skip(groupSize* i).Take(groupSize);
Parallel.ForEach(subList, (item) => RecordUri(item));
//I want to wait here until process further data to avoid overload
while (activeThreads > 30) Thread.Sleep(100);
}
}
private static async Task RecordUri(Uri uri)
{
using (WebClient wc = new WebClient())
{
Interlocked.Increment(ref activeThreads);
wc.DownloadStringCompleted += (sender, e) => Interlocked.Decrement(ref iterationsCount);
var jsonData = "";
RootObject root;
jsonData = await wc.DownloadStringTaskAsync(uri);
var root = JsonConvert.DeserializeObject<RootObject>(jsonData);
RecordData(root)
}
}