C# 4.0 Feature (1) Concurrent Collections

Before C# 4.0 comming out, there is no concurrent collections. So there is thread safety issue about the collections if mutiple thread access these collections.

Traditional way we do is lock the collection's syncroot property.

For example:

Hashtable ht=new Hashtable();
lock (ht.SyncRoot)
{
...//do somthing
}

Notice that the lock of "this" (lock(this)). or lock of an object instance is not a thread safe method. And Syncronized method is not thread safe for collections as well.

After C# 4.0 comming out, there is a new data structure: Concurrent Collection, and this data type makes thread safe.

We write an sample code below to use Concurrent Collection:

using System;
using System.Net;
using System.Threading.Tasks;
using System.Collections.Concurrent;

class Program
{
static void Main(string[] args)
{
var links = new ConcurrentQueue();
links.Enqueue("http://....aa.jpg");
links.Enqueue("http://....bb.jpg");
links.Enqueue("http://....cc.jpg");


Parallel.For(0, links.Count, delegate(int i)
{
string url;
string filename = "Product"+i + "test.jpg";
if (links.TryDequeue(out url))
{

DownLoad(url, filename);
}
});
}

private static void DownLoad(string url,string filename)
{
using (WebClient wc = new WebClient())
{
wc.DownloadFile(url, filename);
}
}
}

For the other Concurrent Data Type, such as ConcurrentBag, ConcurrentStack, ConcurrentList, please check msdn for use.