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Preamble: it's a self-assigned and pure syntetic task to learn (and remember what I already knew) C# threads and synchronization and data structures.

The original question was here http://stackoverflow.com/questions/16458937/synchronization-of-remote-files-download

The short task explanation:

Let's say I have a dictionary <string, string> that represents a path (http) to a file by some key, ie:

foo => http://domain.tld/file1
bar => http://domain2.tld/file2

And I'd like to implement a class that will implement an interface with 2 methods:

String Rand();
String Get(String key);

The first method would pick the file randomly from all the available, and the Get would return a particular file, or to be precise - local path to a downloaded file.

The class should be thread-safe, so that if several threads request the same key with Get() or the Rand() picks the same item - then only one thread should actually download a file to a local drive, or the path should be retrieved immediately if a file has already been downloaded.

My proof of concept solution is:

class Downloader
{
 private readonly IDictionary<string, string> _map;
 private IDictionary<string, string> _storage = new Dictionary<string, string>();
 private ConcurrentDictionary<string, Task<string>> _progress = new ConcurrentDictionary<string,Task<string>>();
 public Downloader(IDictionary<string, string> map)
 {
 _map = map ?? new Dictionary<string, string>();
 }
 public string Get(string key)
 {
 if (_storage.ContainsKey(key))
 {
 return _storage[key];
 }
 Task<string> task;
 if (_progress.TryGetValue(key, out task))
 {
 return task.Result;
 }
 task = _retrieveFile(key);
 if (!_progress.TryAdd(key, task))
 {
 return Get(key);
 }
 task.Start();
 _storage[key] = task.Result;
 return task.Result;
 }
 private Task<string> _retrieveFile(string key)
 {
 string path;
 if (!_map.TryGetValue(key, out path))
 {
 throw new ArgumentException("The specified key wasn't found");
 }
 return new Task<string>(k =>
 {
 Console.WriteLine("Started retrieving {0}", k.ToString());
 System.Threading.Thread.Sleep(3000);
 Console.WriteLine("Finished retrieving {0}", k.ToString());
 return k.ToString() + " local path";
 }, path);
 }
}

The copy is at http://pastebin.com/LJnK7FB6

How could I make it better? Any particular problems with the code?

In the comments @I4V pointed out to some problems at http://stackoverflow.com/questions/16458937/synchronization-of-remote-files-download#comment23617420_16460673 but seems like I'm missing the point