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 https://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 https://stackoverflow.com/questions/16458937/synchronization-of-remote-files-download#comment23617420_16460673 but seems like I'm missing the point
