C# Asynchronous IO: Is there a way of ensuring ordering of tasks?

I would like to use asynchronous io for socket communication with a distributed has开发者_如何转开发h server. Environment is C# 3.5, but could use 4.0 if necessary.

Suppose I issue the following asynchronous commands (in pseudocode):

socket.set_asynch("FOO","bar");
string rc = socket.get_asynch("FOO");

Since asynchronous io uses the system thread pool, the two commands may be run on two different threads. How can I ensure that rc equals "bar"? i.e. that the first command is issued before the second command is issued?

Thanks!

You could design your API around the Task<T> class (Task Parallel Library):

class DistributedHashTable
{
    public Task SetAsync(string key, string value);

    public Task<string> GetAsync(string key);
}

Both methods would use asynchronous IO to perform the respective operation, and set the returned Task<T> to completed when the operation has finished.

Then you could use your class synchronously like this:

var dht = new DistributedHashTable(...);

dht.SetAsync("FOO", "bar").Wait();

Console.WriteLine(dht.GetAsync("FOO").Result);

or asynchronously like this:

var dht = new DistributedHashTable(...);

dht.SetAsync("FOO", "bar")
   .ContinueWith(t => dht.GetAsync("FOO"))
   .Unwrap()
   .ContinueWith(t => {
                          Console.WriteLine(t.Result);
                      });

or asynchronously using the Async CTP like this:

var dht = new DistributedHashTable(...);

await dht.SetAsync("FOO", "bar");

Console.WriteLine(await dht.GetAsync("FOO"));