Question

What would be the best way to do multithreading or asynchronous task in the following situation in C#?

The simplified situation:

A http request needs to make 5 or more
web service calls. Upon completion
each web service call will receive and
return a string list as a result. The
caller (of 5 web service calls) need
to merge the 5 results into a single
string list and return it to the http
caller.

Because each thread needs to return a value in the end so I am wondering if Asynchronous Delegates is the way to go. Because I am not so experienced in this area so I am asking this questions and/or suggestions.

Thank you!

Answer 1

You should have a look at QueueUserWorkItem. This would allow you to do each call on a separate thread and get the string value based on the particular call e.g.

ManualResetEvent[] calls = new ManualResetEvent[5];
string[] results = new string[5];

calls[0] = new ManualResetEvent(false);
ThreadPool.QueueUserWorkItem(t => 
{
    results[0] = // do webservice call
    calls[0].Set();
});

calls[1] = new ManualResetEvent(false);
ThreadPool.QueueUserWorkItem(t => 
{
    results[1] = // do webservice call
    calls[1].Set();
});

....
// wait for all calls to complete
WaitHandle.WaitAll(calls);
// merge the results into a comma delimited string
string resultStr = String.Join(", ", results);