Question

I have a page that needs to combine data from four different webrequests into a single list of items. Currently, I’m running these sequentially, appending to a single list, then binding that list to my repeater.

However, I would like to be able to call these four webrequests asynchronously so that they can run simultaneously and save load time. Unfortunately, all the async tutorials and articles I’ve seen deal with a single request, using the finished handler to continue processing.

How can I perform the four (this might even increase!) simultaneously, keeping in mind that each result has to be fed into a single list?

many thanks!

EDIT: simplified example of what i’m doing:

var itm1 = Serialize(GetItems(url1));
list.AddRange(itm1);
var itm2 = Serialize(GetItems(url2));
list.AddRange(itm2); 

string GetItems(string url)
{
     var webRequest = WebRequest.Create(url) as HttpWebRequest;
     var response = webRequest.GetResponse() as HttpWebResponse;

     string retval;
     using (var sr = new StreamReader(response.GetResponseStream()))
     { retval = sr.ReadToEnd(); }
     return retval;
} 

Answer 1

This should be really simple since your final data depends on the result of all the four requests.

What you can do is create 4 async delegates each pointing to the appropriate web method. Do a BeginInvoke on all of them. And then use a WaitHandle to wait for all. There is no need to use call backs, in your case, as you do not want to continue while the web methods are being processed, but rather wait till all web methods finish execution.

Only after all web methods are executed, will the code after the wait statement execute. Here you can combine the 4 results.

Here’s a sample code I developed for you:

class Program
    {
        delegate string DelegateCallWebMethod(string arg1, string arg2);

        static void Main(string[] args)
        {
            // Create a delegate list to point to the 4 web methods
            // If the web methods have different signatures you can put them in a common method and call web methods from within
            // If that is not possible you can have an List of DelegateCallWebMethod
            DelegateCallWebMethod del = new DelegateCallWebMethod(CallWebMethod);

            // Create list of IAsyncResults and WaitHandles
            List<IAsyncResult> results = new List<IAsyncResult>();
            List<WaitHandle> waitHandles = new List<WaitHandle>();

            // Call the web methods asynchronously and store the results and waithandles for future use
            for (int counter = 0; counter < 4; )
            {
                IAsyncResult result = del.BeginInvoke("Method ", (++counter).ToString(), null, null);
                results.Add(result);
                waitHandles.Add(result.AsyncWaitHandle);
            }

            // Make sure that further processing is halted until all the web methods are executed
            WaitHandle.WaitAll(waitHandles.ToArray());

            // Get the web response
            string webResponse = String.Empty;
            foreach (IAsyncResult result in results)
            {
                DelegateCallWebMethod invokedDel = (result as AsyncResult).AsyncDelegate as DelegateCallWebMethod;
                webResponse += invokedDel.EndInvoke(result);
            }
        }


        // Web method or a class method that sends web requests
        public static string CallWebMethod(string arg1, string arg2)
        {
            // Code that calls the web method and returns the result

            return arg1 + " " + arg2 + " called\n";
        }

    }