Question

I am working on a async web request. and need to depends on the response to do a message return.

was thinking to do sth like following

// creating request

string messageToReturn = string.empty;
request.BeginGetResponse(ar => 
            { 
                HttpWebRequest req2 = (HttpWebRequest)ar.AsyncState; 
                var response = (HttpWebResponse)req2.EndGetResponse(ar);

                // is it safe to do this?
                messageToReturn = "base on respone, assign different message"; 


            }, request);

// will i get any response message? i will always get empty right?
// since response is handle in another thread
return messageToReturn;

what is the best way to do that?

Answer 1

You are right, that variable will always be empty because you fired off an asyncronous request with the BeginGetResponse method. So really you have a few options here. You can either block the executing thread until the response comes back (probably a really bad idea unless you have a very strong argument for doing this), or you could use an event based asynchronous pattern to alert callers when your response returns…

Consider some of your code wrapped in a method

public void GetMessageAsync()
{
    string messageToReturn = string.empty;
    request.BeginGetResponse(ar => 
            { 
                HttpWebRequest req2 = (HttpWebRequest)ar.AsyncState; 
                var response = (HttpWebResponse)req2.EndGetResponse(ar);

                // is it safe to do this?
                messageToReturn = "base on respone, assign different message"; 


            }, request);
}

To wire up an event based pattern here. We define a custom EventArgs class and a custom event which callers can listen for and which we will fire when the response comes back.

public class StringEventArgs : EventArgs
{
   public string Message { get; set; }
}

public event EventHandler<StringEventArgs> MessageReturned;

public void GetMessageAsync()
{
    //string messageToReturn = string.empty;
    request.BeginGetResponse(ar => 
            { 
                HttpWebRequest req2 = (HttpWebRequest)ar.AsyncState; 
                var response = (HttpWebResponse)req2.EndGetResponse(ar);

                //messageToReturn = "base on respone, assign different message"; 
                this.MessageReturned(this, new StringEventArgs { Message = response.ToString() });

            }, request);
}