Sri Lanka Institute of Information Technology System Programming and Design II Year 3 Tutorial 06

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1 Sri Lanka Institute of Information Technology System Programming and Design II Year 3 Tutorial What is an asynchronous call? How does it differ from a synchronous call? In synchronous call, the caller must wait until the called function completes and returns (The caller is blocked until the called function returns). Serial processing. In asynchronous call, the caller is not blocked when an function is being invoked. Invoked function executes in the background and later it'll be returned back to the caller with the results. Parallel processing. 2. What are the three methods of.net to provide asynchronous communication? Blocking and polling with delegates Completion callbacks with delegates Background thread with function call 3. Describe the usage of.net delegates and how they are used. Use example C# code as part of your explanation. Delegates are like function pointers in C, C++. But unlike those function pointers, delegates are object oriented, type safe and secure. Using a delegate we can encapsulate a reference to a method. So instead of calling the original method, we can call the delegate and start executing the method and take the results back.

2 public delegate double calctotaldelegate(); calctotaldelegate del; IAsyncResult res; public double calculatebalance() //implementation of the method public void asynccalculatebalance() del = calculatebalance; res = del.begininvoke(null, null); public double getasyncbalance() if (!res.iscompleted) return 0.0; else double balance = del.endinvoke(res); return balance; 4. What is the purpose of the BeginInvoke() and EndInvoke() functions provided by every.net delegate? BeginInvoke() is used to initiate an asynchronous method invocation. EndInvoke() is used to retrieve out-going parameters and return values. 5. Suppose you have a method called gettotal() in a remote server, which calculates the total of accounts and return results as a double value. Write the coding required if you want this method to be called asynchronously, a. Using blocking approach, b. Using polling approach, c. Using completion callback approach and print the answer. (Consider scenarios where the asynchronous code is with either the client or the server)

3 a. balancedelegate del; IAsyncResult isync; public void calltotalreomtely() deli = srv.gettotal; async = del.begininvoke(null, null); public double gettot() double tot = deli.endinvoke(async); return tot; b. balancedelegate del; IAsyncResult isync; public void calltotalreomtely() del = srv.gettotal; async = del.begininvoke(null, null); public double gettot() if (!async.iscompleted) Console.WriteLine("still executing"); return 0.0; else double tot = deli.endinvoke(async); return tot; c. Server Side public void calltotalreomtely(callbacks c) balancedelegate del = srv.gettotal; AsyncCallback async = OnCompletionTotal; IAsyncResult res = del.begininvoke(async,c);

4 public void OnCompletionTotal(IAsyncResult res) AsyncResult asyncres = (AsyncResult)res; CalcTotalDelegate initialdel = (CalcTotalDelegate)asyncRes.AsyncDelegate; //through asyncres we're accesssing the balancedelegate del deligate //gets the deligate object which called the BeginInvoke Callbacks obj = (Callbacks)asyncRes.AsyncState; //Gets the Callbacks object which was passed as the last parameter to the BeginInvoke (here c) double tot = initialdel.endinvoke(res); //Take the result from the delegate object obj.oncalctotal(result); //call the callback method in the client through obj Callbacks object Client Side public class Callbacks : MarshalByRefObject public void OnCalcTotal(double tot) MessageBox.Show("Total = "+ tot); 6. What are the advantages and disadvantages of blocking and polling and completion callbacks? Blocking advantages All thread synchronization performed for you by.net Very little extra coding over a normal synchronous call Simple: no callback function involved Blocking disadvantages Will block calling thread if async call hasn t finished by the time EndInvoke() is called May cause GUI to freeze if the thread is the event loop thread Polling advantages All thread synchronization performed for you by.net Simple: no callback function involved Polling disadvantages Inefficient use of CPU to repeatedly check if call is complete takes CPU time away from useful processing Requires coding to periodically check (poll) if complete Completion Callback advantages Will be notified as soon as async call is finished This avoids unnecessary waiting on the thread (blocking) and resource wasting (polling)

5 Completion Callback disadvantages Complex: requires a second function as the callback Callback occurs on a different thread, so user must perform thread synchronization themselves