Unlike its CLR counter part, Silverlight’s HttpWebRequest does not provide a synchronous GetResponse method that allows to receive an HTTP response in a blocking operation. As a result, you are forced to use the asynchronous BeginGetResponse method, and handle the results on a callback method.

This makes sense if the response is requested on the UI thread (otherwise, it would freeze the UI), but it might be a problem in certain scenarios (e.g. if you want to submit multiple requests in an orderly fashion).

However, you can get around the issue by using wait handles, which allow you to easily block a running operation. I encapsulated the required functionality in a simple extension method that brings GetResponse method back into Silverlight.

The usage is simple – you just invoke the GetResponse() extension method with an optional timeout. Here’s a sample that submits five synchronous HTTP requests:

private void RunRequests()
{
  for (int i = 0; i < 5; i++)
  {
    Uri uri = new Uri("http://localhost/users?user=" + i);
    HttpWebRequest request = 
      (HttpWebRequest)WebRequestCreator.ClientHttp.Create(uri);

    //get response as blocking operation which times out after 10 secs
    HttpWebResponse response = request.GetResponse(10000);
  }
}

You will run into timeout issues if you run execute this method on the UI thread because internally, the requested response accesses the UI thread (for whatever reasons). Only invoke this extension method on background threads!

Here’s the implementation:

public static class RequestHelper
{
  /// <summary>
  /// A blocking operation that does not continue until a response has been
  /// received for a given <see cref="HttpWebRequest"/>, or the request
  /// timed out.
  /// </summary>
  /// <param name="request">The request to be sent.</param>
  /// <param name="timeout">An optional timeout.</param>
  /// <returns>The response that was received for the request.</returns>
  /// <exception cref="TimeoutException">If the <paramref name="timeout"/>
  /// parameter was set, and no response was received within the specified
  /// time.</exception>
  public static HttpWebResponse GetResponse(this HttpWebRequest request,
                                            int? timeout)
  {
    if (request == null) throw new ArgumentNullException("request");

    if (System.Windows.Deployment.Current.Dispatcher.CheckAccess())
    {
      const string msg = "Invoking this method on the UI thread is forbidden.";
      throw new InvalidOperationException(msg);
    }

    AutoResetEvent waitHandle = new AutoResetEvent(false);
    HttpWebResponse response = null;
    Exception exception = null;

    AsyncCallback callback = ar =>
       {
         try
         {
           //get the response
           response = (HttpWebResponse)request.EndGetResponse(ar);
         }
         catch(Exception e)
         {
           exception = e;
         }
         finally
         {
           //setting the handle unblocks the loop below
           waitHandle.Set(); 
         }
       };


    //request response async
    var asyncResult = request.BeginGetResponse(callback, null);
    if (asyncResult.CompletedSynchronously) return response;

    bool hasSignal = waitHandle.WaitOne(timeout ?? Timeout.Infinite);
    if (!hasSignal)
    {
      throw new TimeoutException("No response received in time.");
    }

    //bubble exception that occurred on worker thread
    if (exception != null) throw exception;

    return response;
  }
}

This is just a little snippet that generates a random string, containing both upper and lower case characters and special chars, so there’s a range of 93 possible characters, taken from the UTF-8 character table. I thought I’d post it as the snippets I’ve seen after a quick search usually just generate strings with characters ranging from ‘A’ to ‘Z’, or do not work with properly initialized seeds.

Note that this method uses a static int field (seedCounter) when in comes to creating a random seed. This counter is incremented in a thread safe manner every time the method is being invoked. This simple trick is a simple workaround to the notoriously unreliable Random class, and effectively prevents double seeds (and thus: duplicate “random strings”) if the GetRandomString method is being invoked several times immediately. As an alternative, you could also use a static Random field, which would only have to be initialized once. My implementation has the smaller footprint (and integer variable), while a Random field would perform much better (currently, every invocation calculates a seed and creates a new Random instance). The better choice depends on the context I guess:

/// <summary>
/// A counter that is being incremented with every invocation of
/// <see cref="GetRandomString"/>.
/// </summary>
private static int seedCounter = (int)DateTime.Now.Ticks;


/// <summary>
/// Generates a random string of a given length, which consists
/// of characters taken from the UTF-8 table (0x21 - 0x7e).
/// </summary>
/// <param name="length">The length of the random string.</param>
/// <returns>Random characters.</returns>
public static string GetRandomString(int length)
{
  const int lower = 0x21;
  const int upper = 0x7e;

  StringBuilder builder = new StringBuilder();

  //increment thread-safe
  seedCounter = Interlocked.Increment(ref seedCounter);

  //create random with the seed (make sure it's not int.MinValue)
  Random rnd = new Random(seedCounter %int.MinValue);

  for (int i = 0; i < length; i++)
  {
    builder.Append((char) rnd.Next(lower, upper));
  }

  return builder.ToString();
}

Accordingly, invoking the method like this:

string random = GetRandomString(10);

…generates you a string comparable to this one: 2#,R`6>Cz{

I’m currently working on VFS, a virtual file system. For running transfers, VFS internally maintains locks that do have expiration time. Accordingly, I found myself in need for a job scheduling mechanism in order to properly release expired locks. I looked around for a few alternatives, but eventually ended up writing my own lightweight version.

Features:

• Simple scheduling and callback mechanisms
• Silverlight compatible
• Lightweight
• Fluent API
• Detection of system time changes with optional rescheduling
• Optional forwarding of exceptions during job execution
• Open Source (Ms-PL)

Download library and sample

Current Version: 1.0.2, 2010.01.12

What Does it Do?

Basically, the library allows you to create a job, and submit that job to a scheduler, along with a callback action. This callback action is invoked as soon (or every time) the job is due.

Before going into the details, here’s a first code snippet that creates a simple Job that is supposed to run repeatedly (every 1.5 seconds) for a minute. Once the job is created, it is submitted to a Scheduler instance which processes the job and makes sure the submitted callback action is being invoked every time the job is due:

private Scheduler scheduler = new Scheduler();

public void RunOnce()
{
  //define a start / end time
  DateTime startTime = DateTime.Now.AddSeconds(5);
  DateTime endTime   = startTime.AddSeconds(60);

  //configure the job
  Job consoleJob = new Job();
  consoleJob.Run.From(startTime)
                .Every.Seconds(1.5)
                .Until(endTime);

  //submit the job with the callback to be invoked
  scheduler.SubmitJob(consoleJob, j => Console.Out.WriteLine("hello world"));
}

Silverlight

The project provides class libraries for .NET 3.5 and Silverlight 3, along with a Silverlight sample application that shows how to add scheduling functionality to your SL application with just a few lines of code.

While long-term scheduling isn’t probably something you need to do in a Silverlight application, the scheduler simplifies the management of periodic jobs, such as polling a server for updates. Below is a snippet from the Silverlight sample application. This job starts immediately, and runs indefinitely with an interval of 2 seconds:

private void CreateJob2()
{
  //create job
  Job<int> job = new Job<int>("Job 2");
  job.Data = 0;
  job.Run.Every.Seconds(2);

  //submit to scheduler
  scheduler.SubmitJob(job, LogJobExecution);
}


private void LogJobExecution(Job<int> job, int data)
{
  //updates user interface
}

Jobs

A job is a simple configuration item for the scheduler. There’s two built-in job types: Job and Job<T>. The only difference between the two is that the latter provides a Data property which allows you to attach state information directly to the job and have it delivered back to you when the job runs.

Read more…

I’ve been working on a new project of mine for a while now, and opened the project at CodePlex: VFS, the Virtual File System, is basically an abstraction to arbitrary hierarchical resources, which can be handled like a file system.

VFS comes with a set of file system providers and clients (including Silverlight support), and allows you to easily write your own providers (e.g. to expose the contents of a ZIP file on a remote server). Additional offerings are flexible security (e.g. to access your Azure blobs via VFS in order to plug-in a custom authorization scheme), auditing and reliable file transfers.

VFS is currently in early Alpha, but I’m working full steam ahead. Comments and wish lists are appreciated! For a more detailed introduction and a few code snippets, visit the project home:
http://vfs.codeplex.com

Interoperability in WCF is a great thing, but it requires us to rethink our exception handling strategy: The simple paradigm of throwing exceptions whenever something goes wrong doesn’t cut it with distributed systems.

Thankfully, WCF allows us to communicate errors (or faults) through FaultExceptions. We can just convert internal exceptions into a FaultException, and WCF will take care about everything else. However: This conversion strategy is tedious and error prone, especially if you want to do more sophisticated exception handling. Even a simplified example as the one below mainly consists of exception handling code:

//Simplified example
public string Foo(string userName)
{
  try
  {
     Bar.FooBar(userName);
  }
  catch(ArgumentException e)
  {
    //convert exception into a WCF friendly FaultException
    throw new FaultException("Info about specific error");
  }
  catch(Exception e)
  {
    //convert exception into a WCF friendly FaultException
    throw new FaultException("Info about unspecific error");
  }
}

I’m currently finding myself having to write a set of WCF services, and I didn’t want to go through try/catch blocks all the time just in order to wrap up the same exceptions over and over again. Accordingly, I wrote helper methods that do the exception handling for me.

• SecureAction is used by service methods that do not have a return value.
• SecureFunc<T> is used by service methods that do have a return value.

These helper methods allow me to easily delegate the exception handling from all my service methods, and if anything goes wrong, I can trust that the internal exception is safely converted into a FaultException. Here’s is the rewritten Foo method from above, now remarkably smaller:

public string Foo(string userName)
{
  return SecureFunc(() => Bar.FooBar(userName));
}

Below is the (simplified) snippet with the helper methods. Do note that the above sample methods are just starting points. My productive helper methods provide a more sophisticated exception handling than shown above (they build detailed FaultContracts which are specific to my application) and come with a few overloads.

Furthermore, you might think about adding additional parameters to your helper methods (such as fault IDs) in order to customize the generated faults.

public static void SecureAction(Action action)
{
  try
  {
    action();
  }
  catch (Exception e)
  {
    //productive code provides more specific
    //exception handling, but you should get the idea...
    throw new FaultException(e.Message);
  }
}

public static T SecureFunc<T>(Func<T> func)
{
  try
  {
    return func();
  }
  catch (Exception e)
  {
    //productive code provides more specific
    //exception handling, but you should get the idea...
    throw new FaultException(e.Message);
  }
}

I just released a minor update for NetDrives, which fixes an issue that caused settings not to be persisted correctly. NetDrives is a Windows utility that helps you manage your network shares and mapped network drives.

Download and further infos: http://www.hardcodet.net/netdrives