Avoiding Problems with the Using Statement and WCF Service Proxies

Artikel
03/18/2010

I encountered a rather nasty issue yesterday on my current project -- a customer portal built on Microsoft Office SharePoint Server (MOSS) 2007 that integrates with multiple external systems via Web services.

The database indexes for one of the external systems that we use to display data on the portal somehow got corrupted. Consequently we encountered an error when trying to view the corresponding portal page.

Fortunately, this didn't happen in the Production environment -- only the Test environment -- and therefore only the Development and Test teams were effected (no actual customers).

Unfortunately, the crux of the issue was not readily apparent due to an issue with the proxy class used to interface with the Web service.

Being good .NET 3.0 citizens, we are using Windows Communication Foundation (WCF) to generate the service proxies for accessing various Web services. In other words, we are using "Service References" instead of the older "Web References" from the original days of the .NET Framework.

Being good Web developers, we also try to hide the details about errors from end users and instead log the exception details to the Windows event log and show a generic error message on the page where the error occurred.

Shortly after starting to troubleshoot the problem, I discovered the following error in the event log:

The communication object, System.ServiceModel.Channels.ServiceChannel, cannot be used for communication because it is in the Faulted state.

I also noticed from the stack trace that the error occurred while calling the Dispose method of the Web service proxy class.

After a quick Internet search, I discovered that there's a well-known problem with WCF service proxies and the using statement in C#. [It obviously wasn't well-known to me until yesterday.]

There are a number of blog posts that describe the issue as well as various hacks around the problem. However, the following MSDN article is the one that really captured my attention:

Avoiding Problems with the Using Statement

https://msdn.microsoft.com/en-us/library/aa355056.aspx

Here's the first sentence from that article:

This sample demonstrates how you should not use the C# "using" statement to automatically clean up resources when using a typed client.

Huh?

I have to admit, I was completely dumbfounded by this statement. An MSDN article telling me that I should not use the C# "using" statement on an object that implements IDisposable?

What's that old movie quote about "dogs and cats living together" and "mass hysteria"? According to my calendar, it's definitely still March -- so no April Fool's joke here.

So, seriously, what gives? We are using "using" statements throughout our solution. In my mind, this is a best practice. Heck, I even blogged once about how I'm still waiting for that Utopian FxCop rule that barks at you whenever an object that implements IDisposable is not wrapped in a using block.

That's when it occurred to me: "Here we go again..."

It seems -- at least in my mind -- that there's a bug in System.ServiceModel.ClientBase, and instead of admitting it's a bug and fixing the fundamental problem (which I know can be painful), some "prescriptive guidance" is put out on MSDN on how to workaround the bug.

Honestly, I can't see how this is any different than not properly freeing memory in IDisposables containing IDisposables and instead forcing the caller to cleanup the memory.

The problem -- at least in my opinion -- is that the Dispose method in ClientBase is currently implemented as follows:

         void IDisposable.Dispose()
        {
            this.Close();
        }

However, as noted in the above MSDN article (and corresponding code sample), this can cause problems when the C# "using" statement is used on the WCF service proxy:

         using (CalculatorClient client = new CalculatorClient())
        {
            // Call Divide and catch the associated Exception.  This throws because the
            // server aborts the channel before returning a reply.
            try
            {
                client.Divide(0.0, 0.0);
            }
            catch (CommunicationException e)
            {
                Console.WriteLine("Got {0} from Divide.", e.GetType());
            }
        }

        // The previous line calls Dispose on the client.  Dispose and Close are the
        // same thing, and the Close is not successful because the server Aborted the
        // channel.  This means that the code after the using statement does not run.
        Console.WriteLine("Hope this code wasn't important, because it might not happen.");

The problem is that if an error occurs while calling the Web service, then when this.Close() is subsequently called from the Dispose method, a CommunicationObjectFaultedException is thrown ("The communication object, System.ServiceModel.Channels.ServiceChannel, cannot be used for communication because it is in the Faulted state.").

The recommendation -- at least according to the above MSDN article -- is to forego the C# "using" statement and instead rely on try/catch blocks (and explicitly calling client.Close() or client.Abort() where appropriate. [Excuse me while I go get a drink, because I've got this really bad taste in my mouth right now.]

I completely understand the recommendation to catch TimeoutExceptions and CommunicationExceptions when calling Web services. However, that shouldn't necessarily mandate that I cleanup the service proxy in various catch blocks -- or a finally block. If a class implements IDisposable, then I want to call Dispose() on it! [Well, actually, I don't want to explicitly call Dispose() -- what I really want is to wrap the object in a using block.]

Wouldn't it be great if ClientBase actually implemented the Dispose method as follows?

         void IDisposable.Dispose()
        {
            if (this.State == CommunicationState.Faulted)
            {
                this.Abort();
            }
            else if (this.State != CommunicationState.Closed)
            {
                this.Close();
            }
        }

Wouldn't this avoid the CommunicationObjectFaultedException that occurs when the service proxy is wrapped in a using block, but an exception occurs when calling the Web service?

Let's see...

How about creating a new "wrapper" class for the WCF service proxy?

 using System;
using System.ServiceModel;

namespace Microsoft.ServiceModel.Samples
{
    public class CalculatorClientWithDisposeFix : CalculatorClient, IDisposable
    {
        #region IDisposable Members

        void IDisposable.Dispose()
        {
            if (this.State == CommunicationState.Faulted)
            {
                this.Abort();
            }
            else if (this.State != CommunicationState.Closed)
            {
                this.Close();
            }
        }

        #endregion
    }
}

With a couple of tweaks to the WCF sample code -- specifically replacing all instances of "new CalculatorClient()" with "new CalculatorClientWithDisposeFix()" -- the output changes from this:

 =
= Demonstrating problem:  closing brace of using statement can throw.
=
Got System.ServiceModel.EndpointNotFoundException from Divide.
Got System.ServiceModel.CommunicationObjectFaultedException
=
= Demonstrating problem:  closing brace of using statement can mask other Exceptions.
=
Got System.ServiceModel.EndpointNotFoundException from Divide.
Got System.ServiceModel.CommunicationObjectFaultedException
=
= Demonstrating cleanup with Exceptions.
=
Calling client.Add(0.0, 0.0);
Got System.ServiceModel.EndpointNotFoundException from Divide.

...to this:

 =
= Demonstrating problem:  closing brace of using statement can throw.
=
Got System.ServiceModel.EndpointNotFoundException from Divide.
Hope this code wasn't important, because it might not happen.
=
= Demonstrating problem:  closing brace of using statement can mask other Exceptions.
=
Got System.ServiceModel.EndpointNotFoundException from Divide.
We do not come here because the ObjectDisposedException is masked.
=
= Demonstrating cleanup with Exceptions.
=
Calling client.Add(0.0, 0.0);
Got System.ServiceModel.EndpointNotFoundException from Divide.

The reality is that the WCF team might never fix this bug -- heck, for all I know, this behavior could be marked as "by design." However, I'd like to think that WCF service proxies will eventually be "fixed" (which is to say "function as I -- and many other .NET developers -- would expect them to). I suppose another option would be to remove the IDisposable interface from ClientBase altogether (but I certainly don't like that idea).

And so continues my neverending quest to make the world of software a simpler -- and happier -- place to live in ;-)

Comments

Anonymous
April 21, 2010
I am new to WCF (trial by fire actually) and I've learned alot in the past couple of months. I've also become intimate with all the code we've written and we have not written our proxy code to leverage the "using" statement. We are explicitly closing the proxy. It is likely that the code is like that because the original authors were unaware of the more elegant solution with the "using" statement. However, based on your analysis, I think Microsoft needs to put out a patch pronto and not force thousands of developers to go back into their WCF code and replace the "using" statement.

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Avoiding Problems with the Using Statement and WCF Service Proxies

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