twzheng's cppblog

The last two Nuts and Bolts articles focused on database related topics. This month we'll change gears and focus on something different. The focus of this month's article will be .NET remoting. We'll discuss what is .NET Remoting, how it compares to DCOM and Web services, and go through an example.

What is .NET Remoting?

.NET Remoting is an enabler for application communication. It is a generic system for different applications to use to communicate with one another. .NET objects are exposed to remote processes, thus allowing interprocess communication. The applications can be located on the same computer, different computers on the same network, or even computers across separate networks.

.NET Remoting versus Distributed COM

In the past interprocess communication between applications was handled through Distributed COM, or DCOM. DCOM works well and the performance is adequate when applications exist on computers of similar type on the same network. However, DCOM has its drawbacks in the Internet connected world. DCOM relies on a proprietary binary protocol that not all object models support, which hinders interoperability across platforms. In addition, have you tried to get DCOM to work through a firewall? DCOM wants to communicate over a range of ports that are typically blocked by firewalls. There are a ways to get it to work, but they either decrease the effectiveness of the firewall (why bother to even have the firewall if you open up a ton of ports on it), or require you to get a firewall that allows support for binary traffic over port 80.

.NET Remoting eliminates the difficulties of DCOM by supporting different transport protocol formats and communication protocols. This allows .NET Remoting to be adaptable to the network environment in which it is being used.

.NET Remoting versus Web Services

Unless you have been living in a cave, or are way behind in your reading, you have probably read something about Web services. When you read the description of .NET Remoting it may remind you a lot of what you're read about Web services. That is because Web services fall under the umbrella of .NET Remoting, but have a simplified programming model and are intended for a wide target audience.

Web services involve allowing applications to exchange messages in a way that is platform, object model, and programming language independent. Web services are stateless and know nothing about the client that is making the request. The clients communicate by transferring messages back and forth in a specific format known as the Simple Object Access Protocol, or SOAP. (Want to get some funny looks in the hallway? Stand around in the hallway near the marketing department with your colleagues and discuss the benefits of using SOAP).

The following list outlines some of the major differences between .NET Remoting and Web services that will help you to decide when to use one or the other:

• ASP.NET based Web services can only be accessed over HTTP. .NET Remoting can be used across any protocol.
  
• Web services work in a stateless environment where each request results in a new object created to service the request. .NET Remoting supports state management options and can correlate multiple calls from the same client and support callbacks.
  
• Web services serialize objects through XML contained in the SOAP messages and can thus only handle items that can be fully expressed in XML. .NET Remoting relies on the existence of the common language runtime assemblies that contain information about data types. This limits the information that must be passed about an object and allows objects to be passed by value or by reference.
  
• Web services support interoperability across platforms and are good for heterogeneous environments. .NET Remoting requires the clients be built using .NET, or another framework that supports .NET Remoting, which means a homogeneous environment.

Channels

Remote objects are accessed through Channels. Channels physically transport the messages to and from remote objects. There are two existing channels TcpChannel and HttpChannel. Their names give away the protocols that they use. In addition, the TcpChannel or HttpChannel can be extended, or a new channel created if you determine the existing channels do not meet your needs.

Create a Remotable Object

A remotable object is nothing more than an object that inherits from MarshalByRefObject. The following sample demonstrates a simple class to expose the omnipresent hello world. This object exposes a single method HelloWorld that will return a string. The only values that can be returned from methods are the classes in the .NET Framework that are serializable such as string and DataSet. In addition, if you need to return a user-defined object then the object needs to be marked as serializable.

Create a new C# class library project. Add a class called SampleObject and put in the following code. Add a reference to System.Runtime.Remoting in the project, otherwise the TcpChannel will not be found. Compile the class to make sure you have everything correct.

using System;
using System.Runtime.Remoting;
using System.Runtime.Remoting.Channels;
using System.Runtime.Remoting.Channels.Tcp;
namespace CodeGuru.Remoting
{
/// <remarks>
/// Sample object to demonstrate the use of .NET Remoting.
/// </remarks>
public class SampleObject : MarshalByRefObject
{
/// <summary>
/// Constructor
/// </summary> 
public SampleObject()
{
}

/// <summary>
/// Return a hello message
/// </summary>
/// <returns>Hello world message</returns>
public string HelloWorld()
{
return "Hello World!";
}
}
}

Create a Server To Expose the Remotable Object

We need to create a server object that will act as a listener to accept remote object requests. For this example we will use the TCP/IP channel. We first create an instance of the channel and then register it for use by clients at a specific port. The service can be registered as WellKnownObjectMode.SingleCall, which results in a new instance of the object for each client, or as WellKnownObjectMode.Singleton, which results in one instance of the object used for all clients.

It is not necessary to create the server listener if you are planning to use IIS. For obvious reasons, IIS only supports the use of the HttpChannel. Create a virtual directory for your application and then put code to register your service in the Application_Start event.

For our example, we'll go ahead and create a server listener in case you don't have IIS. Since the service needs to be bound to an available port, for our example I chose 8080, which is a port that I know to be unused on my computer. You may need to choose a different port depending upon what ports you have available. To see a list of the used ports on your computer open a command prompt and issue the command "netstat --a". It may produce a long listing so make sure the command prompt buffer sizes are set to allow scrolling. Compile the class to make sure you have everything correct.

Create a new C# console application project. Add a class called SampleServer and paste in the following code. Add a reference to System.Runtime.Remoting in the project, otherwise the TcpChannel will not be found. In addition, add a reference to the project containing the SampleObject, otherwise the code will not compile because it won't know how to find a reference to SampleObject.

using System;
using System.Runtime.Remoting;
using System.Runtime.Remoting.Channels;
using System.Runtime.Remoting.Channels.Tcp;
namespace CodeGuru.Remoting
{
/// <remarks>
/// Sample server to demonstrate the use of .NET Remoting.
/// </remarks>
public class SampleServer
{
public static int Main(string [] args)
{
// Create an instance of a channel
TcpChannel channel = new TcpChannel(8080);
ChannelServices.RegisterChannel(channel);
// Register as an available service with the name HelloWorld
RemotingConfiguration.RegisterWellKnownServiceType(
typeof(SampleObject),
"HelloWorld",
WellKnownObjectMode.SingleCall );
System.Console.WriteLine("Press the enter key to exit...");
System.Console.ReadLine();
return 0;
}
}
}
Create a Client To Use the Remotable Object
Now that we have our remotable object and a server object to listen for requests, let's create a client to use it. Our client will be very simple. It will connect to the server, create an instance of the object using the server, and then execute the HelloWorld method. 
Create a new C# console application project. Add a class called SampleClient and paste in the following code. Add a reference to System.Runtime.Remoting in the project, otherwise the TcpChannel will not be found. In addition, add a reference to the project containing the SampleObject, otherwise the code will not compile because it won't know how to find a reference to SampleObject. Compile the class to make sure you have everything correct. 
using System;
using System.Runtime.Remoting;
using System.Runtime.Remoting.Channels;
using System.Runtime.Remoting.Channels.Tcp;
namespace CodeGuru.Remoting
{
/// <remarks>
/// Sample client to demonstrate the use of .NET Remoting.
/// </remarks>
public class SampleClient
{
public static int Main(string [] args)
{
// Create a channel for communicating w/ the remote object
// Notice no port is specified on the client
TcpChannel chan = new TcpChannel();
ChannelServices.RegisterChannel(chan);
// Create an instance of the remote object
SampleObject obj = (SampleObject) Activator.GetObject(
typeof(CodeGuru.Remoting.SampleObject),
"tcp://localhost:8080/HelloWorld" );
// Use the object
if( obj.Equals(null) )
{
System.Console.WriteLine("Error: unable to locate server");
}
else
{
Console.WriteLine(obj.HelloWorld());
}
return 0;
}
}
}

Test the Remoting Sample
Once you have created the projects and successfully compiled each of them you are ready to try it out. Assuming you chose a free TCP/IP port for the service, start the server executable. After the server successfully starts it will result in a console window being displayed with the message "Press the enter key to exit". The server is listening so you are now ready to run the client. Executing the client should result in "Hello World!" being displayed in a separate console window. The client window will then close while the server remains open and available. 
If you have multiple computers available to you on a network you could execute the server on one machine and the client on another just to prove to yourself that it really is remoting. In order to run on separate machines you would need to change the reference to localhost in the sample client to point to the appropriate location. 
Summary
.NET Remoting is a powerful way to enable interprocess communication. It is more complicated to program against than Web services. You need to decide for yourself whether your standard architecture is to use .NET Remoting or Web services. 
Future Columns
The next column will be on the use of encryption in the .NET framework. We'll take a look at some of the encryption algorithms available in the framework and ways to use them. If you have a particular topic in mind please email me at mstrawmyer@crowechizek.com
About the Author
Mark Strawmyer, MCSD, MCSE (NT4/W2K), MCDBA is a Senior Architect of .NET applications for large and mid-size organizations. Mark is a technology leader with Crowe Chizek in Indianapolis, Indiana. He specializes in architecture, design and development of Microsoft-based solutions. You can reach Mark at mstrawmyer@crowechizek.com.