Implementing IDispatchMessageInspector

Introduction

Have you ever needed to change the contract used by WCF service, but were held back by the impact it would have on existing clients? Sure, you could just ask your clients to update their service references and tweak the code which calls your service, but that requires that all your clients (of which there might be many) to do a code release on the same day you release your new server - not ideal.

One way to resolve this would be to leave the existing contract alone, but to create a second modified contract exposed via a new endpoint. That way clients could upgrade to the new contract (and the new endpoint) at their leisure. This will probably create some redundancy in the server-side code, as you'd have very similar code at both endpoints, although you could probably refactor any common code such that it was shared. There's also a risk that this approach would lead to end-point proliferation over time as you make various tweaks to the contract. So, is there an an alternative?

One approach which I used recently, and which is the subject of this post, is to intercept the calls which use the old contract and transform them into a call to the new contract on-the-fly. WCF has an extensibility interface designed just for this purpose - IDispatchMessageInspector. Note that, despite its name, implementing this interface allows us to both inspect and modify inbound and outbound messages.

To demonstrate this approach I'm going to:

• create a WCF Service exposing a particular interface,
• create a client which consumes this service,
• modify the service in a manner which is not backwards compatible,
• show that the client can no longer consume the service,
• implement IDispatchMessageInspector in the service to modify calls on the fly, and
• show that the client can now consume the service again.

Creating the WCF Service

I'm not going to go through this step-by-step. If you're interested in implementing IDispatchMessageInspector to modify WCF messages on the fly, then I'm sure you'll have created a WCF service or two in your time. So I'll just be providing source code listing of the various files:

• IEmployeeService (the service contract)
• EmployeeService (an implementation of that contract)
• Employee (a data contract exposed via the service)
• Server (contains the console application's entry point and service host provider)

IEmployeeService

  using System.ServiceModel;

  namespace WcfService
  {
    [ServiceContract]
    public interface IEmployeeService
    {
      [OperationContract]
      void SaveEmployee(Employee employee);
    }
  }

EmployeeService

  using System;

  namespace WcfService
  {
    public class EmployeeService : IEmployeeService
    {
      public void SaveEmployee(Employee employee)
      {
        Console.WriteLine("Saved {0}", employee);
      }
    }
  }

Employee

  using System;
  using System.Runtime.Serialization;

  namespace WcfService
  {
    [DataContract]
    public class Employee
    {
      [DataMember]
      public string Name { get; set; }

      [DataMember]
      public DateTime DateOfBirth { get; set; }

      [DataMember]
      public char Gender { get; set; }

      public override string ToString()
      {
        return string.Format("{0} (DOB:{1} Gender:{2})", Name, DateOfBirth.ToShortDateString(), Gender);
      }
    }
  }

Server

  using System;
  using System.ServiceModel;
  using System.ServiceModel.Description;

  namespace WcfService
  {
    class Server
    {
      static void Main()
      {
        // create a host and add an end-point to it
        ServiceHost serviceHost = new ServiceHost(typeof(EmployeeService));
        ContractDescription contractDescription = ContractDescription.GetContract(typeof(IEmployeeService), typeof(EmployeeService));
        NetTcpBinding netTcpBinding = new NetTcpBinding();
        EndpointAddress endpointAddress = new EndpointAddress("net.tcp://localhost:8001/EmployeeService");
        ServiceEndpoint serviceEndpoint = new ServiceEndpoint(contractDescription, netTcpBinding, endpointAddress);
        serviceHost.Description.Endpoints.Add(serviceEndpoint);

        // add a meta-data end-point
        ServiceMetadataBehavior serviceMetadataBehavior = new ServiceMetadataBehavior();
        serviceMetadataBehavior.HttpGetEnabled = true;
        serviceMetadataBehavior.HttpGetUrl = new Uri("http://localhost:8002/EmployeeService/mex");
        serviceHost.Description.Behaviors.Add(serviceMetadataBehavior);

        // open the host
        serviceHost.Open();
        Console.WriteLine("Press ENTER to terminate service.");

        // close the host once the user hits ENTER
        Console.ReadLine();
        serviceHost.Close();
      }
    }
  }

Okay, so that the server-side code sorted. As you can see it's all fairly basic stuff: the service merely accepts an Employee into the SaveEmployee method of its IEmployeeService and writes details of that Employee to the console.

Creating the WCF Client

The client is just as trivial.

Continue reading this post here »

Serialising Enumerations with WCF

Introduction

There are occasions when the exceptions thrown by Windows Communication Foundation (WCF) are clear and unambiguous. There are also occassions where they are not. In this post I'll describe a real-world problem which a colleague encountered recently together which the steps which you might reasonably perform to diagnose it.

The problem manifested itself in a System.ServiceModel.CommunicationException being thrown with nothing particularly useful in the Exception message itself.

System.ServiceModel.CommunicationException: An error occurred while receiving th
e HTTP response to http://localhost:8731/Design_Time_Addresses/WcfServiceLibrary
/Service1/. This could be due to the service endpoint binding not using the HTTP
 protocol. This could also be due to an HTTP request context being aborted by th
e server (possibly due to the service shutting down). See server logs for more d
etails. ---> System.Net.WebException: The underlying connection was closed: An u
nexpected error occurred on a receive. ---> System.IO.IOException: Unable to rea
d data from the transport connection: An existing connection was forcibly closed
 by the remote host. ---> System.Net.Sockets.SocketException: An existing connec
tion was forcibly closed by the remote host

Clearly something very bad happened. But the WCF service itself was still up and running, so one of the theories put forward in the message above "possibly due to the service shutting down" can be quickly dismissed. I'm sure it's obvious, but the above message was generated by my mock-up of the problem rather than the original problem itself.

Mock-up of Original Problem

Let me quickly run through the mock-up of the problem which I put together. It consists of three projects: WcfServiceLibrary contains a single WCF service called Service1, WcfConsoleHost provides a host for Service1 in the form of a Console Application, and WcfClient consumes Service1.

Continue reading this post here »

System.Data Serialisation Bug - Scenario 2

Introduction

In my last post I explained that System.Data contains a bug which can cause it to throw an exception during serialisation or deserialisation of a DataSet which contains a column that uses System.Object as its .NET type.

The bug manifests itself in two known scenarios:

• Serialising a DataSet containing a column of type System.Object using a System.Xml.XmlTextWriter instance created via the static Create method; this throws a System.ArgumentException.
• Deserialising a DataSet containing a column of type System.Object after having successfully passed it across the wire via Windows Communication Foundation (WCF) using netTcp binding; this throws a System.Data.DataException.

This post covers the second of the two scenarios – deserialising a DataSet which has been successfully transmitted via WCF's netTcp binding.

System.Data.DataException

A couple of years ago I was asked by a client to port an existing ASMX-based Web Service to one using Windows Communication Foundation. The brief was to change as little of the client and service as possible, and to just replace the communications interface. In addition to passing simple and complex types across the wire, the application would often pass both typed and untyped DataSets (clearly a bad idea, but that's another story). One of the typed DataSets was based upon a SQL Server 2005 table containing a column of type SQL_VARIANT. Attempting to pass an instance of this DataSet from server to client produced the following exception on the client (i.e. during deserialisation):

System.Data.DataException, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Undefined data type: 'xs:int'.

The XML fragment below represents an instance of the SQL_VARIANT column within the serialized typed DataSet. Note the use of the xsi:type="xs:int" attribute to define the data type of this column used for this particular row: although the column can contain data of any type, a given row must explicitly define which data type is being used.

<Data xsi:type="xs:int" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">1</Data>

Note: Much of the investigation detailed herein was carried out with the help of .NET Reflector to study what some of the classes in the .NET Framework were actually doing.

Continue reading this post here »