Moving along through our .NET Exception Handling series today we're going to take a gander at the System.ServiceModel.FaultException. The System.ServiceModel.FaultException is the first error we've dealt with in this series that is tied directly to the Windows Communication Foundation (WCF) platform of services. These services act as a remote interface, such as an API, that can be accessed by clients and other applications to perform some actions, without the client knowing about the underlying logic of the service.
In this article we'll dig into just what the System.ServiceModel.FaultException is including where it sits in the .NET exception hierarchy. We'll also explore some functional C# code examples which will illustrate how System.ServiceModel.FaultExceptions are thrown and how to properly handle them, so let's get started!
System.Exception base class, or derived from another inherited class therein.System.SystemException is inherited from the System.Exception class.System.ServiceModel.CommunicationException inherits directly from System.SystemException.System.ServiceModel.FaultException inherits from System.ServiceModel.CommunicationException.Working with WCF services can be a little confusing compared to "normal", localized code, so we'll start out with the full working code example and then walk through it afterward:
using System.Runtime.Serialization;
using System.ServiceModel;namespace LibraryService
{
[DataContract]
public class InvalidBookFault
{
[DataMember]
public bool Result { get; set; }
[DataMember]
public string Message { get; set; }
[DataMember]
public string Description { get; set; }
}[ServiceContract]
public interface ILibraryService
{
[OperationContract]
[FaultContract(typeof(InvalidBookFault))]
bool ReserveBook(string title, string author);
}
}using System;
using System.ServiceModel;
using Utility;namespace LibraryService
{
public interface IBook
{
string Author { get; set; }
string Title { get; set; }
bool Reserved { get; set; }
}public class Book : IBook
{
public string Title { get; set; }
public string Author { get; set; }
public bool Reserved { get; set; }public Book() { }
public Book(string title, string author)
{
Title = title;
Author = author;
}
}public class LibraryService : ILibraryService
{
public bool ReserveBook(string title, string author)
{
try
{
// Check if title value is null or has no characters.
// Null must be checked prior to length to avoid checking an invalid object.
if (title is null || title.Length == 0)
{
throw new System.ArgumentException($"CheckoutBook() parameter 'title' must be a valid string.");
}
// Check if author value is null or has no characters.
// Null must be checked prior to length to avoid checking an invalid object.
if (author is null || author.Length == 0)
{
throw new System.ArgumentException($"CheckoutBook() parameter 'author' must be a valid string.");
}// Create book record and reserve it (in a real service
// a database record would likely be modified.
Book book = new Book(title, author);
book.Reserved = true;
Logging.Log("RESERVATION SUCCESSFUL");
Logging.LineSeparator();
Logging.Log(book);
return true;
}
catch (ArgumentException e)
{
// Log the exception.
Logging.Log(e);// Generate new fault and set details.
var fault = new InvalidBookFault();
fault.Description = e.Message;
fault.Message = e.Message;
fault.Result = false;// Throw newly created FaultException of appropriate type.
throw new FaultException<InvalidBookFault>(fault, new FaultReason(e.Message));
}
catch (Exception e)
{
Logging.Log(e, false);
}
return false;
}
}
}using System;
using System.ServiceModel;
using Utility;namespace LibraryClient
{
class Program
{
static void Main(string[] args)
{
LibraryServiceReference.LibraryServiceClient client = new LibraryServiceReference.LibraryServiceClient(); ;try
{
// Open client connection.
client.Open();// Reserve a valid book.
client.ReserveBook("The Stand", "Stephen King");
// Reserve an invalid book.
client.ReserveBook("The Stand", null);// Close client connection.
client.Close();
}
// Catch our SOAP fault type.
catch (FaultException<LibraryServiceReference.InvalidBookFault> e)
{
Logging.Log(e);
client.Abort();
}
catch (FaultException e)
{
Logging.Log(e, false);
client.Abort();
}
catch (Exception e)
{
Logging.Log(e, false);
client.Abort();
}
}
}
}using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Reflection;
using System.Text;namespace Utility
{
/// <summary>
/// Houses all logging methods for various debug outputs.
/// </summary>
public static class Logging
{
/// <summary>
/// Outputs to <see cref="System.Diagnostics.Debug.WriteLine"/> if DEBUG mode is enabled,
/// otherwise uses standard <see cref="Console.WriteLine"/>.
/// </summary>
/// <param name="value">Value to be output to log.</param>
public static void Log(string value)
{
#if DEBUG
Debug.WriteLine(value);
#else
Console.WriteLine(value);
#endif
}/// <summary>
/// When <see cref="Exception"/> parameter is passed, modifies the output to indicate
/// if <see cref="Exception"/> was expected, based on passed in `expected` parameter.
/// <para>Outputs the full <see cref="Exception"/> type and message.</para>
/// </summary>
/// <param name="exception">The <see cref="Exception"/> to output.</param>
/// <param name="expected">Boolean indicating if <see cref="Exception"/> was expected.</param>
public static void Log(Exception exception, bool expected = true)
{
string value = $"[{(expected ? "EXPECTED" : "UNEXPECTED")}] {exception.ToString()}: {exception.Message}";
#if DEBUG
Debug.WriteLine(value);
#else
Console.WriteLine(value);
#endif
}/// <summary>
/// Outputs to <see cref="System.Diagnostics.Debug.WriteLine"/> if DEBUG mode is enabled,
/// otherwise uses standard <see cref="Console.WriteLine"/>.
/// </summary>
/// <param name="value">Value to be output to log.</param>
public static void Log(object value)
{
#if DEBUG
Debug.WriteLine(ObjectDumper.Dump(value));
#else
Console.WriteLine(ObjectDumper.Dump(value));
#endif
}/// <summary>
/// Outputs a dashed line separator to <see cref="System.Diagnostics.Debug.WriteLine"/>
/// if DEBUG mode is enabled, otherwise uses standard <see cref="Console.WriteLine"/>.
/// </summary>
public static void LineSeparator()
{
#if DEBUG
Debug.WriteLine(new string('-', 20));
#else
Console.WriteLine(new string('-', 20));
#endif
}
}
}
Ignoring our Utility functions for logging, our service application example consists of three basic components: An ILibraryService interface, a LibraryService class, and then a LibraryClient that uses the service to make something happen. Therefore we start with a basic (albeit very simple) service interface:
namespace LibraryService
{
[DataContract]
public class InvalidBookFault
{
[DataMember]
public bool Result { get; set; }
[DataMember]
public string Message { get; set; }
[DataMember]
public string Description { get; set; }
}
[ServiceContract]
public interface ILibraryService
{
[OperationContract]
[FaultContract(typeof(InvalidBookFault))]
bool ReserveBook(string title, string author);
}
}
Here we're using a lot of [attributes] to specify the expected behavior of our service interface. We really just have one invokable service method called ReserveBook, which finds a book by title and author before trying to reserve it. What's particularly important here is the InvalidBookFault class, which we use to communicate the potential error from the LibraryService to our client. This allows the client to be informed of what specifically went wrong.
Next is our LibraryService class which implements the ILibraryService interface. To make things a bit more interesting we've specified that the ReserveBook() method should be provided valid title and author argument strings, otherwise it'll throw an ArgumentException error. However, this is still all "server-side" so we need to catch the produced ArgumentException and generate a contractual fault that can be relayed to the invoking client. This is where our previously-defined InvalidBookFault comes into play. Within the ArgumentException catch we create a new InvalidBookFault instance, populate it with values from our exception (such as the basic error message), then throw a new FaultException<InvalidBookFault>:
namespace LibraryService
{
public interface IBook
{
string Author { get; set; }
string Title { get; set; }
bool Reserved { get; set; }
}public class Book : IBook
{
public string Title { get; set; }
public string Author { get; set; }
public bool Reserved { get; set; }public Book() { }
public Book(string title, string author)
{
Title = title;
Author = author;
}
}public class LibraryService : ILibraryService
{
public bool ReserveBook(string title, string author)
{
try
{
// Check if title value is null or has no characters.
// Null must be checked prior to length to avoid checking an invalid object.
if (title is null || title.Length == 0)
{
throw new System.ArgumentException($"CheckoutBook() parameter 'title' must be a valid string.");
}
// Check if author value is null or has no characters.
// Null must be checked prior to length to avoid checking an invalid object.
if (author is null || author.Length == 0)
{
throw new System.ArgumentException($"CheckoutBook() parameter 'author' must be a valid string.");
}// Create book record and reserve it (in a real service
// a database record would likely be modified.
Book book = new Book(title, author);
book.Reserved = true;
Logging.Log("RESERVATION SUCCESSFUL");
Logging.LineSeparator();
Logging.Log(book);
return true;
}
catch (ArgumentException e)
{
// Log the exception.
Logging.Log(e);// Generate new fault and set details.
var fault = new InvalidBookFault();
fault.Description = e.Message;
fault.Message = e.Message;
fault.Result = false;
// Throw newly created FaultException of appropriate type.
throw new FaultException<InvalidBookFault>(fault, new FaultReason(e.Message));
}
catch (Exception e)
{
Logging.Log(e, false);
}
return false;
}
}
}
Finally our simple LibraryClient application actually invokes our LibraryService (via a service reference in Visual Studio) to acquire a client connection. From this we can open the connection and then call the ReserveBook() service method:
namespace LibraryClient
{
class Program
{
static void Main(string[] args)
{
LibraryServiceReference.LibraryServiceClient client = new LibraryServiceReference.LibraryServiceClient(); ;try
{
// Open client connection.
client.Open();// Reserve a valid book.
client.ReserveBook("The Stand", "Stephen King");
// Reserve an invalid book.
client.ReserveBook("The Stand", null);
// Close client connection.
client.Close();
}
// Catch our SOAP fault type.
catch (FaultException<LibraryServiceReference.InvalidBookFault> e)
{
Logging.Log(e);
client.Abort();
}
catch (FaultException e)
{
Logging.Log(e, false);
client.Abort();
}
catch (Exception e)
{
Logging.Log(e, false);
client.Abort();
}
}
}
}
We begin with a valid set of title and author arguments for our first ReserveBook() call, so the result is as expected and our service "reserves" our book for us as show in the output:
RESERVATION SUCCESSFUL
--------------------
{LibraryService.Book(HashCode:37916227)}
Title: "The Stand"
Author: "Stephen King"
Reserved: True
However, the second ReserveBook() call includes a null value for the second argument. As you'll recall this isn't acceptable and triggers our chain of exception and fault events: The LibraryService throws a new ArgumentException, which is caught and then creates and throws a new InvalidBookFault. Our LibraryClient then catches the FaultException<LibraryServiceReference.ArgumentExceptionFault> and can safely report that issue to the end-user/client, as we see in the output:
[EXPECTED] System.ServiceModel.FaultException`1[LibraryClient.LibraryServiceReference.InvalidBookFault]: CheckoutBook() parameter 'author' must be a valid string. (Fault Detail is equal to LibraryClient.LibraryServiceReference.InvalidBookFault).: CheckoutBook() parameter 'author' must be a valid string.
We've successfully created a System.ServiceModel.FaultException of type LibraryClient.LibraryServiceReference.InvalidBookFault and it's populated by relevant error information that came from our service. This powerful technique allows clients and services to securely communicate exception information without breaking any contracts.
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