Externalising dependencies with Dependency Injection in .NET part 7: emailing

Introduction

In the previous post of this series we looked at how to hide the implementation of file system operations. In this post we’ll look at abstracting away emailing. Emails are often sent out in professional applications to users in specific scenarios: a new user signs up, the user places an order, the order is dispatched etc.

If you don’t know who to send emails using .NET then check the out mini-series on this page.

The first couple of posts of this series went through the process of making hard dependencies to loosely coupled ones. I won’t go through that again – you can refer back to the previous parts of this series to get an idea. You’ll find a link below to view all posts of the series. Another post on the Single Responsibility Principle includes another example of breaking out an INotificationService that you can look at.

We’ll extend the Infrastructure layer of our demo app we’ve been working on so far. So have it ready in Visual Studio and let’s get to it.

The interface

As usual we’ll need an abstraction to hide the concrete emailing logic. Emailing has potentially a considerable amount of parameters: from, to, subject, body, attachments, HTML contents, embedded resources, SMTP server and possibly many more. So the interface method(s) should accommodate all these variables somehow. One approach is to create overloads of the same method, like…

void Send(string to, string from, string subject, string body, string smtpServer);
void Send(string to, string from, string subject, string body, string smtpServer, bool isHtml);
void Send(string to, string from, string subject, string body, string smtpServer, bool isHtml, List<string> attachments);
.
.
.

…and so on including all combinations, e.g. a method with and without “isHtml”. I think this is not an optimal and future-proof interface. It is probably better to give room to all those arguments in an object and use that object as the parameter to a single method in the interface.

Also, we need to read the response of the operation. Add a new folder call Email to the Infrastructure.Common library. Insert the following object into the folder:

public class EmailSendingResult
{
	public bool EmailSentSuccessfully { get; set; }
	public string EmailSendingFailureMessage { get; set; }
}

The parameters for sending the email will be contained by an object called EmailArguments:

public class EmailArguments
{
	private string _subject;
	private string _message;
	private string _to;
	private string _from;
	private string _smtpServer;
	private bool _html;
		
	public EmailArguments(string subject, string message, string to, string from, string smtpServer, bool html)
	{
		if (string.IsNullOrEmpty(subject))
			throw new ArgumentNullException("Email subject");
		if (string.IsNullOrEmpty(message))
			throw new ArgumentNullException("Email message");
		if (string.IsNullOrEmpty(to))
			throw new ArgumentNullException("Email recipient");
		if (string.IsNullOrEmpty(from))
			throw new ArgumentNullException("Email sender");
		if (string.IsNullOrEmpty(smtpServer))
			throw new ArgumentNullException("Smtp server");
		this._from = from;
		this._message = message;
		this._smtpServer = smtpServer;
		this._subject = subject;
		this._to = to;
		this._html = html;
	}

	public List EmbeddedResources { get; set; }

	public string To
	{
		get
		{
			return this._to;
		}
	}

	public string From
	{
		get
		{
			return this._from;
		}
	}

	public string Subject
	{
		get
		{
			return this._subject;
		}
	}

	public string SmtpServer
	{
		get
		{
			return this._smtpServer;
		}
	}

	public string Message
	{
		get
		{
			return this._message;
		}
	}

	public bool Html
	{
		get
		{
			return this._html;
		}
	}
}

…where EmbeddedEmailResource is a new object, we’ll show it in a second.

So 6 parameters are made compulsory: to, from, subject, smtpServer, message body and whether the message is HTML. We can probably make this list shorter by excluding the subject and isHtml parameters but it’s a good start.

Embedded email resources come in many different forms. Insert the following enum in the Email folder:

public enum EmbeddedEmailResourceType
{
	Jpg
	, Gif
	, Tiff
	, Html
	, Plain
	, RichText
	, Xml
	, OctetStream
	, Pdf
	, Rtf
	, Soap
	, Zip
}

Embedded resources will be represented by the EmbeddedEmailResource object:

public class EmbeddedEmailResource
{
	public EmbeddedEmailResource(Stream resourceStream, EmbeddedEmailResourceType resourceType
		, string embeddedResourceContentId)
	{
		if (resourceStream == null) throw new ArgumentNullException("Resource stream");
		if (String.IsNullOrEmpty(embeddedResourceContentId)) throw new ArgumentNullException("Resource content id");
		ResourceStream = resourceStream;
		ResourceType = resourceType;
		EmbeddedResourceContentId = embeddedResourceContentId;
	}

	public Stream ResourceStream { get; set; }
	public EmbeddedEmailResourceType ResourceType { get; set; }
	public string EmbeddedResourceContentId { get; set; }
}

If you’re familiar with how to add embedded resources to an email then you’ll know why we need a Stream and a content id.

The solution should compile at this stage.

We’re now ready for the great finale, i.e. the IEmailService interface:

public interface IEmailService
{
	EmailSendingResult SendEmail(EmailArguments emailArguments);
}

Both the return type and the single parameter are custom objects that can be extended without breaking the code for any existing callers. You can add new public getters and setters to EmailArguments instead of creating the 100th different version of SendEmail in the version with primitive parameters.

Implementation

We’ll of course use the default emailing techniques built into .NET to implement IEmailService. Add a new class called SystemNetEmailService to the Email folder:

public EmailSendingResult SendEmail(EmailArguments emailArguments)
{
	EmailSendingResult sendResult = new EmailSendingResult();
	sendResult.EmailSendingFailureMessage = string.Empty;
	try
	{
		MailMessage mailMessage = new MailMessage(emailArguments.From, emailArguments.To);
		mailMessage.Subject = emailArguments.Subject;
		mailMessage.Body = emailArguments.Message;
		mailMessage.IsBodyHtml = emailArguments.Html;
		SmtpClient client = new SmtpClient(emailArguments.SmtpServer);

		if (emailArguments.EmbeddedResources != null && emailArguments.EmbeddedResources.Count > 0)
		{
			AlternateView avHtml = AlternateView.CreateAlternateViewFromString(emailArguments.Message, Encoding.UTF8, MediaTypeNames.Text.Html);
			foreach (EmbeddedEmailResource resource in emailArguments.EmbeddedResources)
			{
				LinkedResource linkedResource = new LinkedResource(resource.ResourceStream, resource.ResourceType.ToSystemNetResourceType());
				linkedResource.ContentId = resource.EmbeddedResourceContentId;
				avHtml.LinkedResources.Add(linkedResource);
			}
			mailMessage.AlternateViews.Add(avHtml);
		}

		client.Send(mailMessage);
		sendResult.EmailSentSuccessfully = true;
	}
	catch (Exception ex)
	{
		sendResult.EmailSendingFailureMessage = ex.Message;
	}

	return sendResult;
}

The code won’t compile due to the extension method ToSystemNetResourceType(). The LinkedResource object cannot work with our EmbeddedEmailResourceType type directly. It needs a string instead like “text/plain” or “application/soap+xml”. Those values are in turn stored within the MediaTypeNames object.

Add the following static class to the Email folder:

public static class EmailExtensions
{
	public static string ToSystemNetResourceType(this EmbeddedEmailResourceType resourceTypeEnum)
	{
		string type = MediaTypeNames.Text.Plain;
		switch (resourceTypeEnum)
		{
			case EmbeddedEmailResourceType.Gif:
				type = MediaTypeNames.Image.Gif;
				break;
			case EmbeddedEmailResourceType.Jpg:
				type = MediaTypeNames.Image.Jpeg;
				break;
			case EmbeddedEmailResourceType.Html:
				type = MediaTypeNames.Text.Html;
				break;
			case EmbeddedEmailResourceType.OctetStream:
				type = MediaTypeNames.Application.Octet;
				break;
			case EmbeddedEmailResourceType.Pdf:
				type = MediaTypeNames.Application.Pdf;
				break;
			case EmbeddedEmailResourceType.Plain:
				type = MediaTypeNames.Text.Plain;
				break;
			case EmbeddedEmailResourceType.RichText:
				type = MediaTypeNames.Text.RichText;
				break;
			case EmbeddedEmailResourceType.Rtf:
				type = MediaTypeNames.Application.Rtf;
				break;
			case EmbeddedEmailResourceType.Soap:
				type = MediaTypeNames.Application.Soap;
				break;
			case EmbeddedEmailResourceType.Tiff:
				type = MediaTypeNames.Image.Tiff;
				break;
			case EmbeddedEmailResourceType.Xml:
				type = MediaTypeNames.Text.Xml;
				break;
			case EmbeddedEmailResourceType.Zip:
				type = MediaTypeNames.Application.Zip;
				break;
		}

		return type;
	}
}

This is a simple converter extension to transform our custom enumeration to media type strings.

There you are, this is a good starting point to cover the email purposes in your project.

In the next part of this series we’ll look at hiding the cryptography logic or our application.

View the list of posts on Architecture and Patterns here.

Externalising dependencies with Dependency Injection in .NET part 6: file system

Introduction

In the previous post we looked at logging with log4net and saw how easy it was to switch from one logging strategy to another. By now you probably understand why it can be advantageous to remove hard dependencies from your classes: flexibility, testability, SOLID and more.

The steps to factor out your hard dependencies to abstractions usually involve the following steps:

• Identify the hard dependencies: can the class be tested in isolation? Does the test result depend on an external object such as a web service? Can the implementation of the dependency change?
• Identify the tasks any reasonable implementation of the dependency should be able to perform: what should a caching system do? What should any decent logging framework be able to do?
• Build an abstraction – usually an interface – to represent those expected functions: this is so that the interface becomes as future-proof as possible. As noted before this is easier said than done as you don’t know in advance what a future implementation might need. You might need to revisit your interface and add an extra method or an extra parameter. This can be alleviated if you work with objects as parameters to the interface functions, e.g. LoggingArguments, CachingArguments – you’ll see what I mean in the next post where we’ll take up emailing
• Inject the abstraction into the class that depends on it through one of the Dependency Injection patterns where constructor injection should be your default choice if you’re uncertain
• The calling class will then inject a concrete implementation for the interface – alternatively you can use of the Inversion-of-control tools, like StructureMap

In this and the remaining posts of this series we won’t be dealing with the Console app in the demo anymore. The purpose of the console app was to show the goal of abstractions and dependency injection through examples. We’ve seen enough of that so we can instead concentrate on building the infrastructure layer. So open the demo solution in VS let’s add file system operations to Infrastructure.Common.

File system

.NET has an excellent built-in library for anything you’d like to do with files and directories. In the previous post on log4net we saw an example of checking if a file exists like this:

FileInfo log4netSettingsFileInfo = new FileInfo(_contextService.GetContextualFullFilePath(_log4netConfigFileName));
if (!log4netSettingsFileInfo.Exists)
{
	throw new ApplicationException(string.Concat("Log4net settings file ", _log4netConfigFileName, " not found."));
}

You can have File.WriteAllText, File.ReadAllBytes, File.Copy etc. directly in your code and you may not think that it’s a real dependency. It’s admittedly very unlikely that you don’t want to use the built-in features of .NET for file system operations and instead take some other library. So the argument of “flexibility” might not play a big role here.

However, unit testing with TDD shows that you shouldn’t make the outcome of your test depend on external elements, such as the existence of a file if the method being tested wants in fact to perform some operation on a physical file. Instead, you should be able to declare the outcome of those operations through TDD tools such as Moq which is discussed in the TDD series referred to in the previous sentence. If you see that you must create a specific file before a test is run and delete it afterwards then it’s a brittle unit test. Most real-life business applications are auto-tested by test runners in continuous integration (CI) systems such as TeamCity or Jenkins. In that case you’ll need to create the same file on the CI server(s) as well so that the unit test passes.

Therefore it still makes sense to factor out the file related stuff from your consuming classes.

The abstraction

File system operations have many facets: reading, writing, updating, deleting, copying, creating files and much more. Therefore a single file system interface is going to be relatively large. Alternatively you can break out the functions to separate interfaces such as IFileReaderService, IFileWriterService, IFileInformationService etc. You can also have a separate interface for directory-specific operations such as creating a new folder or reading the drive name.

Here we’ll start with out easy. Add a new folder called FileOperations to the Infrastructure.Common C# library. Insert an interface called IFileService:

public interface IFileService
{		
	bool FileExists(string fileFullPath);		
	long LastModifiedDateUnix(string fileFullPath);		
	string RetrieveContentsAsBase64String(string fileFullPath);		
	byte[] ReadContentsOfFile(string fileFullPath);		
	string GetFileName(string fullFilePath);		
	bool SaveFileContents(string fileFullPath, byte[] contents);		
	bool SaveFileContents(string fileFullPath, string base64Contents);			
	string GetFileExtension(string fileName);		
	bool DeleteFile(string fileFullPath);
}

That should be enough for starters.

The implementation

We’ll of course use the standard capabilities in .NET to implement the file operations. Add a new class called DefaultFileService to the FileOperations folder:

public class DefaultFileService : IFileService
{
	public bool FileExists(string fileFullPath)
	{
		FileInfo fileInfo = new FileInfo(fileFullPath);
		return fileInfo.Exists;
	}

	public long LastModifiedDateUnix(string fileFullPath)
	{
		FileInfo fileInfo = new FileInfo(fileFullPath);
		if (fileInfo.Exists)
		{
			DateTime epoch = new DateTime(1970, 1, 1, 0, 0, 0);
			TimeSpan timeSpan = fileInfo.LastWriteTimeUtc - epoch;
			return Convert.ToInt64(timeSpan.TotalMilliseconds);
		}

		return -1;
	}

	public string RetrieveContentsAsBase64String(string fileFullPath)
	{
		byte[] contents = ReadContentsOfFile(fileFullPath);
		if (contents != null)
		{
			return Convert.ToBase64String(contents);
		}
		return string.Empty;
	}

	public byte[] ReadContentsOfFile(string fileFullPath)
	{
		FileInfo fi = new FileInfo(fileFullPath);
		if (fi.Exists)
		{
			return File.ReadAllBytes(fileFullPath);
		}

		return null;
	}

	public string GetFileName(string fullFilePath)
	{
		FileInfo fi = new FileInfo(fullFilePath);
		return fi.Name;
	}

	public bool SaveFileContents(string fileFullPath, byte[] contents)
	{
		try
		{
			File.WriteAllBytes(fileFullPath, contents);
			return true;
		}
		catch
		{
			return false;
		}
	}

	public bool SaveFileContents(string fileFullPath, string base64Contents)
	{
		return SaveFileContents(fileFullPath, Convert.FromBase64String(base64Contents));
	}

	public string GetFileExtension(string fileName)
	{
		FileInfo fi = new FileInfo(fileName);
		return fi.Extension;
	}

	public bool DeleteFile(string fileFullPath)
	{
		FileInfo fi = new FileInfo(fileFullPath);
		if (fi.Exists)
		{
			File.Delete(fileFullPath);
		}

		return true;
	}
}

There you have it. Feel free to break out the file system functions to separate interfaces as suggested above.

The next post in this series will take up emailing.

View the list of posts on Architecture and Patterns here.

4 design patterns to learn with C# .NET

Here come 4 well-known design patterns that I think most developers will benefit from, even those that are by nature anti-designpattern. Note that the list is biased and only shows the ones that I personally use the most often in my work.

If you don’t find the solution to your design problem here then check out the full list of patterns discussed on this blog here.

• Adapter: this pattern helps you hide the functionality of a class which is not under your control behind an abstraction
• Strategy: this pattern will help you with cleaning up anti-SOLID code where you check for certain properties, especially the type of an object to tweak your code. You will end up with proper objects instead of brittle switch or if-else statements
• Decorator: if you’d like to extend the functionality of a class without changing its implementation then this pattern is something to consider. You can build compound objects by nesting decorators where the individual elements are still standalone classes.
• Factory: this pattern will help you build objects using parameters whose values are not known beforehand. E.g. if you don’t which concrete type of an abstract class to return then hide that build functionality behind an abstract factory

Externalising dependencies with Dependency Injection in .NET part 4: logging part 1

Introduction

In this post of this series we looked at ways to remove caching logic from our ProductService class. In the previous post we saw how to hide reading from a configuration file. We’ll follow in a similar fashion in this post which takes up logging. A lot of the techniques and motivations will be re-used and not explained again.

We’ll build upon the demo app we started on so have it ready in Visual Studio.

Logging

Logging is a wide area with several questions you need to consider. What needs to be logged? Where should we save the logs? How can the log messages be correlated? We’ll definitely not answer those questions here. Instead, we’ll look at techniques to call the logging implementation.

I’d like to go through 3 different ways to put logging into your application. For the demo we’ll first send the log messages to the Console window for an easy start. After that, we’ll propose a way of implementing logging with log4net. The material is too much for a single post so I’ve decided to divide this topic into 2 posts.

Starting point

If you ever had to log something in a professional .NET project then chances are that you used Log4Net. It’s a mature and widely used logging framework. However, it’s by far not the only choice. There’s e.g. NLog and probably many more.

If you recall the first solution for the caching problem we put the caching logic directly inside ProductService.GetProduct:

public GetProductResponse GetProduct(GetProductRequest getProductRequest)
{
	GetProductResponse response = new GetProductResponse();
	try
	{
		string storageKey = "GetProductById";
		ObjectCache cache = MemoryCache.Default;
		Product p = cache.Contains(storageKey) ? (Product)cache[storageKey] : null;
		if (p == null)
		{
			p = _productRepository.FindBy(getProductRequest.Id);
			CacheItemPolicy policy = new CacheItemPolicy() { AbsoluteExpiration = DateTime.Now.AddMinutes(5) };
			cache.Add(storageKey, p, policy);
		}
		response.Product = p;
		if (p != null)
		{
			response.Success = true;
		}
		else
		{
			response.Exception = "No such product.";
		}
	}
	catch (Exception ex)
	{
		response.Exception = ex.Message;
	}
	return response;
}

We then also discussed the disadvantages of such a solution, like diminished testability and flexibility. We could follow a similar solution for the logging with log4net with direct calls from ProductService like this:

var logger = logSource.GetLogger();
var loggingEvent = new LoggingEvent(logSource.GetType(), logger.Logger.Repository, logger.Logger.Name, Level.Info, "Code starting", null);
logger.Logger.Log(loggingEvent);

…which would probably be factored out to a different method but the strong dependency on and tight coupling to log4net won’t go away. We could have similar code for NLog and other logging technologies out there. Then some day the project leader says that the selected technology must change because the one they used on a different project is much more powerful. Then the developer tasked with changing the implementation at every possible place in the code won’t have a good day out.

The logging interface

As we saw in the previous post the key step towards an OOP solution is an abstraction which hides the concrete implementation. This is not always a straightforward step. In textbooks it always looks easy to create these interfaces because they always give you a variety of concrete implementations beforehand. Then you’ll know what’s common to them and you can put those common methods in an interface. In reality, however, you’ll always need to think carefully about the features that can be common across a variety of platforms: what should every decent logging framework be able to do at a minimum? What should a caching technology be able to do? Etc., you can ask similar questions about every dependency in your code that you’re trying to factor out. The interface should find the common features and not lean towards one specific implementation so that it becomes as future-proof as possible.

Normally any logging framework should be able to log messages, message levels – e.g. information or fatal – and possibly exceptions. Insert a new folder called Logging to the Infrastructure layer and add the following interface into it:

public interface ILoggingService
{
	void LogInfo(object logSource, string message, Exception exception = null);
	void LogWarning(object logSource, string message, Exception exception = null);
	void LogError(object logSource, string message, Exception exception = null);
	void LogFatal(object logSource, string message, Exception exception = null);
}

This should be enough for starters: the source where the logging happens, the message and an exception. The level is represented by the method names.

Logging to the console

As promised above, we’ll take it easy first and send the log messages directly to the console. So add the following implementation to the Logging folder:

public class ConsoleLoggingService : ILoggingService
{
	private ConsoleColor _defaultColor = ConsoleColor.Gray;

	public void LogInfo(object logSource, string message, Exception exception = null)
	{
		Console.ForegroundColor = ConsoleColor.Green;
		Console.WriteLine(string.Concat("Info from ", logSource.ToString(), ": ", message));
		PrintException(exception);
		ResetConsoleColor();
	}

	public void LogWarning(object logSource, string message, Exception exception = null)
	{
		Console.ForegroundColor = ConsoleColor.Yellow;
		Console.WriteLine(string.Concat("Warning from ", logSource.ToString(), ": ", message));
		PrintException(exception);
		ResetConsoleColor();
	}

	public void LogError(object logSource, string message, Exception exception = null)
	{
		Console.ForegroundColor = ConsoleColor.DarkMagenta;
		Console.WriteLine(string.Concat("Error from ", logSource.ToString(), ": ", message));
		PrintException(exception);
		ResetConsoleColor();
	}

	public void LogFatal(object logSource, string message, Exception exception = null)
	{
		Console.ForegroundColor = ConsoleColor.Red;
		Console.WriteLine(string.Concat("Fatal from ", logSource.ToString(), ": ", message));
		PrintException(exception);
		ResetConsoleColor();
	}

	private void ResetConsoleColor()
	{
		Console.ForegroundColor = _defaultColor;
	}

	private void PrintException(Exception exception)
	{
		if (exception != null)
		{
			Console.WriteLine(string.Concat("Exception logged: ", exception.Message));
		}
	}
}

That should be quite straightforward I believe.

Solution 1: decorator

We saw an example of the Decorator pattern in the post on caching referred to in the intro and we’ll build on that. We’ll extend the original ProductService implementation to include both caching and logging. Add the following decorator to the Services folder of the Console app layer:

public class LoggedProductService : IProductService
{
	private readonly IProductService _productService;
	private readonly ILoggingService _loggingService;

	public LoggedProductService(IProductService productService, ILoggingService loggingService)
	{
		if (productService == null) throw new ArgumentNullException("ProductService");
		if (loggingService == null) throw new ArgumentNullException("LoggingService");
		_productService = productService;
		_loggingService = loggingService;
	}

	public GetProductResponse GetProduct(GetProductRequest getProductRequest)
	{
		GetProductResponse response = new GetProductResponse();
		_loggingService.LogInfo(this, "Starting GetProduct method");
		try
		{
			response = _productService.GetProduct(getProductRequest);
			if (response.Success)
			{
				_loggingService.LogInfo(this, "GetProduct success!!!");
			}
			else
			{
				_loggingService.LogError(this, "GetProduct failure...", new Exception(response.Exception));
			}
		}
		catch (Exception ex)
		{
			response.Exception = ex.Message;
			_loggingService.LogError(this, "Exception in GetProduct!!!", ex);
		}
		return response;
	}
}

You’ll recall the structure from the Caching decorator. We hide both the product and logging service behind interfaces. We delegate the product retrieval to the product service and logging to the logging service. So if you’d like to add logging to the original ProductService class then you can have the following code in Main:

IProductService productService = new ProductService(new ProductRepository());
IProductService loggedProductService = new LoggedProductService(productService, new ConsoleLoggingService());
GetProductResponse getProductResponse = loggedProductService.GetProduct(new GetProductRequest() { Id = 2 });
if (getProductResponse.Success)
{
	Console.WriteLine(string.Concat("Product name: ", getProductResponse.Product.Name));
}
else
{
	Console.WriteLine(getProductResponse.Exception);
}

If you run this code then you’ll see an output similar to this:

If you run the code with a non-existent product ID then you’ll see an exception as well:

If you’d then like to add both caching and logging to the plain ProductService class then you can have the following test code:

IProductService productService = new ProductService(new ProductRepository());
IProductService cachedProductService = new CachedProductService(productService, new SystemRuntimeCacheStorage());
IProductService loggedCachedProductService = new LoggedProductService(cachedProductService, new ConsoleLoggingService());
GetProductResponse getProductResponse = loggedCachedProductService.GetProduct(new GetProductRequest() { Id = 2 });
if (getProductResponse.Success)
{
	Console.WriteLine(string.Concat("Product name: ", getProductResponse.Product.Name));
}
else
{
	Console.WriteLine(getProductResponse.Exception);
}

getProductResponse = loggedCachedProductService.GetProduct(new GetProductRequest() { Id = 2 });
			
Console.ReadKey();

Notice how we build up the compound decorator loggedCachedProductService from productService and cachedProductService in the beginning of the code. You can step through the code and you’ll see how we print the log messages, check the cache and retrieve the product from ProductService if necessary.

Solution 2: ambient context

I think the above solution with dependency injection, interfaces and decorators follows SOLID principles quite well. We can build upon the ProductService class using the decorators, pass different implementations of the abstractions and test each component independently.

As discussed in the post on Interception – check the section called Interception using the Decorator pattern – this solution can have some practical limitations, such as writing a large number of tedious code. While you may not want to do caching in every single layer, logging is different. You may want to log from just about any layer of the application: UI, services, controllers, repositories. So you may need to write a LoggedController, LoggedRepository, LoggedService etc. for any component where you want to introduce logging.

In the post on patterns in dependency injection we discussed a technique called ambient context. I suggest you read that section if you don’t understand the term otherwise you may not understand the purpose of the code below. Also, you’ll find its advantages and disadvantages there as well.

Insert the following class into the Logging folder of the infrastructure project:

public abstract class LogProviderContext
{
	private static readonly string _nameDataSlot = "LogProvider";

	public static ILoggingService Current
	{
		get
		{
			ILoggingService logProviderContext = Thread.GetData(Thread.GetNamedDataSlot(_nameDataSlot)) as ILoggingService;
			if (logProviderContext == null)
			{
				logProviderContext = LogProviderContext.DefaultLogProviderContext;
				Thread.SetData(Thread.GetNamedDataSlot(_nameDataSlot), logProviderContext);
			}
			return logProviderContext;
		}
		set
		{
			Thread.SetData(Thread.GetNamedDataSlot(_nameDataSlot), value);
		}
	}

	public static ILoggingService DefaultLogProviderContext = new ConsoleLoggingService();
}

You can call this code from any other layer which has a reference to the infrastructure layer. E.g. you can have the following code directly in ProductService.GetProducts:

LogProviderContext.Current.LogInfo(this, "Log message from the contextual log provider");

Solution 3: injecting logging into ProductService

Another solution which lies somewhere between ambient context and the decorator is having ProductService depend upon an ILoggingService:

public ProductService(IProductRepository productRepository, ILoggingService loggingService)

You can then use the injected ILoggingService to log you messages. We saw a similar example in the post on caching and also in the post on Interception referred to above. Read the section called Dependency injection in the post on Interception to read more why this approach might be good or bad.

However, if you’re faced with such a constructor and still want to test ProductService in isolation you can use a Null Object version of ILoggingService. Add the following class to the Logging folder:

public class NoLogService : ILoggingService
{
	public void LogInfo(object logSource, string message, Exception exception = null)
	{}

	public void LogWarning(object logSource, string message, Exception exception = null)
	{}

	public void LogError(object logSource, string message, Exception exception = null)
	{}

	public void LogFatal(object logSource, string message, Exception exception = null)
	{}
}

In the next post we’ll continue with logging to show a more realistic implementation of ILoggingService with log4net.

View the list of posts on Architecture and Patterns here.