Externalising dependencies with Dependency Injection in .NET part 10: hashing and digital signatures

Introduction

In the previous post of this series we looked at how to hide the implementation of asymmetric encryption operations. In this post we’ll look at abstracting away hashing and digital signatures as they often go hand in hand. Refer back to this post on hashing and this on digital signatures if you are not familiar with those concepts.

We’ll continue directly from the previous post and 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.

Preparation

Add two new subfolders called Hashing and DigitalSignature to the Cryptography folder of the Infrastructure.Common library. We’ll follow the same methodology as in the case of asymmetric encryption, i.e. we’ll communicate with proper objects as parameters and return types instead of writing a large amount of overloaded methods.

Recall that we had a common abstract class to all return types in the IAsymmetricCryptographyService interface: ResponseBase. We’ll reuse it here.

Hashing

In short hashing is a one way encryption. A plain text is hashed and there’s no key to decipher the hashed value. A common application of hashing is storing passwords. Any hashing algorithm should be able to hash plain string and that’s the only expectation we have. Add the below interface to the Hashing folder:

public interface IHashingService
{
	HashResult Hash(string message);
	string HashAlgorithmCode();
}

…where HashResult will hold the readable and byte array formats of the hashed value and has the following form:

public class HashResult : ResponseBase
{
	public string HexValueOfHash { get; set; }
	public byte[] HashedBytes { get; set; }
}

What is HashAlgorithmCode() doing there? The implementation if the interface will return a short description of itself, such as “SHA1” or “SHA512”. We’ll see later that it’s required by the digital signature implementation.

This can actually be seen as a violation of ‘L‘ in SOLID as we’re specifying the type of the implementation in some form. However, I think it serves a good purpose here and it will not be misused in some funny switch statement where we check the string returned by HashAlgorithmCode() in order to direct the logic. Let’s insert two implementations of IHashingService to the Hashing folder. Here comes SHA1:

public class Sha1ManagedHashingService : IHashingService
{
	public HashResult Hash(string message)
	{
		HashResult hashResult = new HashResult();
		try
		{
			SHA1Managed sha1 = new SHA1Managed();
			byte[] hashed = sha1.ComputeHash(Encoding.UTF8.GetBytes(message));
			string hex = Convert.ToBase64String(hashed);
			hashResult.HashedBytes = hashed;
			hashResult.HexValueOfHash = hex;
		}
		catch (Exception ex)
		{
			hashResult.ExceptionMessage = ex.Message;
		}
		return hashResult;
	}

	public string HashAlgorithmCode()
	{
		return "SHA1";
	}
}

…and here’s SHA512:

public class Sha512ManagedHashingService : IHashingService
{
	public HashResult Hash(string message)
	{
		HashResult hashResult = new HashResult();
		try
		{
			SHA512Managed sha512 = new SHA512Managed();
			byte[] hashed = sha512.ComputeHash(Encoding.UTF8.GetBytes(message));
			string hex = Convert.ToBase64String(hashed);
			hashResult.HashedBytes = hashed;
			hashResult.HexValueOfHash = hex;
		}
		catch (Exception ex)
		{
			hashResult.ExceptionMessage = ex.Message;
		}
		return hashResult;
	}

	public string HashAlgorithmCode()
	{
		return "SHA512";
	}
}

That’s enough for starters.

Digital signatures

Digital signatures in a nutshell are used to electronically sign the hash of a message. The receiver of the message can then check if the message has been tampered with in any way. Also, a digital signature can prove that it was the sender who sent a particular message. E.g. a receiver may only accept signed messages so that the sender cannot claim that it was someone else who sent the message.

It is not “compulsory” to encrypt a signed message. You can sign the hash of an unencrypted message. However, for extra safety the hash is also often encrypted. The process is the following:

• The Sender encrypts a message with the Receiver’s public key
• The Sender then hashes the cipher text
• The Sender signs the hashed cipher with their private key
• The Sender transmits the cipher text, the hash and the signature to the Receiver
• The Receiver verifies the signature with the Sender’s public key
• If the signature is correct then the Receiver deciphers the cipher text with their full asymmetric key

The following interface will reflect what we need from a digital signature service: sign a message and then verify the signature:

public interface IEncryptedDigitalSignatureService
{
	DigitalSignatureCreationResult Sign(DigitalSignatureCreationArguments arguments);
	DigitalSignatureVerificationResult VerifySignature(DigitalSignatureVerificationArguments arguments);
}

DigitalSignatureCreationResult will hold the signature and the cipher text:

public class DigitalSignatureCreationResult : ResponseBase
{
	public byte[] Signature { get; set; }
	public string CipherText { get; set; }
}

DigitalSignatureCreationArguments will carry the message to be encrypted, hashed and signed. It will also hold the the full key set for the signature and the public key of the receiver for encryption:

public class DigitalSignatureCreationArguments
{
	public string Message { get; set; }
	public XDocument FullKeyForSignature { get; set; }
	public XDocument PublicKeyForEncryption { get; set; }
}

Then we have DigitalSignatureVerificationResult which has properties to check if the signatures match and the decoded text:

public class DigitalSignatureVerificationResult : ResponseBase
{
	public bool SignaturesMatch { get; set; }
	public string DecodedText { get; set; }
}

…and finally we have DigitalSignatureVerificationArguments which holds the properties for the Sender’s public key for signature verification, the Receiver’s full key for decryption, the cipher text and the signature:

public class DigitalSignatureVerificationArguments
{
	public XDocument PublicKeyForSignatureVerification { get; set; }
	public XDocument FullKeyForDecryption { get; set; }
	public string CipherText { get; set; }
	public byte[] Signature { get; set; }
}

RsaPkcs1 digital signature provider

We’ll use the built-in RSA-based signature provider in .NET for the implementation. It will need to hash the cipher text so it’s good that we’ve prepared IHashingService in advance. Here comes the implementation where you’ll see how the HashAlgorithmCode() method is used in specifying the hash algorithm of the signature formatter and deformatter:

public class RsaPkcs1DigitalSignatureService : IEncryptedDigitalSignatureService
{
	private readonly IHashingService _hashingService;

	public RsaPkcs1DigitalSignatureService(IHashingService hashingService)
	{
		if (hashingService == null) throw new ArgumentNullException("Hashing service");
		_hashingService = hashingService;
	}

	public DigitalSignatureCreationResult Sign(DigitalSignatureCreationArguments arguments)
	{
		DigitalSignatureCreationResult res = new DigitalSignatureCreationResult();
		try
		{				
			RSACryptoServiceProvider rsaProviderReceiver = new RSACryptoServiceProvider();
			rsaProviderReceiver.FromXmlString(arguments.PublicKeyForEncryption.ToString());
			byte[] encryptionResult = rsaProviderReceiver.Encrypt(Encoding.UTF8.GetBytes(arguments.Message), false);
			HashResult hashed = _hashingService.Hash(Convert.ToBase64String(encryptionResult));

			RSACryptoServiceProvider rsaProviderSender = new RSACryptoServiceProvider();
			rsaProviderSender.FromXmlString(arguments.FullKeyForSignature.ToString());
			RSAPKCS1SignatureFormatter signatureFormatter = new RSAPKCS1SignatureFormatter(rsaProviderSender);
			signatureFormatter.SetHashAlgorithm(_hashingService.HashAlgorithmCode());
			byte[] signature = signatureFormatter.CreateSignature(hashed.HashedBytes);

			res.Signature = signature;
			res.CipherText = Convert.ToBase64String(encryptionResult);
			res.Success = true;
		}
		catch (Exception ex)
		{
			res.ExceptionMessage = ex.Message;
		}

		return res;
	}

	public DigitalSignatureVerificationResult VerifySignature(DigitalSignatureVerificationArguments arguments)
	{
		DigitalSignatureVerificationResult res = new DigitalSignatureVerificationResult();
		try
		{
			RSACryptoServiceProvider rsaProviderSender = new RSACryptoServiceProvider();
			rsaProviderSender.FromXmlString(arguments.PublicKeyForSignatureVerification.ToString());
			RSAPKCS1SignatureDeformatter deformatter = new RSAPKCS1SignatureDeformatter(rsaProviderSender);
			deformatter.SetHashAlgorithm(_hashingService.HashAlgorithmCode());				
			HashResult hashResult = _hashingService.Hash(arguments.CipherText);
			res.SignaturesMatch = deformatter.VerifySignature(hashResult.HashedBytes, arguments.Signature);
			if (res.SignaturesMatch)
	        	{
        			RSACryptoServiceProvider rsaProviderReceiver = new RSACryptoServiceProvider();
  	        		rsaProviderReceiver.FromXmlString(arguments.FullKeyForDecryption.ToString());
		        	byte[] decryptedBytes = rsaProviderReceiver.Decrypt(Convert.FromBase64String(arguments.CipherText), false);
				res.DecodedText = Encoding.UTF8.GetString(decryptedBytes);
			}

			res.Success = true;
		}
		catch (Exception ex)
		{
			res.ExceptionMessage = ex.Message;
		}

		return res;
	}
}

The whole chain can be tested as follows:

private static void TestDigitalSignatureService()
{
	IAsymmetricCryptographyService asymmetricService = new RsaAsymmetricCryptographyService();
	AsymmetricKeyPairGenerationResult keyPairGenerationResultReceiver = asymmetricService.GenerateAsymmetricKeys(1024);
	AsymmetricKeyPairGenerationResult keyPairGenerationResultSender = asymmetricService.GenerateAsymmetricKeys(1024);

	IEncryptedDigitalSignatureService digitalSignatureService = new RsaPkcs1DigitalSignatureService(new Sha1ManagedHashingService());

	DigitalSignatureCreationArguments signatureCreationArgumentsFromSender = new DigitalSignatureCreationArguments()
	{
		Message = "Text to be encrypted and signed"
		, FullKeyForSignature = keyPairGenerationResultSender.FullKeyPairXml
		, PublicKeyForEncryption = keyPairGenerationResultReceiver.PublicKeyOnlyXml
	};
	DigitalSignatureCreationResult signatureCreationResult = digitalSignatureService.Sign(signatureCreationArgumentsFromSender);
	if (signatureCreationResult.Success)
	{
		DigitalSignatureVerificationArguments verificationArgumentsFromReceiver = new DigitalSignatureVerificationArguments();
		verificationArgumentsFromReceiver.CipherText = signatureCreationResult.CipherText;
		verificationArgumentsFromReceiver.Signature = signatureCreationResult.Signature;
		verificationArgumentsFromReceiver.PublicKeyForSignatureVerification = keyPairGenerationResultSender.PublicKeyOnlyXml;
		verificationArgumentsFromReceiver.FullKeyForDecryption = keyPairGenerationResultReceiver.FullKeyPairXml;
		DigitalSignatureVerificationResult verificationResult = digitalSignatureService.VerifySignature(verificationArgumentsFromReceiver);
		if (verificationResult.Success)
		{
			if (verificationResult.SignaturesMatch)
			{
				Console.WriteLine("Signatures match, decoded text: {0}", verificationResult.DecodedText);
			}
			else
			{
				Console.WriteLine("Signature mismatch!!");
			}
		}
		else
		{
			Console.WriteLine("Signature verification failed: {0}", verificationResult.ExceptionMessage);
		}
	}
	else
	{
		Console.WriteLine("Signature creation failed: {0}", signatureCreationResult.ExceptionMessage);
	}
}

Check how the keys of each party in the messaging conversation are used for encrypting, signing, verifying and decrypting the message.

So that’s it for digital signatures.

This was the last post in the series on object dependencies. We could take up a number of other cross cutting concerns for the infrastructure layer, such as:

• Http calls
• User-level security, such as a role provider, a claims provider etc.
• Session provider
• Random password generation

…and possibly a lot more. By now you have a better understanding how to externalise tightly coupled object dependencies in your classes to make them more flexible and testable.

View the list of posts on Architecture and Patterns here.

Externalising dependencies with Dependency Injection in .NET part 9: cryptography part 2

Introduction

In the previous post of this series we looked at how to hide the implementation of symmetric encryption operations. In this post we’ll look at abstracting away asymmetric cryptography. Refer back to this post in case you are not familiar with asymmetric encryption in .NET.

We’ll continue directly from the previous post and 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.

Preparation

Add a new subfolder called Asymmetric to the Cryptography folder of the Infrastructure.Common library. We’ll follow the same methodology as in the case of emailing and symmetric encryption, i.e. we’ll communicate with proper objects as parameters and return types instead of writing a large amount of overloaded methods.

Recall that we had a common abstract class to all return types in the ISymmetricCryptographyService interface: ResponseBase. We’ll reuse it here. There’s one more class we’ll use again here and that is CipherTextDecryptionResult.

Asymmetric encryption

In summary asymmetric encryption means that you have a pair of keys: a public and a private key. This is in contrast to symmetric encryption where there is only one key. The public key can be distributed to those who wish to send you an encrypted message. The private key will be used to decrypt the incoming cipher text. The public and private keys are in fact also symmetric: you can encrypt a message with the public key and decrypt with the private one and vice versa. As asymmetric encryption is considerably slower – but much more secure – than symmetric encryption it’s usually not used to encrypt large blocks of plain text. Instead, the sender encrypts the text with a symmetric key and then encrypts the symmetric key with the receiver’s public asymmetric key. The receiver will then decrypt the symmetric key using their asymmetric private key and decrypt the message with the decrypted symmetric key. If you want to see that in action I have a sample project which takes up this process starting here.

Our expectations from any reasonable asymmetric encryption mechanism are the same as for symmetric algorithms:

• Create a valid key – actually a pair of keys – of a given size in bits
• Encrypt a text
• Decrypt a cipher text

Add the following interface to the Asymmetric folder to reflect these expectations:

public interface IAsymmetricCryptographyService
{
	AsymmetricKeyPairGenerationResult GenerateAsymmetricKeys(int keySizeInBits);
	AsymmetricEncryptionResult Encrypt(AsymmetricEncryptionArguments arguments);
	CipherTextDecryptionResult Decrypt(AsymmetricDecryptionArguments arguments);
}

We already know CipherTextDecryptionResult, here’s a reminder:

public class CipherTextDecryptionResult : ResponseBase
{
	public string DecodedText { get; set; }
}

AsymmetricKeyPairGenerationResult will represent the keys in XML format. We’ll need to represent the keys in two forms. The full representation is where the private key is included and should be kept with you. Another representation is where only the public key part of the pair is shown which can be used to distribute to your partners. Insert a class called AsymmetricKeyPairGenerationResult to the Asymmetric folder:

public class AsymmetricKeyPairGenerationResult : ResponseBase
{
	private XDocument _fullKeyPairXml;
	private XDocument _publicKeyOnlyXml;

	public AsymmetricKeyPairGenerationResult(XDocument fullKeyPairXml, XDocument publicKeyOnlyXml)
	{
		if (fullKeyPairXml == null) throw new ArgumentNullException("Full key pair XML");
		if (publicKeyOnlyXml == null) throw new ArgumentNullException("Public key only XML");
		_fullKeyPairXml = fullKeyPairXml;
		_publicKeyOnlyXml = publicKeyOnlyXml;
	}

	public XDocument FullKeyPairXml
	{
		get
		{
			return _fullKeyPairXml;
		}
	}

	public XDocument PublicKeyOnlyXml
	{
		get
		{
			return _publicKeyOnlyXml;
		}
	}
}

The keys are of type XDocument to make sure that they are in fact valid XML.

AsymmetricEncryptionResult will have two properties: the cipher text as a base64 string and as a byte array:

public class AsymmetricEncryptionResult : ResponseBase
{
	public string CipherText { get; set; }
	public byte[] CipherBytes { get; set; }
}

To encrypt a message we’ll need a plain text and the public-key-only part of the asymmetric keys:

public class AsymmetricEncryptionArguments
{
	public XDocument PublicKeyForEncryption { get; set; }
	public string PlainText { get; set; }		
}

Finally in order to decrypt a cipher text we’ll need the complete set of keys and the cipher text itself:

public class AsymmetricDecryptionArguments
{
	public XDocument FullAsymmetricKey { get; set; }
	public string CipherText { get; set; }
}

The project should compile at this stage.

The implementation: RSA

We’ll go for one of the most widely used implementation of asymmetric encryption, RSA. .NET has good support for it so the implementation if straightforward. Insert a class called RsaAsymmetricCryptographyService into the Asymmetric folder:

public class RsaAsymmetricCryptographyService : IAsymmetricCryptographyService
{
	public AsymmetricKeyPairGenerationResult GenerateAsymmetricKeys(int keySizeInBits)
	{
		try
		{
			RSACryptoServiceProvider myRSA = new RSACryptoServiceProvider(keySizeInBits);
			XDocument publicKeyXml = XDocument.Parse(myRSA.ToXmlString(false));
			XDocument fullKeyXml = XDocument.Parse(myRSA.ToXmlString(true));
			return new AsymmetricKeyPairGenerationResult(fullKeyXml, publicKeyXml) { Success = true };
		}
		catch (Exception ex)
		{
			return new AsymmetricKeyPairGenerationResult(new XDocument(), new XDocument()) { ExceptionMessage = ex.Message };
		}
	}

	public AsymmetricEncryptionResult Encrypt(AsymmetricEncryptionArguments arguments)
	{
		AsymmetricEncryptionResult encryptionResult = new AsymmetricEncryptionResult();
		try
		{
			RSACryptoServiceProvider myRSA = new RSACryptoServiceProvider();
			string rsaKeyForEncryption = arguments.PublicKeyForEncryption.ToString();
			myRSA.FromXmlString(rsaKeyForEncryption);
			byte[] data = Encoding.UTF8.GetBytes(arguments.PlainText);
			byte[] cipherText = myRSA.Encrypt(data, false);
			encryptionResult.CipherBytes = cipherText;
			encryptionResult.CipherText = Convert.ToBase64String(cipherText);
			encryptionResult.Success = true;
		}
		catch (Exception ex)
		{
			encryptionResult.ExceptionMessage = ex.Message;
		}
		return encryptionResult;
	}

	public CipherTextDecryptionResult Decrypt(AsymmetricDecryptionArguments arguments)
	{
		CipherTextDecryptionResult decryptionResult = new CipherTextDecryptionResult();
		try
		{
			RSACryptoServiceProvider cipher = new RSACryptoServiceProvider();
			cipher.FromXmlString(arguments.FullAsymmetricKey.ToString());
			byte[] original = cipher.Decrypt(Convert.FromBase64String(arguments.CipherText), false);
			decryptionResult.DecodedText = Encoding.UTF8.GetString(original);
			decryptionResult.Success = true;
		}
		catch (Exception ex)
		{
			decryptionResult.ExceptionMessage = ex.Message;
		}
		return decryptionResult;
	}
}

If you’re already familiar with asymmetric cryptography in .NET this should be straightforward. Otherwise consult the above link for the technical details, I won’t repeat them here.

The methods in the interface can be chained together in an example as follows:

private static void TestAsymmetricEncryptionService()
{
	IAsymmetricCryptographyService asymmetricSevice = new RsaAsymmetricCryptographyService();
	AsymmetricKeyPairGenerationResult keyPairGenerationResult = asymmetricSevice.GenerateAsymmetricKeys(1024);
	if (keyPairGenerationResult.Success)
	{
		AsymmetricEncryptionArguments encryptionArgs = new AsymmetricEncryptionArguments()
		{
			PlainText = "Text to be encrypted"
			, PublicKeyForEncryption = keyPairGenerationResult.PublicKeyOnlyXml
		};
		AsymmetricEncryptionResult encryptionResult = asymmetricSevice.Encrypt(encryptionArgs);
		if (encryptionResult.Success)
		{
			Console.WriteLine("Ciphertext: {0}", encryptionResult.CipherText);
			AsymmetricDecryptionArguments decryptionArguments = new AsymmetricDecryptionArguments()
			{
				CipherText = encryptionResult.CipherText
				, FullAsymmetricKey = keyPairGenerationResult.FullKeyPairXml
			};
			CipherTextDecryptionResult decryptionResult = asymmetricSevice.Decrypt(decryptionArguments);
			if (decryptionResult.Success)
			{
				Console.WriteLine("Decrypted text: {0}", decryptionResult.DecodedText);
			}
			else
			{
				Console.WriteLine("Decryption failure: {0}", decryptionResult.ExceptionMessage);
			}
		}
		else
		{
			Console.WriteLine("Encryption failure: {0}", encryptionResult.ExceptionMessage);
		}
	}
	else
	{
		Console.WriteLine("Asymmetric key generation failure: {0}", keyPairGenerationResult.ExceptionMessage);
	}
}

If you run this code you’ll see that the keys are generated, the plain text is encrypted and then decrypted again.

In the next post, which will be last post of this series on dependencies, we’ll look at two other aspects in cryptography: hashing and digital signatures.

View the list of posts on Architecture and Patterns here.

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.