Creating a read-only collection from an array in C#

November 25, 2016 Leave a comment

The Array class has a number of interesting methods. One of them allows you to easily convert an array of T into a read-only collection of T:

string[] bands = new string[5] { "Queen", "ACDC", "Metallica", "Genesis", "INXS" };
IReadOnlyCollection<string> readOnlyBands = Array.AsReadOnly<string>(bands);

Note that readOnlyBands is, well, read-only, so there’s no Add or Remove method that otherwise are often available on lists.

View all various C# language feature related posts here.

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Mixing asymmetric and symmetric encryption, HMAC hash verification and digital signatures in .NET

November 24, 2016 Leave a comment

Introduction

In this post we built a test application where we mixed asymmetric and symmetric encryption with HMAC hash verification. The message sender gets the asymmetric public key of the receiver and uses it to encrypt a symmetric public key. The message is encrypted with a one-time symmetric public key. The symmetric key is also used to calculate the HMAC of the cipher text. The receiver decrypts the symmetric key with her asymmetric private key and calculates the HMAC. If the hashes match then it’s safe to assume that the message hasn’t been tampered with on its way to the sender.

The communication flow is quite secure but we can tighten security even more. In this post we learnt about digital signatures. A digital signature is used to sign the hash of a message with the sender’s private key. The public key that matches the private signature key is sent along all other information to the sender. The sender can then check the validity of the signature using the provided public key. The trust is based on the fact that the public and private keys go hand in hand, therefore signature verification doesn’t need the private key.

The goal of this post is to extend the demo application with digital signatures. The sender will sign the message and the receiver will verify the validity of the signature.

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Extension methods in C#

November 23, 2016 Leave a comment

Introduction

Extension methods in C# allow you to extend the functionality of types that you didn’t write and don’t have direct access to. They look like integral parts of any built-in classes in .NET, e.g.:

DateTime.Now.ToMyCustomDate();
string.ToThreeLetterAbbreviation();

You can extend the following types in C#:

  • Classes
  • Structs
  • Interfaces

You can extend public types of 3rd party libraries. You can also extend generic types, such as List of T and IEnumerable of T. You cannot extend sealed classes.

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A summary of new features in C# 6

November 22, 2016 2 Comments

Here’s a short list of new features in C# 6 and a link to a blog post explaining their usage:

View all various C# language feature related posts here.

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Explicit interface implementation in .NET

November 21, 2016 2 Comments

Introduction

The generic and well-known Dictionary object and its generic and thread-safe counterpart, i.e. the ConcurrentDictionary object both implement the generic IDictionary interface. The IDictionary interface has an Add method where you can insert a new key-value pair into a dictionary:

Dictionary<string, string> singleThreadedDictionary = new Dictionary<string, string>();
singleThreadedDictionary.Add("Key", "Value");

I can even rewrite the above code as follows:

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Lambda expressions in Java

November 20, 2016 Leave a comment

Introduction

If you’re familiar with .NET then you already know what Lambda expressions are and how useful they can be. They were not available in Java before version 8. Let’s investigate how they can be applied in Java.

First example: an interface method with a single parameter

Say you have the following Employee class:

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How to calculate the message digest in Java

November 19, 2016 Leave a comment

A message digest is an important concept in cryptography. A digest is an array of bytes created by a hashing formula. It is used to make sure that some digital information has not been tampered with. In a sense it is a footprint of an object, such as a file. If someone modifies the file then the footprint also changes. Then we know that the file has been changed. Another word for a message digest is checksum. There are various hashing algorithms to perform the calculation. SHA-256 and MD5 are the most common ones.

For an example you can check out the Apacha log4j2 download page here. You’ll see a column called “checksum” for various files. If you click on one of those you’ll see the MD5 hash of the file in a relatively human readable form, such as “31826c19fff94790957d798cb1caf29a”.

Java and other popular programming languages have built-in classes to construct a message digest. Let’s see an example from Java.

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Finding unique values using the LINQ Distinct operator

November 18, 2016 Leave a comment

Extracting unique values from a sequence of objects in LINQ is very easy using the Distinct operator. It comes in two versions: one with a default equality comparer and another which lets you provide a custom comparer.

We’ll use the following collection for the first demo:

string[] bands = { "ACDC", "Queen", "Aerosmith", "Iron Maiden", "Megadeth", "Metallica", "Cream", "Oasis", "Abba", "Blur", "Chic", "Eurythmics", "Genesis", "INXS", "Midnight Oil", "Kent", "Madness", "Manic Street Preachers"
, "Noir Desir", "The Offspring", "Pink Floyd", "Rammstein", "Red Hot Chili Peppers", "Tears for Fears"
, "Deep Purple", "KISS"};

These are all unique values so let’s create some duplicates:

IEnumerable<string> bandsDuplicated = bands.Concat(bands);

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An overview of digital signatures in .NET

November 17, 2016 Leave a comment

Introduction

A digital signature in software has about the same role as a real signature on a document. It proves that a certain person has signed the document thereby authenticating it. A signature increases security around a document for both parties involved, i.e. the one who signed the document – the signee – and the one that uses the document afterwards. The one who signed can claim that the document belongs to them, i.e. it originates from them and cannot be used by another person. If you sign a bank loan request then you should receive the loan and not someone else. Also, the party that takes the document for further processing can be sure that it really originates from the person who signed it. The signee cannot claim that the signature belongs to some other person and they have nothing to do with the document. This latter is called non-repudiation. The signee cannot deny that the document originates from him or her.

Digital signatures in software are used to enhance messaging security. The receiver must be able to know for sure that the message originated with one specific sender and that the sender cannot claim that it was someone else who sent the message. While it is quite possible to copy someone’s signature on a paper document it is much harder to forge a strong digital signature.

In this post we’ll review how digital signatures are implemented in .NET

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Using the StringComparer class for string equality with C# .NET

November 16, 2016 Leave a comment

In this post we saw how to use the generic IEqualityComparer of T interface to indicate equality for our custom types. If you need a similar comparer for strings then there’s a ready-made static class called StringComparer which can construct string comparers for you.

The StringComparer class provides comparers for the common string comparison scenarios: ordinal, locale specific and invariant culture comparisons. This is a good MSDN article on the differences between these.

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