Extension methods in C#

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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Extending class definitions with partial classes in C# .NET

The ‘partial’ keyword helps you divide your classes into multiple files within the same namespace. One obvious usage of partial classes is to split the definition of a large type into smaller chunks. You cannot just use the partial keyword with classes but methods as well.

The partial classes will reside in two – or more – different cs files in the same namespace. Say you have a partial Customer class in the project-name/domains folder:

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TCP level communication with C# .NET

.NET has the necessary objects to enable TCP-level messaging in the System.Net.Sockets namespace. The key objects to build an extremely simple TCP server are TcpListener and Socket. The TCP client and server can communicate in a stream-like fashion over the network using the NetworkStream object.

Here’s an example of how a TCP server can ingest the message of a single client:

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Replacing substrings using Regex in C# .NET: date format example

Say your application receives the dates in the following format:

mm/dd/yy

…but what you actually need is this:

dd-mm-yy

You can try and achieve that with string operations such as IndexOf and Replace. You can however perform more sophisticated substring operations using regular expressions. The following method will perform the required change:

private static string ReformatDate(String dateInput)
{
	return Regex.Replace(dateInput, "\\b(?<month>\\d{1,2})/(?<day>\\d{1,2})/(?<year>\\d{2,4})\\b"
		, "${day}-${month}-${year}");
}

Calling this method with “10/28/14” returns “28-10-14”.

View all posts related to string and text operations here.

Monitor the file system with FileSystemWatcher in C# .NET

In this post we’ll look at how you can use the FileSystemWatcher object to monitor the Windows file system for various changes.

A FileSystemWatcher object enables you to be notified when some change occurs in the selected part of the file system. This can be any directory, such as “c:\” or any subdirectory under the C: drive. So if you’d like to make sure you’re notified if a change occurs on e.g. “c:\myfolder” – especially if it’s editable by your colleagues – then FileSystemWatcher is a good candidate.

Consider the following Console application:

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Join custom objects into a concatenated string in .NET C#

Say you have the following Customer object with an overridden ToString method:

public class Customer
{
	public int Id { get; set; }
	public string Name { get; set; }
	public string City { get; set; }

	public override string ToString()
	{
		return string.Format("Id: {0}, name: {1}, city: {2}", Id, Name, City);
	}
}

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Examining class members through Types and Reflection in .NET C#

Here we saw different ways to get hold of a Type. You can use the Type object to extract different ingredients of a class such as methods, properties, events etc. through various methods. The name of the object that these methods return ends with “Info”, like FieldInfo, MethodInfo etc.

These Info classes all derive from the MemberInfo abstract base class. The Type object also derives from MemberInfo.

Say you have the following Customer class:

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How to hash passwords with a salt in .NET

In this post we learnt about using hashing in .NET. We also saw one of its basic functions in the same post which is message verification. In this post we saw how hashing coupled with a random key can be used for message authentication.

We also mentioned another common usage of hashing which is password storage. A password should never be stored as clear text in your system. Instead we save its hash value and when a user enters a password in a login field then we compare the hashed values instead of the plain string passwords. However, a simple one-way hash is generally still not good enough.

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Using HMACs to authenticate a hash in .NET

In this post we learnt about using hashing in .NET. Hashes provide a one-way encryption of a message where the hash value ideally cannot be “unhashed”, i.e. we cannot build the original string from it. A hash or message digest helps us verify whether the message has been tampered with by a third party after it was sent to us.

We can take a step further and add an extra layer of security on our hashes. After all a message and its hash could originate from anyone. How can we verify the authenticity of the message as well? That’s where Hashed Message Authentication Codes, also called HashMACs or HMACs enter the picture.

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Hashing messages using various hash algorithms in .NET

A hash of a message can be viewed as its unique identifier. A series of bytes, such as a string, can be converted into hash, also called a message digest. A message digest is also a byte array that can be converted into a base 64 string for better readability. There are various hashing algorithms out there that can calculate the hash of a message. Ideally each algorithm should produce a different digest for each message, i.e. “Hello world” should yield a different hash from what “H3llo world” produces. I.e. a single change in the byte sequence will change the resulting hash. Also, it should be impossible to calculate the original message from a hash. Therefore hashing is a one-way cryptographic operation.

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