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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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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How to create custom string formatters with C# .NET

.NET has a fairly large number of built-in string formatters that you can pass into the string.Format method. Here are some examples from the MSDN page about formatting:

String.Format("{0,-12}{1,8:yyyy}{2,12:N0}{3,8:yyyy}{4,12:N0}{5,14:P1}",
                                city.Item1, city.Item2, city.Item3, city.Item4, city.Item5,
                                (city.Item5 - city.Item3)/ (double)city.Item3);
String.Format("{0,-12}{1,8}{2,12}{1,8}{2,12}{3,14}\n",
                                    "City", "Year", "Population", "Change (%)");
String.Format("{0,-10:C}", 126347.89m);         

The IFormatProvider and ICustomFormatter interfaces will provide you with the methods required to create your own formats.

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Calculate standard deviation of integers with C# .NET

Here comes a simple extension method to calculate the standard deviation of a list of integers:

public static double GetStandardDeviation(this IEnumerable<int> values)
{
	double standardDeviation = 0;
	int[] enumerable = values as int[] ?? values.ToArray();
	int count = enumerable.Count();
	if (count > 1)
	{
		double avg = enumerable.Average();
		double sum = enumerable.Sum(d => (d - avg) * (d - avg));
		standardDeviation = Math.Sqrt(sum / count);
	}
	return standardDeviation;
}

Here’s how you can call this method:

List<int> ints = new List<int>() { 4, 7, 3, 9, 13, 90, 5, 25, 13, 65, 34, 76, 54, 12, 51, 23, 3, 1, 7 };
double stdev = ints.GetStandardDeviation();

View all various C# language feature related posts here.

How to pass any number of parameters into a method in C# .NET

You must have come across built-in methods in .NET where you can send any number of arguments into a method. E.g. string.Format has an overload where you can pass in a format string and then an array with the “params” modifier.

There’s nothing stopping you from using the same keyword to write a similar method, here’s an example:

public void MethodWithParams(int toBeMultiplied, params int[] multipliers)
{
	foreach (int m in multipliers)
	{ 
		Console.WriteLine(string.Format("{0} x {1} = {2}", toBeMultiplied, m, toBeMultiplied * m));
	}
}

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