Extension methods in .NET

What are Extension methods?

The most complete description of what are extension methods can be found in following quotes:

Extension methods enable you to “add” methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type. For client code written in C# and Visual Basic, there is no apparent difference between calling an extension method and the methods that are actually defined in a type. MSDN Documentation

Extension methods allow developers to add new methods to the public contract of an existing CLR type, without having to sub-class it or recompile the original type. Extension Methods help blend the flexibility of “duck typing” support popular within dynamic languages today with the performance and compile-time validation of strongly-typed languages. Scott Gu

Info Notice: You can add an extension method to the Object type so it can be used in all the other types of the framework.

Warning Notice: Extension methods present no specific security vulnerabilities. They can never be used to impersonate existing methods on a type, because all name collisions are resolved in favor of the instance or static method defined by the type itself. Extension methods cannot access any private data in the extended class.

When to use

Consider the following to determined wheter or not a method should be used as an extension method:

• Its primary parameter is an instance?
• The method logically operates on that instance?
• Do you want it to appear to others when using that instance?

How to create

The creation of an Extension Method is quite simple, just follow this steps:

1. Define a static class to contain the extension method. The class must be visible to client code.

2. Implement the extension method as a static method with at least the same visibility as the containing class.

3. The first parameter of the method specifies the type that the method operates on; it must be preceded with the this modifier.

4. In the calling code, add a using directive to specify the namespace that contains the extension method class.

5. Call the methods as if they were instance methods on the type.

Example

The following code shows you how to add an extension method to the Datetime type. It allows you to retrieve the number of the week according to the international standard ISO 8601.

public static class GlobalFunctions
{
    public static int WeekOfYearISO8601(this DateTime date)
    {
        var day = (int)CultureInfo.CurrentCulture.Calendar.GetDayOfWeek(date);
        var week = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date.AddDays(4 - (day == 0 ? 7 : day)), CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

        return week;
    }
}

Two conditions were added to validate the method. As you can see calling the extension method is straight forward once you have created.

public void TestISO8601WeekDates()
{
    // According to wikipedia
    // Monday 29 December 2008 is week 1
    // Sunday 3 January 2010 is written week 53

    // Ready 
    DateTime dt1 = new DateTime(2008, 12, 29);
    DateTime dt2 = new DateTime(2010, 1, 3);

    // Set
    int week4dt1 = dt1.WeekOfYearISO8601();
    int week4dt2 = dt2.WeekOfYearISO8601();

    // Go
    Assert.AreEqual(1, week4dt1);
    Assert.AreEqual(53, week4dt2);
}

You can download the full solution for VS2015 here:

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