Extension Methods

Extension methods are a special kind of static method.
Listing 7-2. Library Pattern Introduces the IsWeekend Helper Method

   1: public static class DateTimeHelper

   2: {

   3:     using System;

   4:     public static bool IsWeekend(DateTime dateTime)

   5:     {

   6:         switch (dateTime.DayOfWeek)

   7:         {

   8:             case DayOfWeek.Saturday:

   9:             case DayOfWeek.Sunday:

  10:             return true;

  11:             default:

  12:             return false;

  13:         }

  14:     }

  15: }

Listing 7-3. Helper Method Approach to Calling IsWeekend

   1: DateTime importantDate = new DateTime(2012, 5, 7);

   2: if (DateTimeHelper.IsWeekend(importantDate))

   3:     WeekendProcessing();

   4: else

   5:     WeekdayProcessing();

Listing 7-4. Declaring the IsWeekend Extension Method

   1: namespace Lender.Slos.Extensions

   2: {

   3:     using System;

   4:     public static class DateTimeExtensions

   5:     {

   6:         public static bool IsWeekend(this DateTime dateTime)

   7:         {

   8:             switch (dateTime.DayOfWeek)

   9:             {

  10:                 case DayOfWeek.Saturday:

  11:                 case DayOfWeek.Sunday:

  12:                 return true;

  13:                 default:

  14:                 return false;

  15:             }

  16:         }

  17:     }

  18: }

Writing the IsWeekend method as an extension method is shown in Listing 7-4. The only real difference between the helper method in Listing 7-2 and the extension method in List 7-4 is that the method parameter starts with the this keyword. The static keyword is applied in the same way and the logic is identical.

Listing 7-5. Extension Method Approach to Calling IsWeekend

   1: DateTime importantDate = new DateTime(2011, 5, 7);

   2: if (importantDate.IsWeekend())

   3:     WeekendProcessing();

   4: else

   5:     WeekdayProcessing();

In many ways this is remarkable. From the code listing and through Visual Studio IntelliSense, the IsWeekend method appears to be defined within System.DateTime, a .NET type that is defined in another assembly. The extension method approach has the following benefits:

• Code reuse through a helper method
• Straightforward and readable code
• Visual Studio IntelliSense support
• Code semantics are clear and convenient