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Notice in the code sample that we insert line breaks and whitespace between method calls. We ll do this often in our code samples in order to improve code readability. This isn t a new feature offered by C# 3.0, because it s supported by all versions of C#.
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You ll see more examples of chaining constructs in the next sections. As you ll see in the next chapter, this is a key feature for writing LINQ queries. For the moment, let s see how to declare extension methods in VB.NET. Declaring extension methods in VB.NET In VB.NET, extension methods are shared methods decorated with a custom attribute (System.Runtime.CompilerServices.ExtensionAttribute) that allow them to be invoked with instance-method syntax. (An extension method can be a Sub procedure or a Function procedure.) This attribute is provided by the new System.Core.dll assembly.
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In VB.NET, extension methods should be declared in a module.
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The first parameter in a VB.NET extension method definition specifies which data type the method extends. When the method is run, the first parameter is bound to the instance of the data type against which the method is applied. Listing 2.17 shows how we would declare our TotalMemory extension method in VB.NET.
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Listing 2.17 Sample extension method in VB.NET (ExtensionMethods.vbproj)
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<System.Runtime.CompilerServices.Extension()> _ Public Function TotalMemory( _ ByVal processes As IEnumerable(Of ProcessData)) _ As Int64
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C# and VB.NET language enhancements
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Dim result As Int64 = 0 For Each process In processes result += process.Memory Next Return Result End Function
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NOTE
Extension members of other kinds, such as properties, events, and operators, are being considered by Microsoft for the future but are currently not supported in C# 3.0 and VB.NET 9.0.
To give you a better idea of what can be done with extension methods and why they are useful, we ll now use some standard extension methods provided with LINQ.
More examples using LINQ s standard query operators
LINQ comes with a set of extension methods you can use like any other exten-
sion method. We ll use some of them to show you more extension methods in action and give you a preview of the standard query operators, which we ll cover in the next chapter. OrderByDescending Let s say that we d like to sort the list of processes by their memory consumption, memory hogs first. We can use the OrderByDescending extension method defined in the System.Linq.Enumerable class. Extension methods are imported through using namespace directives. For example, to use the extension methods defined in the Enumerable class, we need to add the following line of code to the top of our code file if it s not already there:
using System.Linq;
NOTE
Your project also needs a reference to System.Core.dll, but this is added by default for new projects.
We re now able to call OrderByDescending as follows to sort our processes:
ObjectDumper.Write( processes.OrderByDescending(process => process.Memory));
You can see that we provide the extension method with a lambda expression to decide how the sort operation will be performed. Here we indicate that we want to compare the processes based on their memory consumption. It s important to note that type inference is used automatically to simplify the code. Although OrderByDescending is defined as a generic method, we don t need to explicitly indicate the types we re dealing with. The C# compiler deduces
Extension methods
from the method call that OrderByDescending works here on Process objects and returns an enumeration of Int64 objects. When a generic method is called without specifying type arguments, a type inference process attempts to infer type arguments for the call. The presence of type inference allows a more convenient syntax to be used for calling a generic method, and allows the programmer to avoid specifying redundant type information. Here is how OrderByDescending is defined:
public static IOrderedSequence<TSource> OrderByDescending<TSource, TKey>( this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
Here is how we would have to use it if type inference weren t occurring:
processes.OrderByDescending<Process, Int64>( (Process process) => process.Memory));
The code would be more difficult to read without type inference because we d have to specify types everywhere in LINQ queries. Let s now look at other query operators. Take If we re interested only in the two processes that consume the most memory, we can use the Take extension method:
ObjectDumper.Write( processes .OrderByDescending(process => process.Memory) .Take(2));
The Take method returns the first n elements in an enumeration. Here we want two elements. Sum If we want to sum the amount of memory used by the two processes, we can use another standard extension method: Sum. The Sum method can be used in place of the extension method we created, TotalMemory. Here is how to use it:
ObjectDumper.Write( processes .OrderByDescending(process => process.Memory) .Take(2) .Sum(process => process.Memory)/1024/1024);
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