Extension Methods Hello World : CSharp Feeds

Extension Methods Hello World

by gavin via elephantsintheroom.org on 12/19/2007 1:15:00 AM

Extension methods allow you to create new public methods for a type without having to edit the type's code. They have been used by Microsoft in the implementation of Linq.

The following example shows how int[] has been extended with a Min() method.

using System.Linq;//without this, the int.Min extension will not be available
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] intArray = { 12, 24, 45 };
            int i = intArray.Min();//i = 12
        }
    }
}

Extension methods allow a new compile-time error scenario: the correct assembly for a type is referenced and brought into scope through a 'using' statement

using System.IO;
class Program
{
    static void Main(string[] args)
    {
        BinaryReader b;
    }
}

or by fully-qualifying the type

class Program
{
    static void Main(string[] args)
    {
        System.IO.BinaryReader b;
    }
}

but the namespace containing the extension methods that you want to use isn't in scope.
The syntax for calling an extension method (that is, the same syntax as for a normal method) does not allow for full-qualification - so, when the extension method that you want to use is defined in a different namespace from the type itself, you are forced to use a 'using' statement. As far as I'm aware, this is the only situation where you are forced to use a 'using'.

In the following 'Hello World' example, the string type is extended with 2 new methods: AppendWorld and AppendAnything.

Like other C# language changes, for example generics and Linq, extension methods can reduce the number of lines of code, but increase the - already vast - number of ways of doing the simplest thing.

This code shows how extension methods work, but is not an example of when they should be used.

using System;
namespace ExtensionMethodsHelloWorld
{
    //Extension methods must be defined in a non-generic static class
    public static class SomeExtensionMethods//this name is arbitrary
    {
        //The first parameter in the signature of an extension method 
        //must be preceded by the keyword 'this'.
        //Calling "Hello".AppendWorld() uses "Hello" as the value of 
        //theString within the AppendWorld method. Note the empty 
        //parenthesis in the method call.
        public static string AppendWorld(this string theString)
        {
            return theString + " World";
        }
        //Second and subsequent parameters, if present, work in the 
        //normal way - so calling "From ".AppendAnything("Gavin") 
        //uses "From " as theString and "Gavin" as stringToAppend. 
        public static string AppendAnything(this string theString
            , string stringToAppend)
        {
            return theString + stringToAppend;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello".AppendWorld());
            Console.WriteLine("From ".AppendAnything("Gavin"));
            Console.ReadLine();
        }
    }
}

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Original Post: Extension Methods Hello World

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Wednesday, December 19, 2007

Extension Methods Hello World

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