C# 0 The Complete Reference

PART I

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better solution in many cases), it is still important that you understand how they work 

because of their integral importance to LINQ.

An extension method is a static method that must be contained within a static, non-generic 

class. The type of its first parameter determines the type of objects on which the extension 

method can be called. Furthermore, the first parameter must be modified by 

this

. The object on 

which the method is invoked is passed automatically to the first parameter. It is not explicitly 

passed in the argument list. A key point is that even though an extension method is declared 

static

, it can still be called on an object, just as if it were an instance method.

Here is the general form of an extension method:

static

ret-type name

(this

invoked-on-type ob

,

param-list

)

Of course, if there are no arguments other than the one passed implicitly to 

ob,

 then 

param-list

will be empty. Remember, the first parameter is automatically passed the object on which 

the method is invoked. In general, an extension method will be a public member of its class.

Here is an example that creates three simple extension methods:

// Create and use some extension methods.

using System;

static class MyExtMeths {

  // Return the reciprocal of a double.

  public static double Reciprocal(this double v) {

    return 1.0 / v;

  }

  // Reverse the case of letters within a string and

  // return the result.

  public static string RevCase(this string str) {

    string temp = "";

    foreach(char ch in str) {

      if(Char.IsLower(ch)) temp += Char.ToUpper(ch);

      else temp += Char.ToLower(ch);

    }

    return temp;

  }

  // Return the absolute value of n / d.

  public static double AbsDivideBy(this double n, double d) {

    return Math.Abs(n / d);

  }

}

class ExtDemo {

  static void Main() {

    double val = 8.0;

    string str = "Alpha Beta Gamma";

    // Call the Recip() extension method.

    Console.WriteLine("Reciprocal of {0} is {1}",

                      val, val.Reciprocal());

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    // Call the RevCase() extension method.

    Console.WriteLine(str + " after reversing case is " +

                      str.RevCase());

    // Use AbsDivideBy();

    Console.WriteLine("Result of val.AbsDivideBy(-2): " +

                      val.AbsDivideBy(-2));

  }

}

The output is shown here:

Reciprocal of 8 is 0.125

Alpha Beta Gamma after reversing case is aLPHA bETA gAMMA

Result of val.AbsDivideBy(-2): 4

In the program, notice that each extension method is contained in a static class called