Extension Method

Extension Method

The extension method feature allowing to write methods associated with classes other than the class in which they are declared. For example I have a class like following:

class MyData
{
 private double D1;                              // Fields
 private double D2;
 private double D3;

 public MyData(double d1, double d2, double d3)  // Constructor
 {
  D1 = d1; D2 = d2; D3 = d3;
 }

 public double Sum()                             // Method Sum
 {
  return D1 + D2 + D3;
 }
}

When you want to add method to the class, we can use extension method. With following requirements:

• The class in which the extension method is declared must be declared static.
• The extension method itself must be declared static.
• The extension method must contain as its first parameter type the keyword this,followed by the name of the class it is extending.

The syntax will be:

static class [Extension Class]
{ 
 public static [Return Type ][Method Name]( this [Class to be Extended] [Identifier])
 {
  ...
 }
}

Lets add a method to calculate average to MyData class.

static class ExtendMyData
{  
  public static double Average( MyData md )
  {
  return md.Sum() / 3;
 }
} 

And here's our Main method:

class Program
{
 static void Main(string[] args)
 {
  MyData md = new MyData(3, 4, 5); 
  Console.WriteLine("Average: "+ ExtendMyData.Average(md) );
 }
}

The output will be:

Average: 4

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