Why do we put "this" in front of a parameter?

Question

For ex:

public static Domain.Recruitment.Recruitment Map(this Data.Recruitment dv) 
{
 //some code
  return new Domain.Recruitment.Recruitment{} 
}

Answer 1

It marks the method as an 'extension method' that can be called like an instance method on an object and allows you "to 'add' methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type":

• http://msdn.microsoft.com/en-us/library/bb383977.aspx

Answer 2

Adding "this" to a parameter creates an extension method. An extension method acts like an instance method in that you can call it on instances of the type of the first parameter in the extension method.

For example, the following is possible since Map is marked as an extension method

Data.RecruitMent dv = ...;
dv.Map();