c# - “this” in function parameter in web api -

i developing web api, have created htmllhelper class, , see declarations like:

public static string getcountrydomain(this system.web.mvc.htmlhelper htmlhelper, area area)     {         //body     } 

in controller want call above function. expects 2 parameters.

htmlhelper.getcountrydomain(area_id); 

can explain first parameter need pass in order call above function?

this keyword marks extension method. there 2 ways of calling it:

• you can call if instance method on htmlhelper, or
• you can call normal static method passing 2 parameters.

the first way more common (after all, that's point of making method extension)

// prepare parameters htmlhelper helper = ... area area = ... // call function string countrydomain = helper.getcountrydomain(area);