c# - "this" in function parameter

Question

Looking at some code examples for HtmlHelpers, and I see declarations that look like:

public static string HelperName(this HtmlHelper htmlHelper, ...more regular params )

I can't remember seeing this type of construct any where else - can someone explain the purpose of the "this"? I thought that by declaring something public static meant that the class did not need to be instantiated - so what is "this" in this case?

Answer 1

This is the syntax for declaring extension methods, a new feature of C# 3.0.

An extension method is part code, part compiler "magic", where the compiler with the help of intellisense in Visual Studio make it appear that your extension method is actually available as an instance method on the object in question.

Let me give an example.

There's no method on the String class that is named GobbleGobble, so let's create an extension method:

public static class StringExtensions
{
    public static void GobbleGobble(this string s)
    {
        Console.Out.WriteLine("Gobble Gobble, " + s);
    }
}

The class name is just my naming convention, it isn't necessary to name it like that, but it has to be static, as do the method.

After declaring the above method, you can, in Visual Studio, type this:

String s = "Turkey Baster!";
s.

after the dot, wait for intellisense, and notice there is a GobbleGobble method there, complete the code like this:

String s = "Turkey Baster!";
s.GobbleGobble();

Important: The class where the extension method is declared must be available to the compiler and the intellisense processor in order for intellisense to show the method. If you type in GobbleGobble manually, and use the Ctrl+. shortcut, it will not help you get the right using directives into the file.

Notice that the parameter to the method has disappeared. The compiler will silently move around the important bits, which are:

String s = "Turkey Baster!";
s.GobbleGobble();
^     ^
|     +-- the compiler will find this in the StringExtensions class
|
+-- will be used as the first parameter to the method

Thus, the above code will be transformed by the compiler to this:

String s = "Turkey Baster!";
StringExtensions.GobbleGobble(s);

So at call-time, there's nothing magical about it, it's just a call to a static method.

Note that if your extension method declares more than one parameter, only the first supports the this modifier, and the rest has to be specified as part of the method call as normal:

public static void GobbleGobble(this string value, string extra)
{                                            |              |
    ...                                      |              |
}                                            |              |
                                             |              |
+--------------------------------------------+              |
|                                                           |
v                                                           |
s.GobbleGobble("extra goes here");                          |
                        ^                                   |
                        |                                   |
                        +-----------------------------------+

Extension methods was added in part due to Linq, where the Linq syntax of C# will look for appropriately named extension methods for the objects in play, which means you can "introduce" Linq-support into any type of class by just declaring the right extension methods. Of course, full Linq support is a lot of work, but it is possible.

Also, extension methods by themselves are really useful, so read up on it.

Here's a few links:

• MSDN: Extension Methods (C# Programming Guide)
• MSDN Magazine - Basic Instincts: Extension Methods
• Wikipedia: Extension Method