I've come across several instances of C# code like the following:
public static int Foo(this MyClass arg)
I haven't been able to find an explanation of what the this keyword means in this case. Any insights?
312
votes
In addition to the accepted answer, for C# Extension Methods one must check out official MSDN documentation.
– vulcan raven
Extension methods are convenient but we must use caution when creating/using them - they can break! Quote from the General Guidelines in the official MSDN documentation: "In general, we recommend that you implement extension methods sparingly and only when you have to. ... When using an extension method to extend a type whose source code you cannot change, you run the risk that a change in the implementation of the type will cause your extension method to break."
– Krishna Gupta
Does that mean this would be possible? Because that's what i'm looking for.
– mekb
6 Answers
299
votes
This is an extension method. See here for an explanation.
Extension methods allow developers to add new methods to the public contract of an existing CLR type, without having to sub-class it or recompile the original type. Extension Methods help blend the flexibility of "duck typing" support popular within dynamic languages today with the performance and compile-time validation of strongly-typed languages.
Extension Methods enable a variety of useful scenarios, and help make possible the really powerful LINQ query framework... .
it means that you can call
MyClass myClass = new MyClass();
int i = myClass.Foo();
rather than
MyClass myClass = new MyClass();
int i = Foo(myClass);
This allows the construction of fluent interfaces as stated below.
70
votes
Scott Gu's quoted blog post explains it nicely.
For me, the answer to the question is in the following statement in that post:
Note how the static method above has a "this" keyword before the first parameter argument of type string. This tells the compiler that this particular Extension Method should be added to objects of type "string". Within the IsValidEmailAddress() method implementation I can then access all of the public properties/methods/events of the actual string instance that the method is being called on, and return true/false depending on whether it is a valid email or not.
12
votes
In addition to Preet Sangha's explanation:
Intellisense displays the extension methods with a blue arrow (e.g. in front of "Aggregate<>"):
You need a
using the.namespace.of.the.static.class.with.the.extension.methods;
for the extension methods to appear and to be available, if they are in a different namespace than the code using them.
8
votes
4
votes
I just learnt this myself the other day: the this keyword defines that method has being an extension of the class that proceeds it. So for your example, MyClass will have a new extension method called Foo (which doesn't accept any parameter and returns an int; it can be used as with any other public method).
1
votes
"this" extends the next class in the parameter list
So in the method signature below "this" extends "String". Line is passed to the function as a normal argument to the method. public static string[] SplitCsvLine(this String line)
In the above example "this" class is extending the built in "String" class.
