Use of this keyword in formal parameters for static methods in C

Question

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

User · Answer

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

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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

User · Answer

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

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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

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They are extension methods   Welcome to a whole new fluent world

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In addition to Preet Sangha s explanation  Intellisense displays the extension methods with a blue arrow  e g  in front of  Aggregate lt         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

[c#] Use of "this" keyword in formal parameters for static methods in C#

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