Extension method is a way to add new functionality to an existing class without modifying its source code or creating a new derived class. It's like teaching an old dog (the existing class) new tricks (the extension method) without having to breed a whole new puppy (derived class).
"I don't have time to wait for the Facebook devs to add this feature, so I'll just whip up an extension method and call it a day."
"Oh great, another 'helpful' extension method from the new hire who thinks he's the next Bill Gates. I'm sure this won't break anything..."
Extension Methods in C#: Microsoft's official documentation on how to create and use extension methods in C#. It's about as exciting as reading the iTunes Terms of Service.
Extension Methods Best Practices: Some tips on using extension methods effectively, like a wise sensei guiding a young grasshopper. Except instead of waxing cars, you're extending classes.
Mads Torgersen on Extension Methods in C# 3.0: A video interview with the lead designer of C# about the reasoning behind extension methods. It's like a behind-the-scenes DVD extra, but for programming language features.
Note: the Developer Dictionary is in Beta. Please direct feedback to skye@statsig.com.