All 11 Extension Methods questions
More in Extension Methods
What is an extension method, and what syntax makes a static method into one? Why must extension methods be defined in a static class, and how does the compiler resolve a call to one? How do you write a generic extension method that works with any collection type, like a custom 'IsEmpty()' check? How do you chain multiple extension methods together in a fluent style, and what design principle makes this possible? What are extension methods in C#, and how do they let you add functionality to a type you can't modify? How would you write an extension method on an interface (like IEnumerable<T>) that provides a default implementation shared across EVERY concrete implementing type? How would you write extension methods to build a lightweight, fluent validation/guard-clause library? Why can't extension methods be used to add STATIC members, operators, or FIELDS to an existing type, and what are the practical implications of this limitation? How do 'extension method ambiguity' conflicts arise when two different NuGet packages or namespaces define extension methods with the identical signature, and how do you resolve them? How would you write generic extension methods leveraging C# 11's generic math interfaces (like INumber<T>) to build a single reusable numeric utility working across int, double, decimal, and other numeric types?
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How does method resolution priority work when an extension method has the SAME name and signature as an actual instance method on the type?

Intermediate

Answer

A genuine INSTANCE method on the type ALWAYS takes priority over an extension method with a matching signature — the compiler only falls back to searching for an applicable extension method if no instance method (or property/field) matches, meaning you can never accidentally 'override' real instance method behavior with an extension method.
public class Calculator {
  public int Add(int a, int b) => a + b; // real instance method
}
public static class CalculatorExtensions {
  public static int Add(this Calculator calc, int a, int b) => 999; // never actually called
}
new Calculator().Add(2, 3); // always calls the instance method, returns 5
Real-world example Understanding why adding an extension method with the same name as an existing instance method silently has no effect on calls to that instance method.

Common follow-ups: What happens if TWO different extension methods (from different namespaces) both match the same call?

Fundamentals
All 11 Extension Methods questions