All 11 Extension Methods questions
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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? 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 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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C# › Extension Methods

How would you write an extension method on an interface (like IEnumerable<T>) that provides a default implementation shared across EVERY concrete implementing type?

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Answer

Extension methods on an interface type automatically become available to ALL classes/structs implementing that interface, without those types needing any changes — this is precisely how virtually all of LINQ works, adding dozens of query methods to every IEnumerable<T> implementation across the entire .NET ecosystem, including your own custom collections.
public static class MyLinqExtensions {
  public static T? SecondOrDefault<T>(this IEnumerable<T> source) {
    using var enumerator = source.GetEnumerator();
    if (enumerator.MoveNext() && enumerator.MoveNext()) return enumerator.Current;
    return default;
  }
}

var list = new List<int> { 1, 2, 3 };
int second = list.SecondOrDefault(); // 2 -- works automatically, since List<T> implements IEnumerable<T>
Real-world example Adding a custom LINQ-style query method that automatically works across every existing and future IEnumerable<T> implementation.

Common follow-ups: Why can interfaces' default interface methods (a C# 8+ feature) sometimes achieve a similar goal, and how do they differ from extension methods?

Interfaces & Abstract Classes
All 11 Extension Methods questions