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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?
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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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Answer

Constrain the extension method's generic type parameter to INumber<T> (or a more specific numeric interface like IFloatingPoint<T>), letting you write ONE generic implementation using standard arithmetic operators that works correctly and efficiently across every built-in (and custom) numeric type implementing that interface, without needing separate overloads for each numeric type.
public static class NumericExtensions {
  public static T Clamp<T>(this T value, T min, T max) where T : INumber<T> =>
    value < min ? min : (value > max ? max : value);
}

int clampedInt = 15.Clamp(0, 10);       // 10
double clampedDouble = 2.5.Clamp(0.0, 5.0); // 2.5
Real-world example Writing a single generic Clamp(), Sum(), or Average() utility that works identically across int, double, decimal, and float without duplicated overloads.

Common follow-ups: What numeric types beyond the built-in ones can implement INumber<T> to also benefit from generic extension methods like this?

Generics
All 11 Extension Methods questions