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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? 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 would you write extension methods to build a lightweight, fluent validation/guard-clause library?

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Answer

Chain extension methods on a generic wrapper (or directly on the value type being validated) where each validation method either RETURNS the original value (for chaining) or THROWS if the condition fails — producing concise, self-documenting validation code without a heavy external dependency.
public static class GuardExtensions {
  public static T NotNull<T>(this T value, string paramName) where T : class =>
    value ?? throw new ArgumentNullException(paramName);

  public static int Positive(this int value, string paramName) =>
    value > 0 ? value : throw new ArgumentOutOfRangeException(paramName, "Must be positive");
}

public Order(Customer customer, int quantity) {
  Customer = customer.NotNull(nameof(customer));
  Quantity = quantity.Positive(nameof(quantity));
}
Real-world example Building a lightweight, dependency-free set of reusable guard-clause validators for constructor and method argument checking.

Common follow-ups: How does this pattern compare to using the newer built-in ArgumentNullException.ThrowIfNull() helper method?

Exception Handling
All 11 Extension Methods questions