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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 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? 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 do you chain multiple extension methods together in a fluent style, and what design principle makes this possible?

Intermediate

Answer

As long as each extension method RETURNS a value of a type that has further applicable extension (or instance) methods, calls can be chained one after another — this is exactly the mechanism that makes LINQ's fluent query syntax (.Where().Select().OrderBy()) work.
public static class StringExtensions {
  public static string Trimmed(this string s) => s.Trim();
  public static string Capitalized(this string s) => char.ToUpper(s[0]) + s[1..];
}

string result = "  hello world  ".Trimmed().Capitalized(); // "Hello world"
Real-world example Building a fluent, chainable string-processing or validation API similar in style to LINQ's own method chaining.

Common follow-ups: How does LINQ itself use exactly this technique to implement its fluent Where/Select/OrderBy chain syntax?

LINQ
All 11 Extension Methods questions