All 11 Extension Methods questions
More in Extension Methods
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? 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?
Advertisement
C# › Extension Methods

What is an extension method, and what syntax makes a static method into one?

Beginner

Answer

An extension method lets you 'add' new methods to an existing type (even one you don't own the source for, like a built-in .NET type) without modifying it — declared as a static method in a static class, with the FIRST parameter prefixed with 'this', specifying which type it extends.
public static class StringExtensions {
  public static bool IsNullOrEmpty(this string? value) => string.IsNullOrEmpty(value);
}

string? name = null;
bool empty = name.IsNullOrEmpty(); // called as if it were an instance method
Real-world example Adding a convenient .IsValidEmail() check directly onto the built-in 'string' type without subclassing or wrapping it.

Common follow-ups: Can you call an extension method using the normal static method syntax instead of instance-method syntax?

Fundamentals
All 11 Extension Methods questions