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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 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 '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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If two 'using' namespaces are both in scope and both define an applicable extension method with an identical signature for the same type, the compiler reports an AMBIGUOUS CALL error — resolved either by removing one of the conflicting 'using' directives, or by explicitly calling the desired method using its full static syntax (ClassName.Method(instance, ...)) to disambiguate.
// Both LibraryA.Extensions and LibraryB.Extensions define: string Truncate(this string s, int length)
using LibraryA.Extensions;
using LibraryB.Extensions;

string result = "hello".Truncate(3); // Error: ambiguous call between the two
// Fix: LibraryA.Extensions.StringExtensions.Truncate("hello", 3); // explicit disambiguation
Real-world example Resolving a real compile error after adding a second NuGet package that happens to define a conflicting extension method name.

Common follow-ups: Does C#'s extension method resolution have any built-in tie-breaking rule based on namespace 'closeness,' or is it always a hard ambiguity error?

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All 11 Extension Methods questions