How are extension methods resolved in C#, and what can break when a library later adds an instance method with the same name?

Extension method resolution and API evolution

Explains static extension binding, instance-member precedence, namespace ambiguity, null receivers, and compatibility when APIs evolve.

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An extension method is a static method made available through instance-like syntax when its namespace is in scope and the receiver can bind to the first parameter. Resolution uses the receiver's compile-time type and normal overload rules; it is not virtual dispatch. An applicable instance member takes precedence over extension methods. If a library later adds an instance method with the same name and compatible arguments, recompiled source may bind to the new instance member instead of the extension.

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Static extension binding, receiver compile-time type, instance precedence, namespace scope, generic inference, null receivers, and source compatibility.

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Extension-call syntax looks like instance dispatch, but the compiler resolves a static method. That distinction matters when receiver types, namespaces, or library versions change.
An extension method is a static method made available through instance-like syntax when its namespace is in scope and the receiver can bind to the first parameter. Resolution uses the receiver's compile-time type and normal overload rules; it is not virtual dispatch. An applicable instance member takes precedence over extension methods. If a library later adds an instance method with the same name and compatible arguments, recompiled source may bind to the new instance member instead of the extension.
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For example, an extension Describe(this Animal value) is selected from an Animal variable even when the runtime object is a , unless an instance virtual method is involved separately. Two imported namespaces can expose equally good extensions and make the call ambiguous. Because the syntax is only rewritten static invocation, a null receiver can enter the extension method; the method may handle null or throw when it dereferences the value. That differs from a normal instance call, which fails before entering the method.

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Describing an extension method as a virtual member added to the target type.
Expecting runtime receiver type to choose a more specific extension overload.
Adding broad extensions that collide across commonly imported namespaces.
Assuming a null receiver can never enter extension method code.

How are extension methods resolved in C#, and what can break when a library later adds an instance method with the same name?

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