LAMA0540: Extension blocks can only be introduced into static classes

Cause

In C#, extension blocks (extension( T receiver ) { ... }) can only be declared in a top-level, non-generic static class. An aspect introduces an extension block with the IntroduceExtensionBlock method, and the target of this advice must therefore be such a class. In the message, <ReceiverType> is the receiver type, that is, the type being extended, and <TargetType> is the type into which the aspect tries to introduce the extension block.

LAMA0540 is a safeguard reported when the advice is implemented. However, Metalama checks the target when the aspect calls IntroduceExtensionBlock, and throws an exception if the target isn't a top-level static class. In current versions, you therefore see LAMA0041 instead of LAMA0540, with a message such as Cannot add an IntroduceExtensionBlock advice to 'StringHelper' because 'StringHelper' must be a top-level static class.

Metalama doesn't report this diagnostic in practice, so it can't be shown in an example.

Resolution

Introduce the extension block into a top-level static class:

• If the aspect is applied to a static class, make sure that the target class is marked static and isn't nested in another type.
• If the aspect is applied to another kind of type, for instance the type being extended, introduce a new static class first with IntroduceClass, setting IsStatic to true in the buildType delegate, then call IntroduceExtensionBlock on the result.
• In the aspect's BuildAspect method, check the target with IsAdviceEligible( AdviceKind.IntroduceExtensionBlock ) before adding the advice when the target may not be a static class.

See also

• Introducing types
• LAMA0041