LA0021: Aspect Serializer Not Derived from AspectSerializer

Cause

PostSharp serializes the aspect instances at build time and deserializes them at run time, as Understanding Aspect Serialization describes. By default, it chooses the serializer from the aspect class: PortableAspectSerializer for a class annotated with PSerializableAttribute. You can choose another serializer with the SerializerType property of an aspect configuration attribute, such as SerializerType or the one of OnMethodBoundaryAspectConfigurationAttribute, or with the SerializerType property of the aspect.

PostSharp creates an instance of this type and calls its methods, so the type must be derived from AspectSerializer. PostSharp reports LA0021 when it is not, for example when the serializer class has Serialize and Deserialize methods but no base class. <SerializerType> is the configured type, and <Aspect> the aspect class.

PostSharp does not stop after reporting LA0021. When the type has a public parameterless constructor, the build then fails with an unhandled exception (PS0099) because PostSharp cannot convert the instance to AspectSerializer; otherwise, it also reports LA0022 and LA0001. This is why no example can show this message alone.

Resolution

• Derive the serializer from AspectSerializer, override its Serialize and Deserialize methods, and give it a public parameterless constructor.
• Unless the aspect needs a serializer of its own, remove the SerializerType property and annotate the aspect class with PSerializableAttribute.
• To build the aspect at run time instead of serializing it, set SerializerType to MsilAspectSerializer.

See Also

Reference

AspectSerializer
AspectConfigurationAttribute
MsilAspectSerializer

Other Resources

Understanding Aspect Serialization