Differences between Metalama and PostSharp

This article covers the major architectural differences between Metalama and PostSharp in a rather theoretical style. You can probably skip it on first reading.

Metalama is a compiler add-in

A significant difference between Metalama and PostSharp is that while PostSharp is a post-compiler, i.e. a process that runs after the compiler and post-processes the output of the compiler, Metalama is a compiler add-in and gets executed both at design- and compile-time.

So how does Metalama execute aspects? Metalama creates a sub-project from your main project, which contains only compile-time code, e.g., aspects, fabrics, and their dependencies. Only this sub-project is compiled and executed at design- or compile-time.

While PostSharp loads the whole project (compiled as an assembly) in the .NET runtime, Metalama only loads the sub-project that contains compile-time code.

Illustrations

PostSharp Architecture

flowchart LR IDE --> Compiler --> input_dll>binary] --> PostSharp --> output_dll>Binary With Aspects] --> Execute subgraph design-time IDE end subgraph compile-time Compiler PostSharp input_dll end subgraph run-time Execute end

Metalama Architecture

flowchart LR IDE --> Compiler --> output_dll>Binary With Aspects] --> Execute IDE <--> Metalama2 Metalama2 <--> compiledAspects Compiler <--> Metalama Metalama <--> compiledAspects>Compiled Aspects] subgraph compile-time Compiler Metalama[Compile-Time\nMetalama] end subgraph design-time IDE Metalama2[Design-Time\nMetalama] end subgraph run-time Execute end

Metalama aspects are compile-time-only

In PostSharp, aspect classes are instantiated at compile time, serialized, stored as a managed resource in the assembly being built, then deserialized at run time and executed. Therefore, in PostSharp, some aspect code is executed at compile time and some at run time.

In Metalama, aspects are never executed at run time. Aspects provide code templates, and templates are expanded at compile time. Templates generate C# code the advice is applied; only this generated code is executed at run time.

Illustration

Aspect lifetime in PostSharp

flowchart LR instantiated[aspect instantiated] --> executed[aspect executed] --> serialized[aspect serialized] --> deserialized[aspect deserialized] --> runTimeExecuted[aspect executed] executed -->|generates| code>code advised\nwith aspect bindings] code --> deserialized subgraph compile-time instantiated executed[executed] serialized end subgraph run-time code deserialized runTimeExecuted end

Aspect lifetime in Metalama

flowchart LR instantiated --> executed -->|generates| code>code advised\nwith inlined templates] subgraph compile-time instantiated executed end subgraph run-time code end

Implications

The difference in aspect lifetime has significant implications for how aspects are designed.

Metalama templates should generate succinct code. In PostSharp, advice methods could be long and complex because they were independent C# methods, compiled and JIT-compiled just once, and executed at run time. However, in Metalama, advice methods are templates. They can be long, but the code they generate must preferably be short. This code must be compiled and JIT-compiled as often as the aspect is applied, so potentially thousands of times. Any logic that may repeat itself should be moved into run-time helper classes.
Aspects can no longer "hold" run-time state. In PostSharp, aspect fields could hold any run-time state required by the aspect. In Metalama, if an aspect needs a run-time state, it has to introduce a field into the target class (see Introducing members for details).

Aspect instances in Metalama can be shared by several declarations

Some aspects are applied to a declaration in a project but affect other projects that reference the main project (as a project or as a package). For instance, an aspect may be applied to a base class in a project. If this aspect is inheritable, it will be automatically applied to all classes derived from this base class. For details, see Applying aspects to derived types.

The way how inheritance is implemented differs between Metalama and PostSharp.

In PostSharp, each inherited aspect instance is instantiated again from the custom attribute from which it stems (i.e., to be exact, it is deserialized from the custom attribute). This mechanism is used for intra-project inheritance as well as for cross-project inheritance.

In Metalama, the mechanism is different inside a project than across projects.

Inside a project, the same aspect instance is shared among all declarations that inherit this aspect. This is why aspect classes should be written in an immutable style.

For cross-project inheritance or validators, inheritable aspect instances are serialized and stored as a managed resource using the Metalama.Framework.Serialization namespace into the assembly is built. In child projects, one new aspect instance is created by deserializing the serialized aspect of the base declaration. This deserialized instance is then shared by all derived declarations inheriting the aspect.

Illustrations

Cross-project aspects in PostSharp

flowchart BT subgraph BaseAssembly subgraph BaseClass BaseAspect CustomAttribute[Aspect Custom Attribute] BaseAspect -->|instantiated from| CustomAttribute end subgraph DerivedClass1 DerivedAspect1 end DerivedClass1 -->|inherited from| BaseClass DerivedAspect1 -->|instantiated from| CustomAttribute end subgraph DependentAssembly subgraph DerivedClass2 DerivedAspect2 end subgraph DerivedClass3 DerivedAspect3 end DerivedAspect2 ----> |instantiated from| CustomAttribute DerivedAspect3 --> |instantiated from| CustomAttribute end DependentAssembly --> BaseAssembly

Cross-project aspects in Metalama

flowchart BT subgraph BaseAssembly subgraph BaseClass BaseAspect CustomAttribute[Custom Attribute] BaseAspect -->|instantiated from| CustomAttribute end subgraph DerivedClass1 DerivedAspect1 end subgraph ManagedResource SerializedAspect(Serialized\nAspect) -->|serialized from| BaseAspect end DerivedAspect1 -->|reuses| BaseAspect end subgraph DependentAssembly subgraph Shared DeserializedAspect[Deserialized\nAspect] -->|deserialized from| SerializedAspect end subgraph DerivedClass2 DerivedAspect2 --> |reuses| DeserializedAspect end subgraph DerivedClass3 DerivedAspect3 --> |reuses| DeserializedAspect end end DependentAssembly --> BaseAssembly

Implications

In Metalama, aspect classes must be written in an immutable style. Since aspect instances may be reused among several declarations, they cannot store state that is specific to a target declaration. For details, see Sharing state with advice.