Proposal: Code interception via source attributes

[alrz]

Currently, attributes are not aware of their targets whatsoever, but with source-only attributes (#6671), we can provide a nice API for interception and additional code generation in the same sense as #5561. This can be used to imitate a macro system but in a more managed fashion.

For example,

[[NPC]]
public partial class Person
{
  [[Property]]
  string Name { get; set; }
}

[AttributeUsage(AttributeTargets.Field)]
class PropertyAttribute : SourceAttribute {
  public override void Intercept(SymbolInjectionContext symbol) { ... }
}

[AttributeUsage(AttributeTargets.Class)]
class NPCAttribute : SourceAttribute {
  public override void Intercept(SymbolInjectionContext symbol) { ... }
}

Obviously executing Intercept happens early in the pipeline like #5561 code injectors.

The provided API should be able to add compilation units (related to #5561 and #5292).

Some scenarios that I'm expecting this to be really useful:

• Implementing libraries like refit.
• Code interception in general, e.g. implementing NPC.
• Logging, mocking, exception handling, dependency injection, metaprogramming, AOP, etc.

Also I want to suggest the API provided be compatible with pattern-matching for easier examining the input (e.g. we can switch on the target symbol).

As a further consideration (like #7875), I would like to suggest preprocessor-like syntax for source-only attributes indicating that it gets evaluated earlier in the pipeline.

#[Mock]
extension IBar { } // #8127

Although, where and how to apply these attributes depend on the implementation details for each of those proposals.

[daveaglick]

👍 - especially for using the same conventions and SymbolInjectionContext as #5561.

For this to be really useful though, I think superseded members (#5292) or some other mechanism for augmenting the intercepted code at specific locations (before the call, after the call) also needs to make it in. For example, if you want to intercept a method to add timing you'll need to create and fire a timer before invocation and then stop and log the cumulative time after invocation.

I'd also like to see a way to apply these attributes more globally: to every method in a given class, to every property in an assembly with a given name, to every method that starts with "Get", etc.

[alrz]

@daveaglick I specifically mentioned #5292 to be used for implementing these attributes. For applying these attribtues globally I can think of assembly attributes, for example, utilizing #1938:

#[assembly: GlobalInterception]

[AttributeUsage(AttributeTargets.Assembly)]
class GlobalInterceptionAttribute : SourceAttribute {
  public override void Intercept(SymbolInjectionContext symbol) {
    from @class in symbol.CompilationUnit.Types
    from member in @class.Members
    where member.Name.StartsWith("Get")
    do { ... }; // #1938
  }
}

or something like that. Similarly this can be used for an attribute applied to a class to intercept all members.

[gafter]

We are now taking language feature discussion on https://github.com/dotnet/csharplang for C# specific issues, https://github.com/dotnet/vblang for VB-specific features, and https://github.com/dotnet/csharplang for features that affect both languages.