CA1812 Shouldn't Fire for DebuggerTypeProxy

[dcwuser]

To change how a class is displayed in the VS debugger, one defines a type proxy and then adds the DebuggerTypeProxy attribute to the original class, so that the proxy class determines what is displayed by VS instead of the original class. It is natural to make the proxy class internal, since it is not intended to be part of the API surface. (VS finds and instantiates it via reflection.) When one does this, the proxy class triggers a CA1812 warning "XXX is an internal class that is apparently never instantiated."

I propose that if an internal class is referenced in any DebuggerTypeProxy attribute in an assembly, it should not trigger CA1812.

[twsouthwick]

As a further suggestion, if a type is referenced via typeof(...) it seems like it shouldn't trigger this as well

[KalleOlaviNiemitalo]

In my ASP.NET Web API (not ASP.NET Core) application, I got a CA1812 warning for internal class RawStringBinder : IModelBinder, which I use only in [FromUri(BinderType = typeof(RawStringBinder))]. So yes, I'd like the analyzer to recognize typeof(...).

[Evangelink]

I think that the DebuggerTypeProxy makes sense. Regarding the FromUri I cannot find the doc for the old api so I am not entirely sure.

It might be too strong but we could also think about considering types declared in a typeof invocation of an attribute to always be instantiated, WDYT?

Tagging @sharwell and @mavasani

[KalleOlaviNiemitalo]

Regarding the FromUri I cannot find the doc for the old api so I am not entirely sure.

In ASP.NET Web API, System.Web.Http.FromUriAttribute inherits the BinderType property from System.Web.Http.ModelBinding.ModelBinderAttribute, which eventually gets an instance of that type from IDependencyResolver.GetService or Activator.CreateInstance.

[Evangelink]

Thanks @KalleOlaviNiemitalo so it also makes sense to support this attribute.

[KalleOlaviNiemitalo]

dotnet/runtime#39126 added [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] to the Type type and string typeName parameters of the DebuggerTypeProxyAttribute constructors. The analyzer could recognize DynamicallyAccessedMembersAttribute and suppress the CA1812 warning, at least if any of the DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.NonPublicConstructors bits is set.

For the sake of code that targets framework versions lower than .NET 5 though, the analyzer should recognize DebuggerTypeProxyAttribute and System.Web.Http.ModelBinding.ModelBinderAttribute as well, or just typeof(…) in all attributes in general.

[Evangelink]

@KalleOlaviNiemitalo I have moved your 2 suggestions as different thread so that this one can move forward.