Why Custom Attributes in .NET Give Me Nightmares

▲ Why Custom Attributes in .NET Give Me Nightmares(blog.washi.dev)

52 points by jandeboevrie 2 days ago | 14 comments

▲ bob1029 2 hours ago | parent | next [-]

I try to avoid using custom attributes to configure things like business logic because they struggle to account for interactions between members. Attributes have no affordance for lambdas, delegates, method references, etc. You would need a separate piece of logic that interprets the primitive attribute data in order to provide any emergent properties between members.

A better approach is often to expose some abstract/interface member that allows for the implementation to define its logic using something like a fluent-style contract. In this arrangement you can pass the type itself as an argument to a lambda making it trivial to define logic that should execute over many members at once. Debugging this also tends to be more pleasant. AspNetCore startup code, LINQ and EF are good examples.

Attributes are useful for things that are definitely pure data and only when the information fully belongs to the thing being annotated regardless of context of use. The moment some kind of per-member custom logic is needed it's no longer appropriate. I think things like [Authorize] are borderline. [JsonIgnore] seems like a good attribute to me.

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xnorswap 2 hours ago | parent | next [-]

This is my experience too, I've worked on code bases which made heavy use of attributes, and they work really well to provide static per-type information, but if you see them as a universal problem solver and try to go too fancy, you'll find yourself in trouble.

Fluent builders are nicer to work with than attributes, although it sometimes feels weird if the defaults are nearly fine but not quite, and you wish you could just reach for a single attribute rather than having to traverse down 3 layers of builders to change a single property.

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fabian2k an hour ago | parent | prev [-]

The only case I've used them was to mark classes that I need to find via reflection and do something with them. For example a migration system where you want to load all migrations that are defined and check if you need to run them. Of course you don't really need an attribute for that, but I find it helpful to leaven a marker on the class that there's something else going on there.

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dh2022 an hour ago | parent | next [-]

To solve this problem I have seen the following pattern:

1. Create an abstract base class named MigrationBaseClass 2. Have all migrations classes inherit from MigrationBaseClass 3. Use .Net Reflection to get all types that inherit from MigrationBaseClass 4. Do something with these types.

	▲	WorldMaker 31 minutes ago \| parent \| next [-]
		Doesn't even need to be an abstract base class. It's just as easy to use reflection to find all implementations in an assembly of an IMigration interface. (ETA: Though my favorite pattern here became using DI for this instead of reflection. For every IMigration have a `services.AddTransient<IMigration, SomeMigrationImplementationClass>()` somewhere and then your service to run all migrations can just request from DI `IEnumerable<IMigration>`. I can then put the Reflection into a unit test to make sure everything that implements IMigration is registered in the DI container. But using DI in the main assembly to register all the migrations rather than Reflection leaves more room to try to AOT compile the assembly in production builds.)
	▲	fabian2k 28 minutes ago \| parent \| prev [-]
		yeah, there's plenty of ways to do that. I like having the attribute there so that there is a strong hint that some magic is happening somewhere with that class.

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tracker1 an hour ago | parent | prev [-]

FastEndpoints does this for you with Validators as an example... It can be abstracted cleanly.

▲ pjc50 3 hours ago | parent | prev | next [-]

.. if you're trying to parse the assemblies by hand for some reason. If you're just trying to handle them with reflection none of this is an issue.

▲ jve 3 hours ago | parent [-]

> If you're just trying to handle them with reflection none of this is an issue

But maybe indicates on how expensive that reflection call can be? Reading multiple .dlls ?

	▲	Rohansi 2 hours ago \| parent \| next [-]
		.NET typically loads dozens of DLLs at boot. It needs to resolve references for almost everything before it can run any code. It doesn't take that long to boot so I have my doubts but they have surely optimized it a lot there.
	▲	EMM_386 an hour ago \| parent \| prev \| next [-]
		Be careful with this sort of logic ("reflection=expensive"). Everything should obviously be measured. I've worked with large .NET code bases that used attributes for things like plugins and it was completely negligible for overall performance in the grand scheme of things.
	▲	pjc50 2 hours ago \| parent \| prev [-]
		I would expect DLL parsing to be a one-off cost at assembly load time. Certainly that handles all the stuff detailed under "Assembly resolution". Assembly resolution is also recursive, so I would expect that to simplify "type tree traversal" by pre-stuffing all the types into a Dictionary. That also necessarily has the parser for all the "System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" machinery. >> "While C# does not support types with weird symbols, it is possible to have type names with spaces, commas, brackets, unprintable characters, and more. This is also heavily (ab)used by code obfuscators." .. hmm. I didn't know that. I will note that there is a milder version of this problem which you might encounter if you're trying to write a dotnet source generator, which is run inside the Roslyn compiler. You then need to remember that the types in the code being compiled are not directly visible from reflection, you have to ask the compiler to look them up for you in the parsed data. These cases are easy: `- types in the netstandard2.0 standard library - types in the assembly currently being compiled` This case is not: `- types in assemblies in the same project which the current assembly depends on` I ended up avoiding handling that at all. It does set some limits on what is easily done with source generators. (by the way, someone sufficiently dedicated should be able to find the corresponding Microsoft loader code: it's all in the dotnet github)

▲ tracker1 an hour ago | parent | prev [-]

FWIW, Custom Attributes in .Net are kind of a pain in geneal, powerful but painfull... Probably why JS still doesnt really have them in practice.

	▲	doctorpangloss an hour ago \| parent [-]
		haha, "use server", use...(use...(...))