[API Proposal]: Get info about generic attributes

[VBAndCs]

Background and motivation

Make it easier to handle generic attributes.

API Proposal

• Allow use open generic as a param to Attribute.GetCustomAttributes Method.. It currently returns an empty array.
• Add an the instance property TypeArgs to the Attribute class that returns all type args if any.

API Usage

var Attrs = Attribute.GetCustomAttributes(this.GetType(), typeof(TestAttribute<>));
var type = Attrs[0].TypeArgs[0];

Alternative Designs

No response

Risks

No response

[ghost]

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Issue Details

---

Background and motivation

Make it easier to handle generic attributes.

API Proposal

• Allow use open generic as a param to Attribute.GetCustomAttributes Method.
• Add an the instance property TypeArgs to the Attribute class that returns all type args if any.

API Usage

var Attrs = Attribute.GetCustomAttributes(this.GetType(), typeof(TestAttribute<>));
var type = Attrs[0].TypeArgs[0];

Alternative Designs

No response

Risks

No response

Author:	VBAndCs
Assignees:	-
Labels:	api-suggestion, area-System.Reflection, untriaged
Milestone:	-

[CyrusNajmabadi]

Add an the instance property TypeArgs to the Attribute class that returns all type args if any.

I'm not certain why this is needed. Why not just do .GetType().GenericTypeArguments?

[buyaa-n]

For adding a new property i agree with @CyrusNajmabadi, doesn't seem we need to add property for TypeArgs

Allow use open generic as a param to Attribute.GetCustomAttributes Method.. It currently returns a null.

Makes sense to me, will discuss with the team. As this is not a new API suggestion and we don't need the other API suggestion removing the api-suggestion label

[madelson]

Just to clarify, is the proposal that calling any GetCustomAttributes() method with an open generic type deriving from Attribute will return all attributes which are or derive from generic instantiations of that type? So if we had:

class GenericAttribute<T> : Attribute { }
class DerivesFromGenericAttribute : GenericAttribute<string> { }

[GenericAttribute<int>, GenericAttribute<string>, DerivesFromGenericAttribute]
class HasAttributes { }

Then typeof(HasAttributes).GetCustomAttributes(typeof(GenericAttribute<>)) would return all 3 attributes?

@buyaa-n I'm potentially interested in working on this but I'd want to be sure of the expected behavior first.

[steveharter]

Then typeof(HasAttributes).GetCustomAttributes(typeof(GenericAttribute<>)) would return all 3 attributes?

That is my understanding of the ask. I think it makes sense, although I don't know where this "open generic" convention is used elsewhere for similar filtering or scoping.

Since the workaround is not that difficult (get all custom attributes and manually filter) this feature is lower-priority.

[steveharter]

@GrabYourPitchforks if we add a new convention for open generic types to be used to mean all closed, does this rise to the level of discussing it in an API review meeting?

[buyaa-n]

Then typeof(HasAttributes).GetCustomAttributes(typeof(GenericAttribute<>)) would return all 3 attributes?

I believe that should depend on if the bool inherit parameter set, as it is false by default, in this case DerivesFromGenericAttribute should not be included.

[buyaa-n]

@GrabYourPitchforks if we add a new convention for open generic types to be used to mean all closed, does this rise to the level of discussing it in an API review meeting?

@GrabYourPitchforks suggested to go through API review

[terrajobst]

Yes, reflection should support retrieving attribute by open generic types. And yes, using a base type should also work, assuming inherit is set to true.

[buyaa-n]

I'm potentially interested in working on this but I'd want to be sure of the expected behavior first.

Thanks @madelson, the expected behavior is approved, do you still interested in working on this?

[madelson]

@buyaa-n sure I'll try to take a look at it this weekend to get a sense of what is involved.

[madelson]

@buyaa-n :

I believe that should depend on if the bool inherit parameter set, as it is false by default, in this case DerivesFromGenericAttribute should not be included.

I'm a bit confused. Are we talking about the inherit parameter on MemberInfo.GetCustomAttributes(bool)? That parameter is supposed to determine whether to to search this member's inheritance chain to find the attributes; I don't think it affects type-based filtering of the attributes.

For example:

typeof(RegexOptions).GetCustomAttributes(typeof(Attribute), inherit: false) // returns the FlagsAttribute

What am I missing?

[buyaa-n]

You are right, I have mistaken about the parameter.

In this case the behavior should be same as how it works for non generic attributes, and Attribute.GetCustomAttributes(,typeof(TestType), typeof(NonGenericAttribute)) returns all derived attribute by default