Generic attributes: Attribute.GetCustomAttributes returns null when passed an unbound generic type

[canton7]

Description

Attribute.GetCustomAttributes normally returns an empty array when no attributes are found (or throws an exception if the parameters don't make sense).

However, Attribute.GetCustomAttributes(someType, typeof(SomeGenericAttribute<>)) returns null. This is unexpected, particularly as the nullable annotations say that it doesn't return null.

It's worth noting that this wasn't previously possible: it was simply not possible to create a generic type which derived from Attribute, and if you passed a type which wasn't derived from Attribute to Attribute.GetCustomAttributes, you got an exception telling you that. So I suspect this is an unhandled case which just fell through the cracks.

Reproduction Steps

Test.M();

public class Test
{
    public static void M()
    {
        var attrs = Attribute.GetCustomAttributes(typeof(Test), typeof(TestAttribute<>));
        Console.WriteLine(attrs == null); // true
    }
}

public class TestAttribute<T> : Attribute { }

SharpLab

Expected behavior

Attribute.GetCustomAttributes either returns an array, or throws an exception. It should not return null.

Actual behavior

Attribute.GetCustomAttributes returns null in this case.

Regression?

No. It was not previously possible to pass an unbound generic attribute type to Attribute.GetCustomAttributes.

Known Workarounds

No response

Configuration

No response

Other information

No response

[ghost]

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Issue Details

---

Description

Attribute.GetCustomAttributes normally returns an empty array when no attributes are found (or throws an exception if the parameters don't make sense).

However, Attribute.GetCustomAttributes(someType, typeof(SomeGenericAttribute<>)) returns null. This is unexpected, particularly as the nullable annotations say that it doesn't return null.

It's worth noting that this wasn't previously possible: it was simply not possible to create a generic type which derived from Attribute, and if you passed a type which wasn't derived from Attribute to Attribute.GetCustomAttributes, you got an exception telling you that. So I suspect this is an unhandled case which just fell through the cracks.

Reproduction Steps

new Test().M();

public class Test
{
    public void M()
    {
        var attrs = Attribute.GetCustomAttributes(this.GetType(), typeof(TestAttribute<>));
        Console.WriteLine(attrs == null); // true
    }
}

public class TestAttribute<T> : Attribute { }

SharpLab

Expected behavior

Attribute.GetCustomAttributes either returns an empty array, or throws an exception.

Actual behavior

Attribute.GetCustomAttributes returns null in this case.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

Author:	canton7
Assignees:	-
Labels:	area-System.Reflection, untriaged
Milestone:	-

[VBAndCs]

I have asked to accept the open generic form and return all generic attributes of this type #64169

[CyrusNajmabadi]

@VBAndCs you didn't address teh question i asked you there. :)

[jkotas]

The bug is caused by ! at this line

runtime/src/coreclr/System.Private.CoreLib/src/System/Attribute.CoreCLR.cs

Line 486 in 03e0187

_ => (element.GetCustomAttributes(typeof(Attribute), inherit) as Attribute[])!,

:

element.GetCustomAttributes returns an empty object[] since it is not possible to create array with open generic element type. Casting object[] to Attribute[] using as is producing null.

[VBAndCs]

@jkotas

since it is not possible to create array with open generic element type

You can return an Attrubute array containing all the generic versions of this open generic attribute.

[Joe4evr]

So I took a look at this, and I think the best solution would be down around here:

runtime/src/coreclr/System.Private.CoreLib/src/System/Reflection/RuntimeCustomAttributeData.cs

Lines 931 to 932 in 03e0187

	bool useObjectArray = (caType.IsValueType || caType.ContainsGenericParameters);
	RuntimeType arrayType = useObjectArray ? (RuntimeType)typeof(object) : caType;

While my gut reaction is to possibly rethink some parts of this implementation, there's probably more involved that I can't see and may end up breaking assumptions elsewhere. In which case, the most effective fix here might be to add a check whether or not caType.IsAssignableTo(typeof(Attribute)) and if so, set arrayType = (RuntimeType)typeof(Attribute) for the array creation.

Effectively:

             bool useObjectArray = (caType.IsValueType || caType.ContainsGenericParameters);
+            bool useAttributeArray = (caType.ContainsGenericParameters && caType.IsAssignableTo(typeof(Attribute)));
-            RuntimeType arrayType = useObjectArray ? (RuntimeType)typeof(object) : caType;
+            RuntimeType arrayType = useAttributeArray ? (RuntimeType)typeof(Attribute) : useObjectArray ? (RuntimeType)typeof(object) : caType;

(and there's a total of 3 places in the file for this.)

[madelson]

@Joe4evr / @buyaa-n I'd be interested in fixing this if it is still up for grabs.

[buyaa-n]

@Joe4evr are working on the fix? if not i will assign to @madelson, thanks!

[Joe4evr]

@buyaa-n I was trying to, but I just ran out of time over the past week, so it's cool.

[stephentoub]

Reopening as #65237 was reverted in #66508 due to #66496.