GetCustomAttributes behavior is different when used as extension method

[Evangelink]

Hi folks,

When doing a PR for fluentassertions/fluentassertions#1095 I have noticed a really strange behavior with GetCustomAttributes when using the inherits option.

Indeed this call CustomAttributeExtensions.GetCustomAttributes(property, true).OfType<TAttribute>().Any() returns true whereas property.GetCustomAttributes(true).OfType<TAttribute>().Any() returns false.

I have made a couple of tests and it seems that the bug happens only for PropertyInfo passed to the MemberInfo overload.

Given the following classes:

[AttributeUsage(AttributeTargets.Property, AllowMultiple = true, Inherited = true)]
internal class DummyPropertyAttribute : Attribute
{
}

internal class TestClassForPropertySelectorWithInheritableAttribute
{
    [DummyProperty]
    public virtual string PublicVirtualStringPropertyWithAttribute { get; set; }
}

internal class TestClassForPropertySelectorWithInheritableAttributeDerived : TestClassForPropertySelectorWithInheritableAttribute
{
}

This code doesn't work properly:

Type type = typeof(TestClassForPropertySelectorWithInheritableAttributeDerived);

// Returns 1
var classicMethodCallCount = type.GetProperties()
    .Count(property =>
        CustomAttributeExtensions.GetCustomAttributes(property, true).OfType<DummyPropertyAttribute>().Any());

// Should return 1 but returns 0
var extensionMethodCallCount = type.GetProperties()
    .Count(property =>
        property.GetCustomAttributes(true).OfType<DummyPropertyAttribute>().Any());

Please let me know if this is not the good place to report this problem.

BTW I found this issue which seems more or less related #17180

[pinkfloydx33]

It's been documented that the inherits parameter does not work for properties or events and that you need to use Attribute.GetCustomAttributes. This probably explains this difference if you were to look at the underlying implementations of the two calls.

This method ignores the inherit parameter for
properties and events. To search the inheritance
chain for attributes on properties and events, use
the appropriate overloads of the
Attribute.GetCustomAttributes method.

MSDN

[Evangelink]

@pinkfloydx33 I have to admit I didn't read MSDN, sorry about that!

I am still not sure to understand why it's working when used like CustomAttributeExtensions.GetCustomAttributes(property, true). If it's really ignoring the inherit parameter for PropertyInfo it should behave the same when not used as extension method, shouldn't it?

[pinkfloydx33]

I'm pretty sure that you're not calling the same method in both versions of your code. PropertyInfo (via MemberInfo) has an instance method GetCustomAttributes. That is what you're calling with the latter form whereas you're explicitly invoking the extension method (which calls Attribute.GetCustomAttributes) in the former.

[GrabYourPitchforks]

A little history behind this. Some frameworks didn't expose the instance method MemberInfo.GetCustomAttributes(bool), so the extension method was added to account for this missing API. Since the extension method couldn't call MemberInfo.GetCustomAttributes(bool) - it was missing, after all - it had to do the next best thing and call Attribute.GetCustomAttributes(MemberInfo, bool). As mentioned in #30219 (comment), there is a subtle behavioral difference between these two methods. It's unfortunate that the behavioral difference is there since it could cause behavioral differences depending on what target framework you compiled against, but the behavior is established by now and I don't see us changing it at this time as it could break people.