The error message and the documentation https://developer.apple.com/documentation/objectivec/1418553-class_copypropertylist say that class_copyPropertyList returns a UnsafeMutablePointer<objc_property_t>?, but you have written UnsafeMutablePointer<objc_property_t?>?, with an extra ? inside the <...>. Could you say a bit more about what you mean by "not getting class property count"?

Also, attaching a complete xcode project or playground that demonstrates the full issue lets us help you better.

Comment by Sandeep (JIRA)

@huonw there is two things in attached screenshot if you seen.

1. In screen shot line number 348, when i am using only UnsafeMutablePointer<objc_property_t>? & i am not getting the length of returned array in outCount(it's zero in all case) of passes class instance.

2. In attached screenshot line number 349 is working with Swift 3 and i am getting length of array and it's working fine for me but after migrating whole project with Swift 4 & running it in Xcode9Beta6, i am getting compile time error mentioned in screenshot.

3. Can we have a call at your time so that i can share my screen and show you.
   and for "not getting class property count" i mean length of array.

Comment by Sandeep (JIRA)

TestCopyProprty.zip
Please find this attached sample project with my problem.

Thanks for uploading a project!

The problem is that the class_copyPropertyList is an Objective-C function, and so only understands Objective-C concepts. The properties of the MockupServer.Factory class are only Swift properties in your code, and need to be be marked with the @objc attribute for that function to know they exist. This can be done by placing @objc on each property (@objc var id: String = "") or by putting @objcMembers on the class itself (@objcMembers class Factory: NSObject) which is like marking everything in the class with @objc (properties and methods).

However, taking a step back, I'm not sure this is a good idea. This sort of reflection may be better handled in Swift with Mirrors, or some other approach that avoids being so dynamic.

(I'm closing as "invalid" because all the parts are behaving correctly here, but it is definitely confusing! It's one of the problems of having lots of Objective-C code around: some of it doesn't work as expected.)

Comment by Sandeep (JIRA)

@huonw Many Thansk Huon