KClass::memberExtensionFunctions always be empty

Question

Code

import kotlin.reflect.full.*

class FooBar(val bar: String)
fun FooBar.baz(): Unit {println(this.bar)}

fun main(args: Array<String>) {
    FooBar::class.declaredMemberExtensionFunctions.forEach {
        println(it)
    }
    FooBar::class.memberExtensionFunctions.forEach {
        println(it)
    }
}

Output is empty

score 2 · Accepted Answer · answered Apr 30 '18 at 13:56

This is because declaredMemberExtensionFunctions only returns extension functions that are declared inside a class (as seen in the docs) and FooBar.baz() is a top level declaration (So it is not declared inside FooBar.

class FooBar(val bar: String) {
    fun FooBar.baz(): Unit {
        println(this.bar)
    }
}

While I imagine this is not what you want, structuring the extension function like this would make your main method output lines.

score 1 · Answer 2 · answered Apr 30 '18 at 14:09

TLDR: You aren't going to be able to do this. Because extension functions can be declared everywhere, you are limited in what the reflection system can do for you.

There is a thread on kotlinlang.org that covers this exact question and why it is not possible.

Essentially, Kotlin's declaredMemberExtensionFunctions function is able to list extension functions which are declared as part of the class, not externally. The docs state:

Returns extension functions declared in this class.

And of course, memberExtensionFunctions behaves similarly:

Returns extension functions declared in this class and all of its superclasses.

Here's what @Yole says in that thread as to why this is not possible:

The task of finding all extension functions for Foo is equivalent to finding all methods which have Foo as the first parameter. Neither of these is possible without accessing every single class in your application through reflection.

@Yole is on here, he might be able to provide a more authoritative answer for you.

KClass::memberExtensionFunctions always be empty

2 Answers2