I'm trying to reflectively access a property value of a data class like this:

it::class.memberProperties.filter { prop -> prop.name == propertyName }.first().get(it)

but getting the following compiler error:

Out-projected type 'KProperty1<out DataClass!, Any?>' prohibits the use of 'public abstract fun get(receiver: T): R defined in kotlin.reflect.KProperty1'

using

it::class.java.getMethod("get${propertyName.capitalize()}").invoke(it)

appears to work, but it seems like there should be a way to do this through KClass directly (and avoid having to generate the getter method name)?

The problem is that the value of

it

is of type

DataClass

or any of its potential subclasses. Due to that

it::class

is not

KClass<DataClass>

(class descriptor for

DataClass

) but

KClass<out DataClass>

(class descriptor for

DataClass

or any of its subclasses).

it::class.memberProperties.filter { prop -> prop.name == propertyName }.first()

thus returns

KProperty1<out DataClass,Any?>

(property of a

DataClass

or any of its subclasses). You cannot call

get()

in that case because the compiler cannot know what class (

DataClass

or one of its subclasses) has declared the property you have found (it could be of a subclass).

get()

requires a value of that subclass to be passed. Because the compiler cannot know,

get(receiver: T)

resolves to

get(receiver: Nothing)

which effectively makes the method uncallable. The way you work around this depends on the context, which is impossible to tell from the code you've shared. One way is to cast

it::class

from

KClass<out DataClass>

to

KClass<DataClass>

which makes

get()

require a receiver of type

DataClass

or any subclass. As long as you use the same value

it

to both,

it::class

and

.get(it)

, and

it

is declared as

it: DataClass

then this should always be safe.

(it::class as KClass<DataClass>)
	.memberProperties
	.first { prop -> prop.name == propertyName }
	.get(it)

You can also cast the resulting

KProperty1

instead:

it::class
	.memberProperties
	.first { prop -> prop.name == propertyName }
	.let { it as KProperty1<DataClass,Any?> }
	.get(it)

The compiler will issue a warning for the cast because it cannot validate it, but that's okay.

interesting, especially that the compiler can't figure out the data class is final, but thanks!