Hi! I have a data class with a couple of properties that should be marked `@Transient` and they are ...

# serialization

[hellman]

Hi! I have a data class with a couple of properties that should be marked @Transient and they are calculated in the init block, but that doesn't seem to be allowed. It works if I change it to a lateinit var but that seems wrong. Is this a bug in kotlinx-serialization or is there a workaround?

[curioustechizen]

It looks like the error is that it should have a default value.
Source: https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/basic-serialization.md#transient-properties

[hellman]

That feels like a mistake. I have three values that are computed at initialization and assigned in the init block

[curioustechizen]

Yeah I don't mean to offer that as a solution. I would not even consider it as a work-around.

[hellman]

I feel like my case would be more common than a transient property you assign with the constructor?

[hfhbd]

Another workaround: store the result of the computation in a private variable and use an initial expression.

[hellman]

Yeah, that's what I do now

[glureau]

"a transient property you assign with the constructor" so that can't be a constructor parameter. data class is coupling property and parameters but here you don't want that. Feels like a simple property (out of the ctor) could be ok but it depends if you need to use those fields in equals/hashcode/toString/copy...

[hellman]

Transient in this case means it won't be serialized by kotlinx-serialization

[glureau]

If a deserialization is done, Ktx wants to call the constructor but there's no default value for those transients parameters, so it can't construct them.

[hellman]

The init block will still be called though

[hellman]

If I remove @Transient it works, but then it serializes those properties

[arve]

Suggestion: use getters for computed values:

[arve]

data class Person(val name: String, val age: Int) {
  val isAdult: Boolean get() = age > 18
}

[arve]

I don’t remember whether @Transient is required here

[hfhbd]

A getter does not need Transient, but the computation will be executed every access.

[hellman]

Ok, computed values works but I don't want it computed every time

[hellman]

The problem here is that I want to assign a property in my init block and add @Transient so it isn't serializaed