SingleAssign.kt

interface SingleAssignType<T> {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T)
}

class SingleAssign<T> : SingleAssignType<T> {

    private var initialized = false
    private var _value: Any? = null

    override operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
        if (!initialized)
            throw Exception("Value has not been assigned yet!")
        @Suppress("UNCHECKED_CAST")
        return _value as T
    }

    override operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        if (initialized) {
            throw Exception("Value has already been assigned!")
        }
        _value = value
        initialized = true
    }

}

//usage 
class Bar {
    var f: Foo by SingleAssign()
  
    fun doSomething() {
        f = createFoo()
        f.foo() //use normally and foo is not nullable type
        f = createAnotherFoo() //throw exception as f has been initialized
    }
}