Kotlin for C# Developers

3.6. open

In C#:

• classes are open for inheritance unless marked sealed
• methods and properties are closed for inheritance unless marked virtual (or abstract)
• overridden members in a non-sealed class are implicitly open but can be closed with sealed

C#

public class BaseClass // You CAN extend this
{
    public string Property1 => "You CANNOT override this";
    public virtual string Property2 => "You CAN override this";
    public virtual string Method() => "You CAN override this";
}

public class OpenClass : BaseClass // You CAN extend this
{
    public sealed override string Property2 => "You CANNOT override this";
    public override string Method() => "You CAN override this";
}

public sealed class ClosedClass : BaseClass // You CANNOT extend this
{
    public override string Property2 => "You CANNOT override this";
    public override string Method() => "You CANNOT override this";
}

In Kotlin:

• everything is implicitly final (sealed) unless marked open (or abstract)
• overridden members in an open class are implicitly open but can be closed with final

Kotlin

open class BaseClass { // You CAN extend this
    val property1 = "You CANNOT override this"
    open val property2 = "You CAN override this"
    open fun method() = "You CAN override this"
}

open class OpenClass : BaseClass() { // You CAN extend this
    final override val property2 = "You CANNOT override this"
    override fun method() = "You CAN override this"
}

class ClosedClass : BaseClass() { // You CANNOT extend this
    override val property2 = "You CANNOT override this"
    override fun method() = "You CANNOT override this"
}

Next: abstract

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