Understanding Downcasting on Swift

mutableCopy() creates replication for that instance. CNMutableGroupKai does not override mutableCopy(), so the parent class implementation is called to create replication for CNMutableGroup.

It is illegal to cast a CNMutableGroup to CNMutableGroupKai, so crashing with as! is a legitimate behavior.

The workaround is to implement mutableCopy(), but

class CNMutableGroupKai:CNMutableGroup{
    varicon: String=""

    override func mutableCopy()->AnyObject{
        let copy = CNMutableGroupKai()
        // Identifier is read-only and cannot be copied
        // copy.identifier=self.identifier
        copy.name = self.name
        copy.icon=self.icon
        return copy
    }
}

Adding your own properties to CNMutableGroup does not even save them in the CNContactStore (CNMutableGroup is a class to manipulate and store group information) and seems to be an undesirable behavior.

If you want a contact book group to have an icon to show it, wouldn't it be better to create a wrapper structure than to extend the Contacts framework class?

structureGroupData{
    varicon:UIimage
    var group —CNGroup
}