I have a lot of existing categories.

Now I want to add a new "VisibleInMenu" bool property to categories, and I want it to be checked by default i all the exisiting categories.

I have tried the SetDefaultValues approach, which did not work:

public override void SetDefaultValues(ContentType contentType)
{
base.SetDefaultValues(contentType);
VisibleInMenu = true;
}

I've tried overwrititng the get/set of the property, and that did not work:

[Display(Name = "Display in navigation", GroupName = SystemTabNames.PageHeader, Order = 1000)]
public virtual bool VisibleInMenu
{
get
{
return (bool)(this["VisibleInMenu"] ?? true);
}
set
{
this["VisibleInMenu"] = value;
}
}

Is this even possible without looping through all the categories and setting the value?