Setting a readonly "Parent" property in child object from parent - properties

Question

I am trying to reproduce the behaviour found in some WinForms controls (such as DataGridView and DataGridViewColumn) where the child object has a property pointing to the parent. This property is normally readonly, but it somehow changes after the parent's Add() method has been called.
In my code I have two classes, DataGroup and DataEntry, with the latter being the child object.
If I simply implemented something like:
public class DataEntry
{
public DataGroup Parent { get; set; }
}
public class DataGroup
{
public List<DataEntry> DataEntries { get; set; }
public DataGroup()
{
DataEntries = new List<DataEntry>();
}
public void Add(DataEntry de)
{
// Check stuff here
// ...
//
DataEntries.Add(de);
de.Parent = this;
}
}
it would certainly work, but with one major drawback: DataEntry.Parent has a public setter, so the property could be modified from anywhere without any of the checks I designed in DataGroup.Add()
I could maybe do the following:
public class DataEntry
{
private DataGroup _parent;
public DataGroup Parent
{
get { return _parent; }
set
{
_parent = value;
_parent.Add(this);
}
}
}
public class DataGroup
{
...
public void Add(DataEntry de)
{
// Check stuff here
// ...
//
_dataEntries.Add(de);
// de.Parent = this; Already set!
}
...
}
which would work fine in case I wanted to add the child by setting its parent property, but would not update DataEntry.Parent if I called DataGroup.Add()
As I already said, it is quite normal for WinForms controls not to be able to change the Parent property directly in the child, but the property is changed after the Add method is called from the parent.
I can't figure out the link, a way for the parent to modify a property in the child, unless that property is public or exposed through a method, both of which would grant a chance to bypass my checks and ultimately produce errors.

Answer 1

if you just want to hide the method from outside parties you could declare the method internal
internal void SetParent(DataGroup dg)
{
//code to set parent
}
I've never tried this but it might work also. I do it with private all the time.
public DataGroup Parent { get; internal set; }
Another option to hide it from yourself a little more is to have an Explicit Interface Implementation. and even combine it with internal to hide it from outside. When you Explicitly implement an interface the only way to call the method is to cast the object to the interface first. I think something like this would work
internal interface ISetParent
{
void SetParent(DataGroup dg);
}
public class DataEntry : ISetParent
{
void ISetParent.SetParent(DataGroup dg)
{
Parent = dg;
}
public DataGroup Parent { get; private set;}
}
public class DataGroup
{
public List<DataEntry> DataEntries { get; set; }
public DataGroup()
{
DataEntries = new List<DataEntry>();
}
public void Add(DataEntry de)
{
// Check stuff here
// ...
//
DataEntries.Add(de);
((ISetParent)de).SetParent(this);
}
}