How to access a value based on siblings or parent in datagrid databinding

Question

Sorry for the wordy title - I am battling to work out if my problems are caused by

1. Bad class design (likely)
2. Ignorance (very likely)
3. Both (most likely)

I have spent ages googling but due to not really understanding my problem it's harder to find the solution.

Basically, I have a "Day" class which has an ObservableCollection of Period called Periods. Each period has a length of time. I have a method/property of my Day class (TotalTimeUpUnTilThisPeriod) that takes in a period and returns the amount of time of all the periods up until that point.

I need to put this Period level though so that my datagrid can bind to it (it's itemssource is set to Day.Periods and it's datacontext is set to Day). However, I have to do the calculation in Day as Period instances don't know anything about their siblings and it feels wrong if a child walked up the tree to parent level and summed the children.

My thinking is this - a period should have no "state" ie. no idea of where it is in the day in relation to it's siblings - this is the job of the Day class.

So, if my class design is correct - then how can I access the datacontext in a bound datacolumn? I tried to write a converter that would take in a period and then get the parent datacontext (Day) and call my method to return a string to bind to at period level) but I could not figure out how to make my converter aware of the datagrid datacontext and I want to know if I am on the right track.

This is probably bad design but I considered replicating the parent property (e.g. TotalTimeUpUnTilThisPeriod) in the child class (Period) and then somehow setting it whenever a duration changed just so that I could bind to it but it got really messy.

Periods can be added/removed from the datagrid/Day so it can't be calculated on rowload - it has to be dynamic (I guess I could use LINQ to sum the lengths of all previous periods)

So I think my questions are:

1. Is my class design above ok? (ie. the children should know nothing of the parent or their fellow siblings?)
2. If so, then how do you bind to parent properties or methods in a datagrid if a value is dependent on the state of the parent?

Thanks for any pointers.

UPDATE:

Here are some more things I have tried in the cold light of day:

1) Text= "{Binding DataContext.SomeTextField, ElementName=LayoutRoot}"

This does not work - it works fine when outside the datagrid but not when inside.

2) I tried setting the DataContext of a textbox in a templated DataGrid column - this works well, but then I need to pass in the row datacontext AND the parent datacontext to a Converter (in order to lookup the total time for all periods before this one) and I can't figure out how to do that.

So another question may be - How do you pass in the the current row datacontext to a converter AND the parent datacontext (or just "how to make a converter aware of the parent datacontext").

Answer 1

Here is what I did in the end - I would love to know if this is good practise or not:

I made the parent class a property on the child class (this felt so wrong?) - now I can reference the parent's properties in the datagrid.