Get children from flat list - silverlight-4.0

Question

I've got a flat list of items in a ObservableCollection. These items have the properties item.Id and item.Parent.Id.
I've been given the id of the parent (top level), now with this id I need to iterate through the list and find the children of this parent. Each child can only have one parent, a parent can have multiple childs.
How can I do this effectively?

Answer 1

You can use:
var childrenOfParent = theCollection.Where(item => item.Parent.Id == parentId);
Edit in response to comments:
Given that you have a hierarchical dataset, I would personally make a routine that checks to see if a given item has a specific item as a parent recursively, like so:
bool HasParent(Item item, int parentId)
{
if (item.Parent == null)
return false;
else if (item.Parent.Id == parentId)
return true;
else
return HasParent(item.Parent, parentId);
}
Given this, you could use:
var childrenOfParent = theCollection.Where(item => HasParnet(item, parentId));

Answer 2

Well I got it, however, if anyone can optimize/refactor this code into something more efficient please let me know and I'll mark your reaction as an answer. As long as I learn :).
foreach (var item in myitemlist) // first put the top level parent in the queue
{
if (parentitem.Id == item.Id)
{
filteredChildrenQueue.Enqueue(item);
}
}
while (!stillsearching)
{
if (filteredChildrenQueue.Count == 0)
{
stillsearching = true;
return;
}
FindParentChild();
}
Keep calling this method and work the first item in the queue
private void FindParentChild()
{
foreach (var item in myitemlist)
{
if (item.Parent.Id == filteredChildrenQueue.ElementAt(0).Id)
{
filteredChildrenQueue.Enqueue(item);
}
}
filteredChildrenList.Add(filteredChildrenQueue.ElementAt(0));
filteredChildrenQueue.Dequeue();
}
filteredChildrenList will contain the top level parent + all childs it contains.