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);

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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));