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POComposite pattern for file and folder structure with parent reference in C#
 

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    <p>I'm currently struggling with the implementation of a set of file system classes. I guess this calls for the composite pattern if I'm not mistaken. So I set up the following classes:</p> <p>An abstract class <code>Node</code> which has a reference to its parent folder and two classes <code>Folder</code> and <code>File</code> that implement <code>Node</code>. A folder contains a collection of all its children and methods to add and remove children.</p> <p>The thing is, I can't figure out how to implement all the methods properly. In all the examples I have seen there is no reference to the parent in the children. How can the <code>AddChild</code> method ensure that the child's parent reference is set correctly? I solved that by checking whether <code>child.Parent</code> has already been set to the folder or it throws an <code>ArgumentException</code>. The matter is further complicated by the fact that <code>AddChild</code> might also throw an exception like <code>DuplicateNameException</code> or something. So my methods look like this now:</p> <pre><code>File.AddTo(Folder folder) { this.Parent = folder; try { folder.AddChild(this); } catch { this.Parent = null; throw; } } Folder.AddChild(Node child) { if(child.Parent != this) throw new ArgumentException(...); ... } </code></pre> <p>Now I have this ugly <code>AddTo</code> method and cannot do something like <code>someFolder.AddChild(new File(...))</code>. I wonder how it was implemented with <code>ListViewItem</code> for instance. There I can just do <code>someListView.Items.Add(new ListViewItem(...))</code>.</p> <p>My solution works, but I'm not convinced that it's the right way to do this. Maybe someone has a better solution or can point me to a good example. Thanks in advance.</p> <p><strong>EDIT</strong>: Minimal full class definitions below.</p> <pre><code>abstract class Node { public Folder Parent { get; protected set; } public string Name { get; private set; } public Node(string name) { Parent = null; Name = name; } } class Folder : Node { private Dictionary&lt;string, Node&gt; _children; public Folder(string name) : base(name) { // Other initializations here... } public void AddChild(Node child) { if(child is Folder) ((Folder)child).Parent = this; // Damn, doesn't work for files!!! else if(child.Parent != this) throw new ArgumentException(); if(_children.ContainsKey(child.Name)) throw new DuplicateNameException(); _children[child.Name] = child; } } class File : Node { public File(string name) : base(name) { // Other initializations here... } public void AddTo(Folder folder) { Parent = folder; try { folder.AddChild(this); } catch { Parent = null; } } } </code></pre>
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