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    <p>If you have the Martin Fowler's Refactoring book, just follow the "Change Unidirectional Association to Bidirectional" refactoring.</p> <p>In case you don't have it, here's how your classes will look like after the refactoring:</p> <pre><code>class C { // Don't to expose this publicly so that // no one can get behind your back and change // anything private List&lt;W&gt; contentsW; public void Add(W theW) { theW.Container = this; } public void Remove(W theW) { theW.Container = null; } #region Only to be used by W internal void RemoveW(W theW) { // do nothing if C does not contain W if (!contentsW.Contains(theW)) return; // or throw an exception if you consider this illegal contentsW.Remove(theW); } internal void AddW(W theW) { if (!contentW.Contains(theW)) contentsW.Add(theW); } #endregion } class W { private C containerC; public Container Container { get { return containerC; } set { if (containerC != null) containerC.RemoveW(this); containerC = value; if (containerC != null) containerC.AddW(this); } } } </code></pre> <p>Take note that I've made the <code>List&lt;W&gt;</code> private. Expose the list of Ws via an enumerator instead of exposing the list directly.</p> <p>e.g. public List GetWs() { return this.ContentW.ToList(); }</p> <p>The code above handles transfer of ownership properly. Say you have two instances of C -- C1 and C2 - and the instances of W -- W1 and W2.</p> <pre><code>W1.Container = C1; W2.Container = C2; </code></pre> <p>In the code above, C1 contains W1 and C2 contains W2. If you reassign W2 to C1</p> <pre><code>W2.Container = C1; </code></pre> <p>Then C2 will have zero items and C1 will have two items - W1 and W2. You can have a floating W</p> <pre><code>W2.Container = null; </code></pre> <p>In this case, W2 will be removed from C1's list and it will have no container. You can also use the Add and Remove methods from C to manipulate W's containers - so C1.Add(W2) will automatically remove W2 from it's original container and add it to the new one.</p>
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