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copied!<p><strong>Sometimes LINQ is not the answer.</strong> This is the kind of problem where I would consider using a <code>HashSet<T></code> with a custom comparer to reduce the work of performing set operations. HashSets are much more efficient at performing set operations than lists - and (depending on the data) can reduce the work considerably:</p> <pre><code>// create a wrapper class that can accomodate either an Alpha or a Bravo class ABItem { public Object Instance { get; private set; } public int Id { get; private set; } public ABItem( Alpha a ) { Instance = a; Id = a.Id; } public ABItem( Bravo b ) { Instance = b; Id = b.Id; } } // comparer that compares Alphas and Bravos by id class ABItemComparer : IComparer { public int Compare( object a, object b ) { return GetId(a).Compare(GetId(b)); } private int GetId( object x ) { if( x is Alpha ) return ((Alpha)x).Id; if( x is Bravo ) return ((Bravo)x).Id; throw new InvalidArgumentException(); } } // create a comparer based on comparing the ID's of ABItems var comparer = new ABComparer(); var hashAlphas = new HashSet<ABItem>(myAlphaItems.Select(x => new ABItem(x)),comparer); var hashBravos = new HashSet<ABItem>(myBravoItems.Select(x => new ABItem(x)),comparer); // items with common IDs in Alpha and Bravo sets: var hashCommon = new HashSet<Alpha>(hashAlphas).IntersectWith( hashSetBravo ); hashSetAlpha.ExceptWith( hashSetCommon ); // items only in Alpha hashSetBravo.ExceptWith( hashSetCommon ); // items only in Bravo </code></pre>

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