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copied!<p><strong>Edit:</strong> Fixed up. This is sloppy but it should work.</p> <pre><code> listA //Start with lisA .Where(a=>!listB.Any(b=>a.Key == b.Key)) // Remove any that are duplicated in listB .Select(a => new TypeAB() { Key=a.Key, ValueA=a.Value}) // Map each A to an AB .Union( listB.Select(b => { var correspondingA = listA.FirstOrDefault(a => a.Key == b.Key); //Is there an a that corresponds? return new TypeAB() { Key=b.Key, ValueB=b.Value, //Value B is easy ValueA= correspondingA!=null ? (int?)correspondingA.Value : null //If there is an A than map its value }; }) ) </code></pre> <p>As an aside, if you're using this as some sort of domain operation, TypeA and TypeB should probably be based on some sort AorBIsAConceptThatHasMeaningInTheDomain base class. That's just a general rule whenever you find yourself combining lists. If no such concept exists, then you probably don't need to be combining the lists.</p> <p>On the other hand, if you're doing this as part of a mapping - such as mapping domain objects to UI - you might be able to simplify your code somewhat by using anonymous types instead of a TypeAB class. (Or maybe not, this one is up to personal preference)</p> <p><strong>Edit Edit</strong> Here's a slightly more intellectually interesting answer using hashes</p> <pre><code> var listAB = listA.Cast<object>().Union(listB.Cast<object>()).ToLookup(x => x is TypeA ? (x as TypeA).Key : (x as TypeB).Key) .Select(kv => { var a = kv.FirstOrDefault(x => x is TypeA) as TypeA; var b = kv.FirstOrDefault(x => x is TypeB) as TypeB; return new TypeAB() { Key = kv.Key, ValueA = a != null ? (int?)a.Value : null, ValueB = b != null ? (int?)b.Value : null }; }).ToList(); </code></pre>

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