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There are 3 ways of adding items to most lists... via a direct public API method, typically Add(SomeType) via the generic IList<T>.Add(T) interface via the non-generic IList.Add(object) interface method and you normally expect them to behave more or less the same. However, LINQ's EntitySet<T> is... peculiar on both 3.5 and 4.0; the IList API does not flag the set as "assigned" - the other two mechanisms do - this sounds trivial, but it is important in that it heavily influences serialization (i.e. causes it to be skipped) in the boilerplate code. Example: EntitySet<string> set1 = new EntitySet<string>(); set1.Add("abc"); Debug.Assert(set1.Count == 1); // pass Debug.Assert(set1.HasLoadedOrAssignedValues, "direct"); // pass EntitySet<string> set2 = new EntitySet<string>(); IList<string> typedList = set2; typedList.Add("abc"); Debug.Assert(set2.Count == 1); // pass Debug.Assert(set2.HasLoadedOrAssignedValues, "typed list"); // pass EntitySet<string> set3 = new EntitySet<string>(); IList untypedList = set3; untypedList.Add("abc"); Debug.Assert(set3.Count == 1); // pass Debug.Assert(set3.HasLoadedOrAssignedValues, "untyped list"); // FAIL Now... this is deeply surprising to me; so much so that it took me over 2 hours of tracking upwards through code to isolate what was happening. So... is there any sane reason for this? Or is this just a bug? (FWIW, there was also an issue in set.Assign(set) in 3.5, but this is now fixed in 4.0.)

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