EntitySet - is there a sane reason that IList.Add doesn't set assigned?

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.)

Solution

Interestingly, this has been identified for several versions now (you stated that a 3.5 issue was fixed in 4.0). Here is a post from 2007. The rest of the IList methods in 4.0 are correctly tied to the IList<T> methods. I think that there are 2 likely explanations (of the bug/feature variety):

This is an actual bug that Microsoft has not yet fixed.
This is a feature that some other Microsoft code is ~~exploiting~~ leveraging to add items without setting the HasLoadedOrAssignedValues.

It is probably both - a bug that other code inside the framework is counting on. Sounds like someone said to themselves:

No one is really going to cast this into an IList and then call the Add method, right?