GetRange range check overflows for large counts: ContiguousCollection tries to allocate an int.MaxValue array

[matt-edmondson]

What's wrong

The range guard in GetRange is

ArgumentOutOfRangeException.ThrowIfGreaterThan(startIndex + count, Count);

in Containers/ContiguousCollection.cs (~line 379), Containers/InsertionOrderCollection.cs (~line 197) and Containers/OrderedCollection.cs (~line 357, the line just edited for #59). startIndex + count overflows to a negative number when count is large, so the guard passes.

new ContiguousCollection<int> { 1, 2, 3 }.GetRange(1, int.MaxValue);
// OutOfMemoryException: Array dimensions exceeded supported range
//   (the new collection is sized with `count` before Array.Copy runs)

new OrderedCollection<int> { 1, 2, 3 }.GetRange(1, int.MaxValue);
// ArgumentException from List<T>.GetRange instead of the documented ArgumentOutOfRangeException

Reproduced against the current main build.

Why it matters

The method is documented to throw ArgumentOutOfRangeException for an invalid range. Instead, ContiguousCollection attempts a multi-gigabyte allocation and fails with OutOfMemoryException, which callers do not expect and typically do not catch. It is also a cheap way for untrusted paging parameters (offset/limit) to trigger a huge allocation.

Suggested fix

After the two non-negative checks, compare in a form that cannot overflow:

ArgumentOutOfRangeException.ThrowIfGreaterThan(count, Count - startIndex);

This still allows the zero-length range at the end (GetRange(Count, 0)) fixed in #59.

Acceptance criteria

• GetRange(1, int.MaxValue) throws ArgumentOutOfRangeException on all three collections.
• GetRange(Count, 0) still returns an empty collection.