fix: let OrderedCollection.GetRange take a zero-length range at the end [patch]

[matt-edmondson]

Fixes #59.

OrderedCollection<T>.GetRange guarded startIndex with ThrowIfGreaterThanOrEqual(startIndex, Count), which rejects startIndex == Count. That made a zero-length range at the end of the collection impossible to request, and made GetRange unusable on an empty collection at all — even though count == 0 makes both requests perfectly valid:

new OrderedCollection<int>().GetRange(0, 0);    // threw ArgumentOutOfRangeException
collection.GetRange(collection.Count, 0);        // threw for any collection

List<T>.GetRange explicitly allows index == Count when count == 0, and the sibling collections in this repo already agree — InsertionOrderCollection.GetRange and ContiguousCollection.GetRange both check only startIndex >= 0, count >= 0 and startIndex + count <= Count. OrderedCollection was the only one carrying the extra check, so this is as much a consistency fix as a correctness one.

The change

The one line is dropped. The remaining three guards still reject every genuinely out-of-range request, which is worth spelling out because it is the reason no narrowing comes with this:

• startIndex negative → ThrowIfNegative(startIndex)
• count negative → ThrowIfNegative(count)
• past the end, e.g. GetRange(3, 1) on a 3-element collection → startIndex + count > Count

That last one is what the existing GetRange_InvalidStartIndex_ThrowsArgumentOutOfRangeException asserts, and it still passes unmodified: the removed check was redundant for every case except the valid one it was rejecting.

The XML doc gains a <remarks> noting the List<T> parity, since the boundary is now part of the method's contract.

Tests

Three added next to the existing GetRange_ValidRange_ReturnsCorrectSubset, which only covered a mid-collection range. Verified by stashing the OrderedCollection.cs change and re-running:

Test	Without the fix
GetRange_ZeroCountAtEnd_ReturnsEmpty	fails — startIndex ('3') must be less than '3'
GetRange_ZeroCountOnEmptyCollection_ReturnsEmpty	fails — startIndex ('0') must be less than '0'
GetRange_ZeroCountMidCollection_ReturnsEmpty	passes — a regression guard for the zero-count case that already worked, not a failing-first test

Full suite: 373 passed, 0 failed, 0 skipped.

🤖 Generated with Claude Code

https://claude.ai/code/session_01T1ntzPwg732TrH7D5vWpSV

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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