Include original index in extraneous item failure messages by dennisdoomen · Pull Request #3203 · fluentassertions/fluentassertions · GitHub

Summary

Fixes #985.

When BeEquivalentTo fails because the subject collection has more items than expected, the error message now includes the original index of each extraneous item, making it easier to diagnose where collections diverge.

Before

Expected actual to contain exactly one item, but found one extraneous item Customer { Age = 16, ... }

After

Single extra item:

Expected actual to contain exactly one item, but found one extraneous item at index 1: Customer { Age = 16, ... }

Multiple extra items:

Expected actual to contain exactly 2 items, but found extraneous items: Customer { Age = 16, ... } (at index 2), Customer { Age = 21, ... } (at index 4)

Implementation

EnumerableEquivalencyValidator: subjects are now wrapped in List<IndexedItem<object>> so original indices survive through the matching pipeline to the reporting stage.
StrictlyOrderedEquivalencyStrategy / LooselyOrderedEquivalencyStrategy: updated to pass List<IndexedItem<object>> instead of List<object> for subjects; raw objects are accessed via .Item.
When only extra items exist, each item is reported with at index N:. When both missing and extra items exist, the compact fallback format is used to avoid expensive per-item formatting.
Braces in formatted item strings (e.g. Customer { Age = 16 }) are escaped before embedding into the FailWith format string to prevent string.Format treating them as placeholders.

Pull request overview

This PR improves BeEquivalentTo failure diagnostics for collection equivalency by including the original index of extraneous (subject-only) items in the assertion failure message (Fixes #985), making it easier to locate where collections diverge.

Changes:

Wrap subjects in IndexedItem<object> so original indices survive the matching pipeline through to reporting.
Update ordered equivalency strategies to operate on List<IndexedItem<object>> and compare via .Item.
Extend/adjust specs and release notes to cover and document the improved failure messages.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/_pages/releases.md	Adds a release note describing the improved BeEquivalentTo extraneous-item messaging.
Tests/FluentAssertions.Specs/Collections/CollectionAssertionSpecs.BeEquivalentTo.cs	Updates an existing assertion message expectation to match the new output shape.
Tests/FluentAssertions.Equivalency.Specs/CollectionSpecs.cs	Updates single-extra-item expectation and adds coverage for multiple extra items including indices.
Tests/FluentAssertions.Equivalency.Specs/BasicSpecs.cs	Updates golden failure-message text to include the extraneous item index.
Src/FluentAssertions/Equivalency/Steps/StrictlyOrderedEquivalencyStrategy.cs	Changes subject inputs to IndexedItem<object> and compares using .Item.
Src/FluentAssertions/Equivalency/Steps/LooselyOrderedEquivalencyStrategy.cs	Changes subject inputs to IndexedItem<object> and compares using .Item throughout matching/scoring.
Src/FluentAssertions/Equivalency/Steps/EnumerableEquivalencyValidator.cs	Wraps subjects with original indices and updates failure reporting to include indices for extraneous-only cases.

Test Results

37 files ± 0 37 suites ±0 2m 47s ⏱️ -1s
6 385 tests + 2 6 384 ✅ + 2 1 💤 ±0 0 ❌ ±0
39 656 runs +12 39 650 ✅ +12 6 💤 ±0 0 ❌ ±0

Results for commit 6059560. ± Comparison against base commit fb42954.

This pull request removes 10 and adds 10 tests. Note that renamed tests count towards both.

FluentAssertions.Specs.Streams.StreamAssertionSpecs+HaveLength ‑ When_a_throwing_stream_should_have_a_length_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
FluentAssertions.Specs.Streams.StreamAssertionSpecs+HaveLength ‑ When_a_throwing_stream_should_have_a_length_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
FluentAssertions.Specs.Streams.StreamAssertionSpecs+HavePosition ‑ When_a_throwing_stream_should_have_a_position_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
FluentAssertions.Specs.Streams.StreamAssertionSpecs+HavePosition ‑ When_a_throwing_stream_should_have_a_position_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
FluentAssertions.Specs.Streams.StreamAssertionSpecs+NotHaveLength ‑ When_a_throwing_stream_should_not_have_a_length_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
FluentAssertions.Specs.Streams.StreamAssertionSpecs+NotHaveLength ‑ When_a_throwing_stream_should_not_have_a_length_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
FluentAssertions.Specs.Streams.StreamAssertionSpecs+NotHavePosition ‑ When_a_throwing_stream_should_not_have_a_position_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
FluentAssertions.Specs.Streams.StreamAssertionSpecs+NotHavePosition ‑ When_a_throwing_stream_should_not_have_a_position_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'GetLengthExceptionMessage'.)
Object name: 'GetPositionExceptionMessage'.)

FluentAssertions.Equivalency.Specs.CollectionSpecs ‑ When_the_subject_contains_multiple_extra_items_it_should_include_the_index_of_each
FluentAssertions.Equivalency.Specs.CollectionSpecs ‑ When_the_subject_contains_multiple_extra_items_the_failure_message_should_include_their_formatted_properties
FluentAssertions.Specs.Streams.StreamAssertionSpecs+HaveLength ‑ When_a_throwing_stream_should_have_a_length_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'GetLengthExceptionMessage'.)
FluentAssertions.Specs.Streams.StreamAssertionSpecs+HaveLength ‑ When_a_throwing_stream_should_have_a_length_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'GetLengthExceptionMessage'.)
FluentAssertions.Specs.Streams.StreamAssertionSpecs+HavePosition ‑ When_a_throwing_stream_should_have_a_position_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'GetPositionExceptionMessage'.)
FluentAssertions.Specs.Streams.StreamAssertionSpecs+HavePosition ‑ When_a_throwing_stream_should_have_a_position_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'GetPositionExceptionMessage'.)
FluentAssertions.Specs.Streams.StreamAssertionSpecs+NotHaveLength ‑ When_a_throwing_stream_should_not_have_a_length_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'GetLengthExceptionMessage'.)
FluentAssertions.Specs.Streams.StreamAssertionSpecs+NotHaveLength ‑ When_a_throwing_stream_should_not_have_a_length_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'GetLengthExceptionMessage'.)
FluentAssertions.Specs.Streams.StreamAssertionSpecs+NotHavePosition ‑ When_a_throwing_stream_should_not_have_a_position_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'GetPositionExceptionMessage'.)
FluentAssertions.Specs.Streams.StreamAssertionSpecs+NotHavePosition ‑ When_a_throwing_stream_should_not_have_a_position_it_should_fail(exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'GetPositionExceptionMessage'.)

♻️ This comment has been updated with latest results.

Coverage Report for CI Build 25427286962

Coverage decreased (-0.002%) to 97.14%

Details

Coverage decreased (-0.002%) from the base build.
Patch coverage: 1 uncovered change across 1 file (31 of 32 lines covered, 96.88%).
No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
Src/FluentAssertions/Equivalency/Steps/LooselyOrderedEquivalencyStrategy.cs	13	12	92.31%

Coverage Regressions

No coverage regressions found.

Coverage Stats


Relevant Lines:	13406
Covered Lines:	13181
Line Coverage:	98.32%
Relevant Branches:	4392
Covered Branches:	4108
Branch Coverage:	93.53%
Branches in Coverage %:	Yes
Coverage Strength:	67966.14 hits per line

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Nice!

When BeEquivalentTo fails because the subject collection has more items than expected, the error message now includes the original index of each extraneous item to help diagnose where collections diverge. Before: found one extraneous item Customer { Age = 16, ... } After: found one extraneous item at index 1 Customer { Age = 16, ... } For multiple extra items: found extraneous items {Customer { Age = 16, ... } (at index 1), ...} Fixes #985 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Escape { and } in Formatter.ToString output before embedding into the FailWith format string, preventing string.Format from treating object dumps like 'Customer { Age = 16 }' as format placeholders - Pass a single item (not a 1-element list) as the {1} argument whenever remainingSubjects.Count == 1, matching the previous behavior for the 'both missing and extra' fallback path Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Multi-item: 'found extraneous items: A (at index 3), B (at index 96)' instead of 'found extraneous items {A (at index 3), B (at index 96)}' - Single-item: 'found one extraneous item at index 1: X' instead of 'found one extraneous item at index 1 X' - Fix TestBeEquivalent to be a proper test with a throw assertion and a descriptive name Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…lidator.cs Co-authored-by: Jonas Nyrup <jnyrup@users.noreply.github.com>

Without escaping curly braces in the formatted items string, string.Format throws a FormatException, causing a **WARNING** fallback instead of the actual failure message. The existing test didn't catch this because the WARNING still contained the expected text from the raw format string. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Jonas Nyrup <jnyrup@users.noreply.github.com>

…lidator.cs Co-authored-by: Jonas Nyrup <jnyrup@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings May 3, 2026 10:35

Copilot started reviewing on behalf of dennisdoomen May 3, 2026 10:36 View session

Copilot AI reviewed May 3, 2026

View reviewed changes

Comment thread Src/FluentAssertions/Equivalency/Steps/EnumerableEquivalencyValidator.cs Outdated

Comment thread Src/FluentAssertions/Equivalency/Steps/EnumerableEquivalencyValidator.cs Outdated

dennisdoomen force-pushed the dennisdoomen/fix-extra-item-index-985 branch from f29c4ec to 3fc7825 Compare May 3, 2026 15:51

dennisdoomen requested a review from jnyrup May 5, 2026 14:06

jnyrup requested changes May 5, 2026

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Comment thread Src/FluentAssertions/Equivalency/Steps/EnumerableEquivalencyValidator.cs Outdated

Comment thread Src/FluentAssertions/Equivalency/Steps/EnumerableEquivalencyValidator.cs Outdated

jnyrup approved these changes May 6, 2026

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Comment thread Tests/FluentAssertions.Equivalency.Specs/CollectionSpecs.cs Outdated

Comment thread Src/FluentAssertions/Equivalency/Steps/EnumerableEquivalencyValidator.cs Outdated

dennisdoomen and others added 10 commits May 6, 2026 10:48

Fix PR number in releases.md

31a2d01

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Remove redundant type argument in ToList call

76c75d4

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Remove redundant System.Linq using directive

81530da

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Update Src/FluentAssertions/Equivalency/Steps/EnumerableEquivalencyVa…

1f71935

…lidator.cs Co-authored-by: Jonas Nyrup <jnyrup@users.noreply.github.com>

Update Tests/FluentAssertions.Equivalency.Specs/CollectionSpecs.cs

0d5ba91

Co-authored-by: Jonas Nyrup <jnyrup@users.noreply.github.com>

Update Src/FluentAssertions/Equivalency/Steps/EnumerableEquivalencyVa…

ad34005

…lidator.cs Co-authored-by: Jonas Nyrup <jnyrup@users.noreply.github.com>

dennisdoomen force-pushed the dennisdoomen/fix-extra-item-index-985 branch from 510afe6 to ad34005 Compare May 6, 2026 08:52

Remove redundant System using directive

6059560

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

dennisdoomen added the enhancement label May 6, 2026

dennisdoomen merged commit 0804dbc into main May 6, 2026
14 checks passed

dennisdoomen deleted the dennisdoomen/fix-extra-item-index-985 branch May 6, 2026 11:48

Include original index in extraneous item failure messages by dennisdoomen · Pull Request #3203 · fluentassertions/fluentassertions · GitHub

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Include original index in extraneous item failure messages - #3203

Include original index in extraneous item failure messages#3203
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Include original index in extraneous item failure messages by dennisdoomen · Pull Request #3203 · fluentassertions/fluentassertions · GitHub

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Include original index in extraneous item failure messages - #3203

Include original index in extraneous item failure messages#3203dennisdoomen merged 11 commits intomainfluentassertions/fluentassertions:mainfrom dennisdoomen/fix-extra-item-index-985fluentassertions/fluentassertions:dennisdoomen/fix-extra-item-index-985Copy head branch name to clipboard

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Include original index in extraneous item failure messages#3203
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