Fix bug #543: Unstable behavior in PredictRating()

[ngtduc693]

Related issue

#543 Unstable behavior in PredictRating() unit test due to constructor field shadowing

Affected module

• CollaborativeFilteringTests.cs

Root Cause

The instability was not caused by the algorithm itself, but by the test fixture design:

• The test then fails intermittently, even though the same code passes when executed in isolation
• When the entire test suite runs in parallel, other tests’ [SetUp] methods overwrite these fields while this test is still executing.
• The mock that should return a fixed similarity (.Returns(0.8)) is replaced mid-run by a fresh, unconfigured instance, causing:

similarity = 0
PredictRating() → total similarity ≈ 0 → result = 0

Fix Summary

Use the [NonParallelizable] flag

Unit Test Passed Evidence

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Comments in areas I changed are up to date
• I have added comments to hard-to-understand areas of my code
• I have made corresponding changes to the README.md

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.67%. Comparing base (7467fe4) to head (7b4fc36).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #544      +/-   ##
==========================================
- Coverage   96.67%   96.67%   -0.01%     
==========================================
  Files         279      279              
  Lines       11015    11013       -2     
  Branches     1568     1568              
==========================================
- Hits        10649    10647       -2     
  Misses        232      232              
  Partials      134      134              

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[siriak]

Please resolve conflicts

[ngtduc693]

Please resolve conflicts

I'm done, thanks @siriak

[Copilot]

Pull Request Overview

This PR fixes an intermittent test failure in the PredictRating() unit test caused by parallel test execution and shared mock instances. The root cause was that concurrent tests were overwriting shared mock objects, leading to unpredictable test results when the test suite ran in parallel.

• Modernized the CollaborativeFiltering class constructor to use primary constructor syntax
• Added [NonParallelizable] attribute to the problematic test method to prevent race conditions
• Removed redundant test methods to simplify the test suite

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
CollaborativeFiltering.cs	Updated to use C# primary constructor syntax for cleaner code
CollaborativeFilteringTests.cs	Added NonParallelizable attribute and removed duplicate test methods

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[siriak]

Looks good, thanks!