Code Coverage — что измеряет и что не измеряет?

public decimal CalculateDiscount(Customer c, decimal total)
{
    if (c.IsVip && total > 1_000_000)         // 3 ветки логики: IsVip=false; IsVip=true & total ≤ 1M; оба true
        return total * 0.10m;
    return 0;
}

[Fact]
public void Vip_with_big_order_gets_10_percent() =>
    new Calc().CalculateDiscount(new Customer { IsVip = true }, 2_000_000).Should().Be(200_000);

# Coverlet collector (подключён в шаблонах xunit по умолчанию)
dotnet test --collect:"XPlat Code Coverage" --results-directory ./coverage

# HTML-отчёт
dotnet tool install -g dotnet-reportgenerator-globaltool
reportgenerator -reports:"coverage/**/coverage.cobertura.xml" -targetdir:"coverage/report" -reporttypes:"Html;Cobertura;MarkdownSummaryGithub"

<!-- coverlet.runsettings -->
<RunSettings>
  <DataCollectionRunSettings>
    <DataCollectors>
      <DataCollector friendlyName="XPlat code coverage">
        <Configuration>
          <Format>cobertura</Format>
          <Exclude>[*]*.Migrations.*,[*]Program</Exclude>
          <ExcludeByAttribute>GeneratedCodeAttribute,CompilerGeneratedAttribute,ExcludeFromCodeCoverageAttribute</ExcludeByAttribute>
        </Configuration>
      </DataCollector>
    </DataCollectors>
  </DataCollectionRunSettings>
</RunSettings>

[Fact]
public void Covers_everything()
{
    var calc = new Calc();
    calc.CalculateDiscount(new Customer { IsVip = true }, 2_000_000);
    calc.CalculateDiscount(new Customer { IsVip = true }, 10);
    calc.CalculateDiscount(new Customer { IsVip = false }, 10);
    // ни одного Assert — все ветки «покрыты», ошибки не будут обнаружены
}

- name: Test
  run: dotnet test --configuration Release --collect:"XPlat Code Coverage" --results-directory ./coverage
- name: Report
  run: reportgenerator -reports:"coverage/**/coverage.cobertura.xml" -targetdir:"coverage/report" -reporttypes:"MarkdownSummaryGithub;Cobertura"
- name: Summary
  run: cat coverage/report/SummaryGithub.md >> $GITHUB_STEP_SUMMARY
- name: Upload to Codecov
  uses: codecov/codecov-action@v4
  with: { files: coverage/report/Cobertura.xml }