Copilot instructions

1---
2applyTo: "src/tests/**,**/tests/**"
3---
4 
5# Tests
6 
7Conventions for test changes. Also apply `conventions` plus the language file for the code under
8test (`csharp` or `native`). Note that test conventions differ across areas: libraries tests
9(`src/libraries/**/tests/`) typically use xUnit with `[Fact]`/`[Theory]` and `Assert.*`;
10JIT/runtime tests (`src/tests/`) often use a `return 100` success convention and have different
11naming/priority requirements. Apply area-specific rules when they conflict with general guidance
12below.
13 
14## Testing
15 
16- **Always add regression tests for bug fixes and behavior changes.** Prefer adding `[InlineData]` test cases to existing test files rather than creating new ones. Ensure new test files are included in the csproj.
17- **Use platform-specific test attributes correctly.** Use `[PlatformSpecific]`, `[ConditionalFact]`, or `[ActiveIssue]` for skip logic rather than runtime if-checks. `ConditionalFact` is required for `SkipTestException` to work.
18- **Test edge cases, error paths, and all affected types.** Include empty strings, negative values, boundary conditions, Turkish 'i', surrogate pairs. Test both true and false for boolean options. Choose inputs that can't accidentally pass if output wasn't touched.
19- **Test assertions must be specific.** Assert exact expected values (exact `OperationStatus`, exact byte counts), not broad conditions. Ensure tests actually fail when the fix is reverted.
20- **Delete flaky and low-value tests rather than patching them.** Do not add tests known to be flaky. If a test relies on fragile runtime details and cannot be made reliable, prefer deletion.
21- **Make test data deterministic and culture-independent.** Create `CultureInfo` with explicit format settings. Use `[Theory]` with `[InlineData]` over individual `[Fact]` methods.
22- **Use `PLACEHOLDER` for test passwords.** Avoids false positives from credential scanning tools.
23- **Use checked builds for CI, lower priority for regression tests.** Use checked (not debug) CoreCLR builds for CI. New JIT regression tests should typically be `CLRTestPriority 1`.
24- **Use `RemoteExecutor` for tests with process-wide shared state.** Tests that modify shared state should use `RemoteExecutor` for isolation. Avoid hardcoded paths; use temp files. Do not add heavy dependencies like `Microsoft.CodeAnalysis.CSharp` to test assemblies.
25- **Catch only expected exceptions in fuzz tests.** Catching all exceptions masks bugs like undocumented exceptions escaping the API.
26- **Use modern xUnit patterns for xUnit-based tests.** In xUnit test projects (for example, most libraries tests), use `Assert.*` instead of the legacy `return 100 == success` pattern, use `[Fact]`/`[Theory]`, prefer `ThrowsAnyAsync<OperationCanceledException>` for cancellation, and name regression test classes after the issue number (e.g., `Runtime_117605`). Legacy non-xUnit tests under `src/tests` may continue to use the existing `return 100` convention.
27- **Reduce test output volume.** Avoid megabytes of console output. Use `Thread.Sleep` with fewer iterations instead of busy loops.
28- **Follow naming conventions for regression test directories.** In `src/tests/Regressions/coreclr/`, use `GitHub_<issue_number>` for the directory and `test<issue_number>` for the test name.
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