.NET: Add FileAccessProvdider and concurrency fix for FileMemoryProvider

[westey-m]

Motivation and Context

Description

• Add a FileAccessProvider with a sample to show it's usage
• Fix a concurrency issues in FileMemoryProvider

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 90%

✓ Correctness

This PR adds a new FileAccessProvider for shared file access, adds thread safety (SemaphoreSlim write lock + IDisposable) to FileMemoryProvider, reorganizes file store classes into a dedicated FileStore directory, and adds a data processing sample. The code is correct: path traversal protection is properly delegated to StorePaths.NormalizeRelativePath, the write lock correctly serializes save/delete operations with index rebuilds, AsyncLocal-backed CurrentRunContext ensures concurrent tests are safe, and the new provider is a clean, minimal wrapper. No correctness issues found.

✓ Security Reliability

This PR adds a new FileAccessProvider for shared file access and improves FileMemoryProvider with a SemaphoreSlim write lock for thread safety. Security-critical areas look solid: path traversal is blocked by StorePaths.NormalizeRelativePath() (rejects .., absolute paths, drive roots) and FileSystemAgentFileStore.ResolveSafePath() (full-path prefix check). Regex from LLM-sourced tool calls has a 5-second timeout in both store implementations to mitigate ReDoS. The SemaphoreSlim is properly guarded with try/finally and disposed via the new IDisposable implementation. No security or reliability issues identified.

✓ Test Coverage

The PR adds a new FileAccessProvider with comprehensive test coverage (577 lines) covering all five tools, constructor validation, path traversal protection, and options. FileMemoryProvider gains IDisposable and a SemaphoreSlim write lock with corresponding thread-safety tests. Test coverage is solid overall with only minor gaps: no test for subdirectory paths through FileAccessProvider tools (e.g., 'reports/summary.md'), and the Dispose test only verifies idempotent dispose without checking post-dispose operation behavior. No blocking issues found.

✓ Design Approach

The change mostly moves the file-store abstraction in a sensible direction, but two design choices look off: the new sample teaches a least-privilege-violating pattern by mounting its bundled input data as a fully mutable store, and the new FileAccessProvider exposes an internally inconsistent contract where agents can write nested paths they cannot later discover via list/search. Both issues are about the chosen abstraction boundary rather than implementation details.

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Automated review by westey-m's agents

[Copilot]

Pull request overview

This PR introduces a new FileAccessProvider (plus a harness sample) to give agents persistent/shared file tooling, and adds new AgentFileStore implementations to support file operations. It also updates FileMemoryProvider to address a concurrency issue during writes/index rebuilds.

Changes:

• Added AgentFileStore abstraction with InMemoryAgentFileStore and FileSystemAgentFileStore, plus shared path/glob utilities (StorePaths) and search result models.
• Added FileAccessProvider (+ options) and comprehensive unit tests, along with a new harness sample (Harness_Step03_DataProcessing).
• Fixed FileMemoryProvider concurrent write/index rebuild behavior via a write lock and added thread-safety tests.

Reviewed changes

Copilot reviewed 11 out of 20 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI/Harness/FileStore/AgentFileStore.cs	New base abstraction for agent file operations
dotnet/src/Microsoft.Agents.AI/Harness/FileStore/StorePaths.cs	Shared normalization + glob matching helpers
dotnet/src/Microsoft.Agents.AI/Harness/FileStore/InMemoryAgentFileStore.cs	In-memory AgentFileStore implementation with search support
dotnet/src/Microsoft.Agents.AI/Harness/FileStore/FileSystemAgentFileStore.cs	Disk-backed AgentFileStore implementation with traversal protection + search
dotnet/src/Microsoft.Agents.AI/Harness/FileStore/FileSearchResult.cs	DTO for search results
dotnet/src/Microsoft.Agents.AI/Harness/FileStore/FileSearchMatch.cs	DTO for per-line matches
dotnet/src/Microsoft.Agents.AI/Harness/FileAccess/FileAccessProvider.cs	New provider exposing persistent file tools to agents
dotnet/src/Microsoft.Agents.AI/Harness/FileAccess/FileAccessProviderOptions.cs	Options for FileAccessProvider instructions
dotnet/src/Microsoft.Agents.AI/Harness/FileMemory/FileMemoryProvider.cs	Concurrency fix: serialize writes/index rebuild; implements IDisposable
dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/FileStore/StorePathsTests.cs	Unit tests for StorePaths normalization/globbing
dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/FileStore/InMemoryAgentFileStoreTests.cs	Unit tests for in-memory store behavior/search
dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/FileStore/FileSystemAgentFileStoreTests.cs	Unit tests for file-system store behavior/search
dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/FileMemory/FileMemoryProviderTests.cs	Added concurrency + dispose tests for FileMemoryProvider
dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/FileAccess/FileAccessProviderTests.cs	Unit tests for FileAccessProvider tools/instructions
dotnet/samples/02-agents/Harness/README.md	Registers new Step03 sample in harness index
dotnet/samples/02-agents/Harness/Harness_Step03_DataProcessing/Program.cs	New sample wiring FileAccessProvider over a data folder
dotnet/samples/02-agents/Harness/Harness_Step03_DataProcessing/README.md	Sample walkthrough and setup
dotnet/samples/02-agents/Harness/Harness_Step03_DataProcessing/data/sales.csv	Sample dataset shipped to output
dotnet/samples/02-agents/Harness/Harness_Step03_DataProcessing/Harness_Step03_DataProcessing.csproj	New sample project
dotnet/agent-framework-dotnet.slnx	Adds the new sample project to the solution