# FileSystemClient — a `System.IO`-style async client for OPC UA file systems

`FileSystemClient` (in `Libraries/Opc.Ua.Client/FileSystem/`, namespace
`Opc.Ua.Client.FileSystem`) is an ergonomic, async-only wrapper around the
OPC UA file-system primitives defined in **Part 5 §C** and **Part 20 §4** —
the `FileType`, `FileDirectoryType`, and `TemporaryFileTransferType`
ObjectTypes. It is layered on top of the source-generated proxies emitted
into `Opc.Ua.Core` (`FileTypeClient`, `FileDirectoryTypeClient`,
`TemporaryFileTransferTypeClient`) and exposes a surface that closely
mirrors `System.IO.{File, Directory, FileInfo, DirectoryInfo, FileStream}`
to make remote file-system navigation feel like working with a local disk.

## Quick reference

| Type | Mirrors | Purpose |
|---|---|---|
| `FileSystemClient` | `System.IO.Directory` + `System.IO.File` | Top-level entry point. Exposes path-based operations (`GetFileAsync`, `CreateDirectoryAsync`, `DeleteAsync`, `MoveAsync`, `CopyAsync`, `ReadAllBytesAsync`, …). Rooted at any `FileDirectoryType` instance. |
| `UaFileSystemInfo` | `System.IO.FileSystemInfo` | Abstract base for `UaFileInfo` and `UaDirectoryInfo`. Carries the resolved `NodeId`, parent reference, browse name, and canonical path. |
| `UaFileInfo` | `System.IO.FileInfo` | A single file. Lazy-loaded `Size` / `Writable` / `MimeType` / etc. metadata; `OpenAsync` / `OpenReadAsync` / `OpenWriteAsync` returning a `UaFileStream`; bulk `ReadAllBytes` / `WriteAllText` shortcuts. |
| `UaDirectoryInfo` | `System.IO.DirectoryInfo` | A single directory. `EnumerateAsync` / `EnumerateFilesAsync` / `EnumerateDirectoriesAsync` (`IAsyncEnumerable<>`); `CreateSubdirectoryAsync`, `CreateFileAsync`, `DeleteAsync(recursive)`. |
| `UaFileStream` | `System.IO.FileStream` | A `Stream`-derived wrapper around an open server file handle. Async members hit the wire via `FileTypeClient`; sync members forward via `GetAwaiter().GetResult()`. |
| `UaPath` | `System.IO.Path` | Helpers for parsing, formatting, combining, and splitting OPC UA file paths. |
| `FileSystemClientOptions` | n/a | Tuning knobs: `ChunkSize`, `MaxBufferedReadSize`, `PathCacheSize`, type-subtype filters. |
| `TemporaryFileTransferClient` + `UaTemporaryWriteFile` | n/a | Separate surface for Part 5 §C.5 atomic temp-file transfers (`GenerateFileForRead/Write` + `CloseAndCommit`). |

## Getting started

### Open the standard server file system

Many OPC UA servers expose the standard `Server.FileSystem` object
(`NodeId i=16314`):

```csharp
using Opc.Ua.Client.FileSystem;

ISession session = /* an active OPC UA client session */;

var fs = FileSystemClient.OpenServerFileSystem(session);

await foreach (UaFileSystemInfo entry in fs.EnumerateAsync("/"))
{
    Console.WriteLine($"{(entry.IsDirectory ? "DIR " : "FILE")} {entry.FullPath}");
}
```

### Open any `FileDirectoryType` instance

```csharp
NodeId vendorRoot = /* result of a Browse / TranslateBrowsePathsToNodeIds */;
var fs = new FileSystemClient(session, vendorRoot);
```

### Read a file

```csharp
string content = await fs.ReadAllTextAsync("/Reports/2024/summary.json");
```

### Stream a large file

```csharp
await using UaFileStream stream = await fs.OpenReadAsync("/Logs/server.log");
using var reader = new System.IO.StreamReader(stream);
string firstLine = await reader.ReadLineAsync();
```

### Create a directory tree and write a file

```csharp
UaDirectoryInfo dir = await fs.CreateDirectoryAsync("/Uploads/2024-05/test", createIntermediate: true);
await fs.WriteAllBytesAsync($"{dir.FullPath}/payload.bin", payload);
```

### Move and delete

```csharp
UaFileInfo file = await fs.GetFileAsync("/Drafts/v1.txt");
await file.MoveToAsync("/Published/v1.txt");

await fs.DeleteAsync("/Drafts", recursive: true);
```

## Path syntax

Paths use the forward slash `'/'` as the segment separator. Each segment
is parsed by `QualifiedName.Parse(...)`, so the standard
`"<ns>:<name>"` form is supported. Examples:

| Path | Meaning |
|---|---|
| `""` or `"/"` | The root directory |
| `"foo"` | The child `foo` (namespace 0) of the root |
| `"/foo/bar"` | Same as `foo/bar`; leading slash is optional |
| `"1:Reports/1:2024/data.csv"` | Qualified path using namespace index 1 for the first two segments and namespace 0 for `data.csv` |

`UaPath.Combine`, `UaPath.GetDirectoryName`, `UaPath.GetFileName`, and
`UaPath.Normalize` mirror the equivalents on `System.IO.Path`. Canonical
paths returned from `UaFileSystemInfo.FullPath` always include the
namespace prefix when it is non-zero, so siblings with the same `Name`
in different namespaces are never collapsed.

## Path resolution and caching

Path → NodeId resolution uses
`TranslateBrowsePathsToNodeIdsAsync` segment-by-segment, with each step
following `ReferenceTypeIds.HierarchicalReferences` (with subtypes).
`(parent NodeId, browse name) → child NodeId` mappings are cached in a
small LRU controlled by `FileSystemClientOptions.PathCacheSize` (default
`1024`; set to zero to disable). The cache is **best-effort**: when a
resolved entry stops working (`BadNodeIdUnknown` from the server), the
entry is evicted and the path is re-resolved before the error propagates.
After any successful create/delete/move/copy, the path-cache entries
rooted at the affected parent are invalidated to avoid serving stale
NodeIds.

## Type classification (FileType vs FileDirectoryType)

Each child returned by `EnumerateAsync` is classified by its
`HasTypeDefinition` reference. Subtype-aware classification uses
`Session.TypeTree.IsTypeOf(...)` — by default subtypes count, so
`TrustListType`, `AddressSpaceFileType`, `ConfigurationFileType` etc.
appear as files. To restrict enumeration to the exact `FileType` /
`FileDirectoryType` set:

```csharp
var fs = new FileSystemClient(session, ObjectIds.FileSystem,
    new FileSystemClientOptions
    {
        IncludeFileTypeSubtypes = false,
        IncludeFileDirectoryTypeSubtypes = false,
    });
```

## File metadata

`UaFileInfo` exposes the seven well-known `FileType` properties (`Size`,
`Writable`, `UserWritable`, `OpenCount`, `MimeType`,
`MaxByteStringLength`, `LastModifiedTime`). They are populated lazily by
`UaFileInfo.RefreshAsync` (a single batched `Read` against the
properties resolved via `TranslateBrowsePathsToNodeIds`). The optional
properties (`MimeType`, `MaxByteStringLength`, `LastModifiedTime`) are
returned as `null` when the server does not expose them
(`BadNoMatch`, `BadNodeIdUnknown`, or empty target lists are tolerated).

`Writable` / `UserWritable` are advisory: callers should not pre-check
them before opening a file. Rely on the server's `Open` response for the
authoritative answer.

## Streams

`UaFileStream` derives from `System.IO.Stream` and supports both async
(`ReadAsync`, `WriteAsync`, `DisposeAsync`) and sync (`Read`, `Write`,
`Dispose`) members. The sync members forward to the async ones via
`GetAwaiter().GetResult()` — this means they can deadlock on
single-threaded synchronization contexts (e.g. WPF UI thread). Prefer
the async overrides.

Reads and writes are chunked at `FileSystemClientOptions.ChunkSize`,
clamped down to `FileType.MaxByteStringLength` when the server advertises
a smaller maximum. Empty `ByteString` returns are interpreted as EOF;
zero-length reads/writes never hit the wire.

`Position` is tracked locally; the server is informed via
`SetPosition` only when the local cursor diverges from the last
successfully transmitted position. `Length` is tracked locally too —
opened from `FileType.Size` at construction and bumped whenever a write
extends past it. Callers that mutate the underlying file through other
handles (or other clients) should call `UaFileInfo.RefreshAsync()`
before relying on `Length`.

`UaFileStream` is not thread-safe in the sense that multiple threads can
share a single instance: concurrent calls are serialised by an internal
`SemaphoreSlim`, matching `FileStream`'s "not thread-safe but doesn't
corrupt" contract. `DisposeAsync` issues `FileType.Close` exactly once
even when called concurrently.

## Error mapping

OPC UA Bad status codes returned by the server are translated into the
familiar `System.IO` exception family at the public boundary:

| StatusCode | Mapped exception |
|---|---|
| `BadNoMatch`, `BadNodeIdUnknown`, `BadNotFound` | `FileNotFoundException` (files) / `DirectoryNotFoundException` (directories) |
| `BadBrowseNameDuplicated` | `IOException("already exists: …")` |
| `BadUserAccessDenied`, `BadNotWritable`, `BadWriteNotSupported`, `BadSecurityChecksFailed` | `UnauthorizedAccessException` |
| `BadInvalidArgument`, `BadOutOfRange`, `BadInvalidState`, `BadResourceUnavailable`, `BadOutOfMemory` | `IOException` |
| Other `Bad…` codes | `ServiceResultException` (preserved unchanged) |

Mapped exceptions wrap the original `ServiceResultException` as
`InnerException`, so callers that need the raw OPC UA status code can
still retrieve it.

`TranslateBrowsePathsToNodeIds` returning more than one target throws
`IOException("ambiguous path: …")`; the client never silently picks the
first match.

## Move, copy, and delete semantics

These three operations are routed through the **source's parent**
directory (per Part 20 §4.3) — the server's `MoveOrCopy` and `Delete`
methods take a NodeId and operate from the directory's perspective.

`DeleteAsync(recursive: false)` on a directory first enumerates the
directory and throws `IOException("directory not empty: …")` if any
child exists. `DeleteAsync(recursive: true)` invokes the server's
`Delete` exactly once and lets the server perform recursive removal
(per spec). The client never walks the tree itself for a recursive
delete — that would weaken the server's atomicity / locking guarantees
and double-traverse.

`CreateDirectoryAsync` and `CreateFileAsync` accept only a plain
**string** for the leaf name; they reject leaf segments with a namespace
prefix (the server picks the BrowseName namespace). After the server
returns the new NodeId, the client reads its actual `BrowseName` and
uses that for the canonical path / cache entry.

`CreateFileAsync` always passes `requestFileOpen: false` to the server
— we never leak a server-allocated handle out of the create call.
Callers that want an immediate stream should call `UaFileInfo.OpenAsync`
afterwards.

## Temporary file transfer (Part 5 §C.5)

The temporary-file-transfer pattern is exposed on a separate surface
because its lifecycle does not fit the `System.IO` abstraction. The
server allocates a transient file, the client streams data through it,
and a final commit (or rollback) tells the server to either publish or
discard the result.

```csharp
using Opc.Ua.Client.FileSystem;

NodeId transferObject = /* TemporaryFileTransferType instance */;
var temp = new TemporaryFileTransferClient(session, transferObject);

// Read flow
await using UaFileStream readStream = await temp
    .GenerateFileForReadAsync(generateOptions: default);
byte[] payload = ReadAll(readStream);

// Write flow
await using UaTemporaryWriteFile write = await temp.GenerateFileForWriteAsync();
await write.Stream.WriteAsync(payload, 0, payload.Length);
NodeId completion = await write.CommitAsync(); // CloseAndCommit
```

`UaTemporaryWriteFile` owns the close lifecycle: exactly one terminal
call — `CommitAsync` (CloseAndCommit) or `DisposeAsync` (Close,
implicit server rollback) — is sent to the server. The wrapped
`Stream` cannot accidentally close the server handle; its `Dispose`
is a no-op.

## What lives where

```
Libraries/Opc.Ua.Client/FileSystem/
├── FileSystemClient.cs                 # entry point
├── FileSystemClientOptions.cs          # tuning knobs
├── FileMetadata.cs                     # internal property snapshot
├── FileSystemErrors.cs                 # status code → IO exception mapper
├── PathCache.cs                        # internal LRU
├── UaPath.cs                           # System.IO.Path mirror
├── UaFileMode.cs                       # OpenFileMode flags wrapper
├── UaFileSystemInfo.cs                 # abstract base
├── UaFileInfo.cs                       # System.IO.FileInfo mirror
├── UaDirectoryInfo.cs                  # System.IO.DirectoryInfo mirror
├── UaFileStream.cs                     # System.IO.Stream mirror
├── TemporaryFileTransferClient.cs      # Part 5 §C.5 surface
└── UaTemporaryWriteFile.cs             # commit/rollback wrapper
```

Tests live under `Tests/Opc.Ua.Client.Tests/FileSystem/`.

## See also

- [SourceGeneratedDataTypes.md](SourceGeneratedDataTypes.md) — for an
  overview of the generated `*TypeClient` proxies the FileSystem client
  builds on.
- OPC UA Part 5 Annex C (FileType, TemporaryFileTransferType).
- OPC UA Part 20 §4 (FileSystem object model).