fix: Map FileAccessMode to appropriate FileMode in StorageFile.OpenStreamAsync

[Copilot]

Issue

StorageFile.OpenStreamAsync was always using FileMode.Open regardless of the FileAccessMode argument, causing FileNotFoundException when attempting to write to files that don't exist yet.

This issue affected Skia.Desktop (macOS, Linux, Windows) and iOS platforms when using APIs like:

• FileIO.WriteTextAsync() on new files
• OpenStreamForWriteAsync() on files that haven't been created yet

Root Cause

The Local and iOS implementations of OpenStreamAsync were hardcoded to use FileMode.Open:

// Before (incorrect)
File.Open(Path, FileMode.Open, ToFileAccess(accessMode), ToFileShare(options))

FileMode.Open requires the file to exist, so when FileAccessMode.ReadWrite was passed (as happens with OpenStreamForWriteAsync), it would throw FileNotFoundException instead of creating the file.

Solution

Added a ToFileMode helper method that maps FileAccessMode to the appropriate FileMode:

• FileAccessMode.Read → FileMode.Open (requires file to exist)
• FileAccessMode.ReadWrite → FileMode.OpenOrCreate (creates file if it doesn't exist)

Updated both StorageFile.Local.cs and StorageFile.iOS.cs implementations to use this mapping:

// After (correct)
File.Open(Path, ToFileMode(accessMode), ToFileAccess(accessMode), ToFileShare(options))

Changes

• Added: ToFileMode helper method in StorageFile.cs
• Fixed: StorageFile.Local.OpenStreamAsync to respect FileAccessMode
• Fixed: StorageFile.iOS.SecurityScopedFile.OpenStreamAsync to respect FileAccessMode
• Added: Runtime test When_OpenStreamForWriteAsync_On_New_File to verify the fix

Testing

✅ Writing to non-existent files now works correctly (creates the file)
✅ Reading from non-existent files still throws FileNotFoundException as expected
✅ Existing tests continue to pass
✅ Manual verification confirms the fix resolves the reported issue

Closes #XXXXX

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>StorageFile.Local.OpenStreamAsync always use File.Open instead of the FileAccessMode argument</issue_title>
<issue_description>### Current behavior 🐛

FileNotFoundException thrown when trying to write new StorageFile in a Skia.Desktop Mac app.

Expected behavior 🎯

No exception thrown with correct entitlements or choice of output folder

How to reproduce it (as minimally and precisely as possible) 🔬

Demo project: https://github.com/baskren/MacDesktop_StorageFile_Write

Relevant configurations:

entitlements.plist [CLICK HERE TO EXPAND]

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
	<dict>
		<key>com.apple.security.cs.allow-jit</key>
		<true/>
		<key>com.apple.security.files.user-selected.read-write</key>
		<true/>
		<key>com.apple.security.files.downloads</key>
		<true/>
		<key>com.apple.security.files.documents</key>
		<true/>
	</dict>
</plist>

• Tried adding either/or/both of the following to the project's .csproj, in the <Properties> group:

<CodesignEntitlements Condition=" $(TargetFramework.Contains('desktop')) " >Platforms/Desktop/MacOS/entitlements.plist</CodesignEntitlements>

<UnoMacOSEntitlements Condition=" $(TargetFramework.Contains('desktop')) " >Platforms/Desktop/MacOS/entitlements.plist</UnoMacOSEntitlements>

Workaround 🛠️

None known at this time

Renderer 🎨

• Skia
• Native

Affected platforms 📱💻🖥️

Desktop (macOS)

Uno.Sdk version (and other relevant versions) 📦

Uno.Sdk: 6.1.23

UnoFeatures:

• CSharpMarkup;
• Lottie;
• Hosting;
• Toolkit;
• Configuration;
• Localization;
• ThemeService;
• SkiaRenderer;

IDE version 🧑‍💻

Click to expand [Mac Dev Environment]

JetBrains Rider

JetBrains Rider 2025.2
Build #RD-252.23892.524, built on August 12, 2025
Source revision: 18364647ddbb4
Licensed to Ben Askren
Subscription is active until July 10, 2026.
Runtime version: 21.0.7+6-b1038.58 aarch64 (JCEF 122.1.9)
VM: OpenJDK 64-Bit Server VM by JetBrains s.r.o.
Toolkit: sun.lwawt.macosx.LWCToolkit
macOS 15.6
.NET Core v8.0.11 ARM64 (Server GC)
GC: G1 Young Generation, G1 Concurrent GC, G1 Old Generation
Memory: 4096M
Cores: 12
Metal Rendering is ON
Registry:
ide.lookup.shrink=true
ide.experimental.ui=true
llm.show.ai.promotion.window.on.start=false
Non-Bundled Plugins:
org.antlr.intellij.plugin (1.24)
com.intellij.resharper.HeapAllocationsViewer (2025.1.0)
com.intellij.copyright (252.23892.464)
com.redhat.devtools.lsp4ij (0.14.2)
lermitage.intellij.iconviewer (1.31.3)
com.github.camork.fileExpander (2.5)
com.wakatime.intellij.plugin (15.0.4)
com.cppcxy.Intellij-SumnekoLua (3.15.0.46-IDEA243)
com.jetbrains.rider.android (252.23892.524)
unoplatform (1.5.19)
Karma (252.25204.0)
me.rafaelldi.aspire (1.8.8)
com.intellij.resharper.azure (4.5.3)

Anything else we need to know? 💬

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

@spouliot https://github.com/baskren/MacDesktop_StorageFile_Write is a 404. Is it public ? @spouliot Your `entitlements.plist` above does not match the one inside the repo.

However that's not the problem. Entitlements are only needed if/when the app is packaged as an app bundle (and in the case of this entitlement the app would have to be sandboxed to need this).

For a dotnet app the file system is accessible with the same permission that the current user (launching the app) has.

The full exception is:

   at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirError)
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, UnixFileMode openPermissions, Int64& fileLength, UnixFileMode& filePermissions, Boolean failForSymlink, Boolean& wasSymlink, Func`4 createOpenException)
   at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
   at System.IO.File.Open(String path, FileMode mode, FileAccess access, FileShare share)
   at Windows.Storage.StorageFile.Local.OpenStreamAsync(CancellationToken ct, FileAccessMode accessMode, StorageOpenOptions options) in C:\a\1\s\src\Uno.UWP\Storage\StorageFile.Local.cs:line 43
   at Windows.Storage.StorageFile.OpenStream(CancellationToken ct, FileAccessMode accessMode, StorageOpenOption...

</details>

Fixes unoplatform/uno#21311

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[Copilot]

Pull Request Overview

This PR fixes a bug where StorageFile.OpenStreamAsync was always using FileMode.Open regardless of the FileAccessMode parameter, causing FileNotFoundException when attempting to write to non-existent files on Skia.Desktop and iOS platforms.

Key Changes:

• Added ToFileMode helper method to properly map FileAccessMode to FileMode
• Updated OpenStreamAsync implementations in both Local and iOS variants to use the new mapping
• Added runtime test to verify the fix

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/Uno.UWP/Storage/StorageFile.cs	Added ToFileMode helper method to map FileAccessMode to appropriate FileMode
src/Uno.UWP/Storage/StorageFile.Local.cs	Updated OpenStreamAsync to use ToFileMode instead of hardcoded FileMode.Open
src/Uno.UWP/Storage/StorageFile.iOS.cs	Updated iOS implementation of OpenStreamAsync to use ToFileMode
src/Uno.UI.RuntimeTests/Tests/Windows_Storage/Given_StorageFile_Native.base.cs	Added test to verify writing to non-existent files works correctly

[Copilot]

The ToFileMode method should include XML documentation explaining the mapping rationale. Specifically, it should document why FileAccessMode.ReadWrite maps to FileMode.OpenOrCreate (to create files that don't exist) and why FileAccessMode.Read maps to FileMode.Open (to ensure the file exists before reading).