Add Process.Start callback overload with ref struct ProcessStartArguments by Copilot · Pull Request #128862 · dotnet/runtime

Copilot · 2026-06-01T17:35:46Z

main PR

Description

Adds Process.Start(ProcessStartInfo, Func<ProcessStartArguments, SafeProcessHandle>) to let callers create processes with custom platform APIs while still using Process infrastructure (ReadAllText, async stream handling, WaitForExit, etc.).

This PR also incorporates review feedback across the callback path and tests, including:

safer callback start-path handling on Unix and Windows
callback test coverage for both stdout and stderr content
Unix callback test gating for supported platforms
API/doc updates to match the finalized callback argument shape
Windows callback stderr assertion updated to match actual cmd /c echo ... 1>&2 output formatting ("error \r\n" with trailing space before \r\n)

Most notably, callback executable-path data and callback argument lifetime behavior were updated based on feedback:

ProcessStartArguments.FileName was replaced with Unix-only ProcessStartArguments.ResolvedPath
ResolvedPath is now byte* and annotated with [UnsupportedOSPlatform("windows")]
Unix callback startup now prepares a UTF-8 null-terminated path pointer without managed byte[] allocation
Unix callback test now consumes args.ResolvedPath directly for posix_spawn
ProcessStartArguments is now a ref struct to constrain usage to callback lifetime by construction

Additional Unix callback-path update from feedback:

resolved-path stack buffer sizing is now debug/release split (2 in DEBUG, 256 otherwise) to exercise both allocation paths in debug test runs
stack allocation now uses a const-size stackalloc buffer and slices to the required length

Customer Impact

Developers who need unsupported/native process creation mechanisms (for example custom CreateProcess*/posix_spawn flows) can still rely on .NET Process features instead of reimplementing process I/O and lifetime handling themselves.

Regression

No known product regression is being fixed. This is a new callback-based API path plus follow-up correctness and API-shape refinements from review feedback.

Testing

Validated with targeted builds in this repo:

./dotnet.sh build src/libraries/System.Diagnostics.Process/src/System.Diagnostics.Process.csproj -c Release -v minimal /p:BuildProjectReferences=false
./dotnet.sh build src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.csproj -c Release -v minimal /p:BuildProjectReferences=false

System.Diagnostics.Process.Tests full project build remains blocked in this environment by existing shared-framework prerequisite errors (eng/targetingpacks.targets), unrelated to these code changes.

Risk

Moderate. This change adds new public API surface and adjusts callback argument shape on Unix (ResolvedPath pointer) and callback-lifetime constraints (ProcessStartArguments as ref struct).
Risk is mitigated by:

focused changes in callback-specific paths
explicit API/ref/doc updates to keep surface consistent
targeted build validation and callback test updates

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

Introduces a new Process.Start(ProcessStartInfo, Func<ProcessStartArguments, SafeProcessHandle>) overload that allows users to create processes using their own system calls while leveraging .NET's Process infrastructure (pipe management, async I/O, WaitForExit). - Added ProcessStartArguments class with platform-specific pointer properties - Added Windows implementation using BuildCommandLine/GetEnvironmentVariablesBlock - Added Unix implementation using ResolvePath/ParseArgv/AllocArgvArray/AllocEnvpArray - Updated ref assembly with new public API surface - Added Windows test using CreateProcess inside the callback - Added Unix test using posix_spawn inside the callback Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

dotnet-policy-service · 2026-06-01T18:28:58Z

Tagging subscribers to this area: @dotnet/area-system-diagnostics-process
See info in area-owners.md if you want to be subscribed.

- reduce code duplication - acquire the locks! - fix Windows tests: enable inheritance - fix Unix implementation: handle "usesTerminal"

Copilot

Pull request overview

This PR adds a new public Process.Start overload that prepares stdio handles + argument/environment buffers and then delegates the actual OS process creation to a user callback, returning a Process wired up to existing Process infrastructure (streams, WaitForExit, etc.).

Changes:

Adds new public API surface: Process.Start(ProcessStartInfo, Func<ProcessStartArguments, SafeProcessHandle>) and ProcessStartArguments.
Extends Windows/Unix start paths and SafeProcessHandle to support invoking a callback-based start flow.
Adds Windows + Unix tests exercising callback-based process creation and output redirection.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
src/libraries/System.Diagnostics.Process/tests/ProcessHandlesTests.Windows.cs	Adds a Windows test using `CreateProcess` inside the callback to validate stdout redirection.
src/libraries/System.Diagnostics.Process/tests/ProcessHandlesTests.Unix.cs	Adds a Unix test using `posix_spawn` inside the callback to validate stdout redirection.
src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessStartArguments.cs	Introduces the new argument carrier type passed to the callback.
src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.Windows.cs	Threads optional callback through the Windows `StartCore` flow.
src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.Unix.cs	Threads optional callback through the Unix `StartCore` flow and updates wait-state ownership logic.
src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.cs	Adds the new public `Process.Start` callback overload and refactors internal start flow.
src/libraries/System.Diagnostics.Process/src/System.Diagnostics.Process.csproj	Adds the new `ProcessStartArguments.cs` to compilation.
src/libraries/System.Diagnostics.Process/src/Resources/Strings.resx	Adds a resource string for invalid callback handle errors.
src/libraries/System.Diagnostics.Process/src/Microsoft/Win32/SafeHandles/SafeProcessHandle.Windows.cs	Adds Windows callback start helper that builds/pins command line + env block and enables handle inheritance.
src/libraries/System.Diagnostics.Process/src/Microsoft/Win32/SafeHandles/SafeProcessHandle.Unix.cs	Adds Unix callback start helper that allocates argv/envp and integrates wait-state registration.
src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.csproj	Enables unsafe blocks for the ref assembly build due to pointer-typed API.
src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.cs	Updates ref surface with new public `Process.Start` overload and `ProcessStartArguments`.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.ForkAndExecProcess.cs	Promotes argv/envp allocation helpers to `internal` for reuse.

adamsitnik

@copilot please address my feedback

…local const, redirect stderr Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

adamsitnik

@copilot address my feedback

adamsitnik · 2026-06-02T18:06:27Z

+                        byte[] pathBytes = System.Text.Encoding.UTF8.GetBytes(args.FileName! + '\0');
+                        fixed (byte* pathPtr = pathBytes)


The users don't need to perform any kind of encoding/decoding for other parameters like args or env vars, they should not need to do it for FileName as well.

@copilot Since FileName is used only on Unix, we should do the following:

rename FileName to ResolvedPath

change the type of the property from string? to byte*

annotate the property with [UnsupportedOSPlatform("windows")]

update SafeProcessHandle.Unix.cs to provide us with pinned byte*, also ensure the encoding is done without allocating any managed memory

Addressed in 3079bdd. ProcessStartArguments.FileName is now ResolvedPath (byte*, [UnsupportedOSPlatform("windows")]), Unix start now provides the UTF-8 null-terminated pointer without managed byte[] allocation, and the Unix callback test now uses args.ResolvedPath directly.

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

jkotas · 2026-06-02T18:52:35Z

        public bool StandardError { get { throw null; } }
        public override string ToString() { throw null; }
    }
+    public sealed partial class ProcessStartArguments


Something to consider: Should this be ref struct to limit the lifetime to the callback by construction?

Sounds very reasonable. But this would force us to introduce a dedicated delegate type?

Yes - if we wanted to pass the structs as in.

@copilot please give this idea a try (make it ref struct, but don't introduce a dedicated delegate if you don't have to), but separate it from other changes (use dedicated commit)

Done in d30a6f62: ProcessStartArguments is now a ref struct, and I kept this as a dedicated commit separate from other changes.

adamsitnik

@copilot please address my feedback

adamsitnik · 2026-06-03T10:56:02Z

        public bool StandardError { get { throw null; } }
        public override string ToString() { throw null; }
    }
+    public sealed partial class ProcessStartArguments


@copilot please give this idea a try (make it ref struct, but don't introduce a dedicated delegate if you don't have to), but separate it from other changes (use dedicated commit)

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot · 2026-06-03T11:04:41Z

@copilot please address my feedback

Addressed in two commits: 26a38000 (Unix resolved-path buffer feedback) and d30a6f62 (ProcessStartArguments as ref struct in a dedicated commit).

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI and others added 2 commits June 1, 2026 17:33

Fix CreateProcess environment parameter cast in Windows test

59234fd

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI review requested due to automatic review settings June 1, 2026 17:35

Copilot AI assigned Copilot and adamsitnik Jun 1, 2026

Copilot AI review requested due to automatic review settings June 1, 2026 17:35

Copilot created this pull request from a session on behalf of adamsitnik June 1, 2026 17:36 View session

Copilot finished work on behalf of adamsitnik June 1, 2026 17:36

Copilot AI requested a review from adamsitnik June 1, 2026 17:36

adamsitnik mentioned this pull request Jun 1, 2026

[API Proposal]: Process constructor from SafeProcessHandle #71595

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github-actions Bot added the area-System.Diagnostics.Process label Jun 1, 2026

This was referenced Jun 1, 2026

"We stopped hearing from agent Azure Pipelines 32. Verify the agent machine is running and has a healthy network connection" dotnet/dnceng#1886

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TestNativeDigits fails for ur-IN on Apple platforms (xunit v3 exposed previously non-running test) #125933

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Copilot AI review requested due to automatic review settings June 2, 2026 10:21

Copilot started reviewing on behalf of adamsitnik June 2, 2026 10:21 View session

address my own feedback:

3163021

- reduce code duplication - acquire the locks! - fix Windows tests: enable inheritance - fix Unix implementation: handle "usesTerminal"

adamsitnik force-pushed the copilot/add-process-start-overload branch from 638b886 to 3163021 Compare June 2, 2026 10:33

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Copilot AI and others added 2 commits June 2, 2026 11:01

Address @adamsitnik test feedback: use ReadAllText, Redirect helper, …

ba27166

…local const, redirect stderr Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Preserve size rationale comment alongside local const

9c12e8e

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot June 2, 2026 11:03

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Copilot AI requested a review from adamsitnik June 2, 2026 11:04

Apply suggestions from code review

8b593b3

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings June 2, 2026 14:43

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Copilot AI requested review from Copilot and removed request for Copilot June 2, 2026 17:59

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Copilot started work on behalf of adamsitnik June 2, 2026 18:08 View session

Update callback path to expose Unix resolved executable pointer

3079bdd

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot June 2, 2026 18:43

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jkotas reviewed Jun 2, 2026

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Comment thread src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.cs

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slow macOS - "##[error]The job running on agent Azure Pipelines 9 ran longer than the maximum time of 60 minutes." dotnet/dnceng#1883

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The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008

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Comment thread src/libraries/System.Diagnostics.Process/tests/ProcessHandlesTests.Windows.cs Outdated

Copilot AI and others added 2 commits June 3, 2026 11:03

Adjust Unix resolved path buffer strategy

26a3800

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Make ProcessStartArguments a ref struct

d30a6f6

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot June 3, 2026 11:04

Copilot AI changed the title ~~Add Process.Start callback overload with ProcessStartArguments~~ Add Process.Start callback overload with ref struct ProcessStartArguments Jun 3, 2026

Copilot finished work on behalf of adamsitnik June 3, 2026 11:05

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Fix Windows callback stderr assertion spacing

39ef8d1

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot June 3, 2026 11:11

Copilot finished work on behalf of adamsitnik June 3, 2026 11:12

		byte[] pathBytes = System.Text.Encoding.UTF8.GetBytes(args.FileName! + '\0');
		fixed (byte* pathPtr = pathBytes)

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Description

Customer Impact

Regression

Testing

Risk

Package authoring no longer needed in .NET 9

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dotnet-policy-service Bot commented Jun 1, 2026

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Pull request overview

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Copilot AI commented Jun 1, 2026 •

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