Add Process constructor accepting SafeProcessHandle and optional stream handles

[Copilot]

Description

Adds a new public Process constructor that creates instances from user-provided safe handles, enabling advanced interop scenarios where users call CreateProcess (or equivalent) directly but still want Process class features like async stream reading and WaitForExit.

API Addition

public Process(
    SafeProcessHandle processHandle,
    SafeFileHandle? standardInput = null,
    SafeFileHandle? standardOutput = null,
    SafeFileHandle? standardError = null,
    ProcessStartInfo? startInfo = null)

On Windows, standardOutput and standardError handles must be opened for asynchronous I/O (IsAsync == true); an ArgumentException is thrown otherwise.

Changes

• Ref assembly (ref/System.Diagnostics.Process.cs): New constructor signature
• Implementation (Process.cs): Constructor wires up handle, process ID, and wraps optional file handles into StreamWriter/StreamReader using existing OpenStream + encoding logic. Validates that standardOutput/standardError handles are async on Windows.
• Tests (ProcessHandlesTests.Windows.cs):
  • Refactored RunWithInvalidHandles → RunWithHandles parameterized by handle values and bInheritHandles
  • Extracted CreateProcessWithHandles static helper that encapsulates the CreateProcess P/Invoke logic, reused by both RunWithHandles and the new test
  • Added INVALID_HANDLE_VALUE constant to replace magic numbers
  • Added ProcessStartedWithAnonymousPipeHandles_CanCaptureOutput test: creates async anonymous pipes via SafeFileHandle.CreateAnonymousPipe(asyncRead: true), enables inheritance via SetHandleInformation, spawns child process using the shared helper, wraps read handles in new Process ctor, uses ReadAllText() to assert captured stdout/stderr
• Test project (.csproj): Added Interop.HandleInformation.cs common source
• Resources (Strings.resx): Added Argument_HandleNotAsync error message

Usage Example

SafeFileHandle.CreateAnonymousPipe(out SafeFileHandle readHandle, out SafeFileHandle writeHandle, asyncRead: true);
// ... CreateProcess with writeHandle as hStdOutput ...

using Process process = new(safeProcessHandle, standardOutput: readHandle);
(string stdout, string stderr) = process.ReadAllText();

[adamsitnik]

@copilot address my feedback

[dotnet-policy-service]

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

[Copilot]

@copilot address my feedback

Done in a53a1fc. All three items addressed:

• Simplified _startInfo assignment (removed unnecessary null check)
• Used INVALID_HANDLE_VALUE constant instead of (nint)(-1) magic numbers
• Extracted CreateProcessWithHandles helper that's reused by both RunWithHandles and the new test

[adamsitnik]

@copilot address my feedback

[Copilot]

@copilot address my feedback

Done in a6488fc. Changes:

• Added asyncRead: true to CreateAnonymousPipe calls
• Replaced ReadToEnd with process.ReadAllText()
• Added IsAsync validation for standardOutput/standardError on Windows (throws ArgumentException if not async)