Catch exception in SerialStream constructor if DTR or RTS is not available (Windows) by jarnedemeulemeester · Pull Request #121468 · dotnet/runtime

jarnedemeulemeester · 2025-11-08T16:18:57Z

Catch IOException in SerialStream constructor when setting DTR or RTS is not possible. This will allow ports that do not support setting DTR or RTS to function normally.

This is the Windows equivalent of #48577.

Fixes #27729.

…fault values Some ports do not support setting DTR or RTS. By catching the exeption when setting the value to their default value we can allow these ports to function.

jarnedemeulemeester · 2025-11-08T16:22:39Z

@dotnet-policy-service agree

huoyaoyuan · 2025-11-08T16:33:58Z

src/libraries/System.IO.Ports/src/System/IO/Ports/SerialStream.Windows.cs

+                try
+                {
+                    DtrEnable = dtrEnable;
+                }
+                catch (IOException) when (dtrEnable == false)


It is not a good approach to throw and immediately catch the exception.

Instead, the setter of DtrEnable/RtsEnable can be refactored into methods of TryEnable, then the callers here can call the TryEnable methods directly and ignores the failure.

Alright, I will try to refactor the setters into methods.

I just took this approach from the unix implementation.
Do you want me to also refactor the unix implementation?

It's up to the area owners for decision. Following existing pattern is also acceptable.

I'll leave it up to the area owners then.

Copilot

Pull Request Overview

This PR adds exception handling in the SerialStream constructor for Windows to gracefully handle IOException when DTR or RTS control signals are not supported by the serial port hardware. This brings Windows implementation in line with the existing Unix behavior.

Key Changes:

Wrapped DtrEnable property assignment in a try-catch block that swallows IOException when setting to false (default value)
Wrapped RTS-related operations (querying and setting RtsEnable) in a try-catch block that swallows IOException when setting to false (default value)
Added explanatory comments matching the Unix implementation's pattern

Copilot · 2025-11-11T14:51:18Z

src/libraries/System.IO.Ports/src/System/IO/Ports/SerialStream.Windows.cs

+                    // An IOException can be thrown when using a port which doesn't implement
+                    // the required termios command for setting DtrEnable, but it still works without setting the value


The comment refers to 'termios command' which is Unix-specific terminology. On Windows, this should refer to Windows API calls like SetCommState or EscapeCommFunction instead. Consider updating to: 'the required Windows API calls for setting DtrEnable'.

Copilot · 2025-11-11T14:51:18Z

src/libraries/System.IO.Ports/src/System/IO/Ports/SerialStream.Windows.cs

+                catch (IOException) when (!rtsEnable)
+                {
+                    // An IOException can be thrown when using a port which doesn't implement
+                    // the required termios command for setting RtsEnable, but it still works without setting the value


The comment refers to 'termios command' which is Unix-specific terminology. On Windows, this should refer to Windows API calls like SetCommState or EscapeCommFunction instead. Consider updating to: 'the required Windows API calls for setting RtsEnable'.

Suggested change

// the required termios command for setting RtsEnable, but it still works without setting the value

// the required Windows API calls (such as SetCommState or EscapeCommFunction) for setting RtsEnable, but it still works without setting the value

see dotnet#121468

Benkol003 · 2025-12-09T00:20:39Z

i was trying this out on a system with COM ports without DTR/DSR, DtrEnable is also set in SerialStream.Dispose() (from port.Close() ) to clear DTR that will throw an exception.
Also I think a more reliable way of doing this would be to check that DTR is actually not supported via the PCF_DTRDSR flag from COMMPROP.dwProvCapabilities / SerialStream._commProp, rather than just catching the exception ?

see dotnet#121468

krwq · 2025-12-11T08:59:15Z

@jarnedemeulemeester @huoyaoyuan @Benkol003 thanks for the engagement here. Current approach given it follows existing patterns is fine:

perf-wise/memory - doesn't matter IMO - serial port operates on fairly low speeds comparing to CPU so IMO this will have very negligible impact
this scenario is fairly uncommon

so I'm OK with this as is but in general I agree with the comments that better approach is to either do feather presence check or detect error on the WinAPI level. I'll leave it for you to decide if you want to pursuit better approach.

krwq

Please write a comment if you'll be pursuing better approach or leaving as is.

…n ctor/dispose (dotnet#121468)

Benkol003 · 2025-12-11T14:03:38Z

See #122454 . I'd also be willing to extend this to check the remaining device capabilities

…n ctor/dispose (dotnet#121468)

krwq · 2026-01-14T09:59:11Z

@jarnedemeulemeester @Benkol003 thank you both for contribution. I just caught up after holidays break. Does it make sense to merge this PR given #122454 seems like an extended version of this? I do prefer capabilities check if that's reliable enough (not talking about single device driver which might report this incorrectly but if most of them does misreport we could do both capability check and try catch). I'd appreciate if @jarnedemeulemeester could test @Benkol003's solution once ready if that's feasible.

Copilot

Pull request overview

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

Copilot · 2026-01-14T10:04:02Z

src/libraries/System.IO.Ports/src/System/IO/Ports/SerialStream.Windows.cs

+                    // An IOException can be thrown when using a port which doesn't implement
+                    // the required termios command for setting RtsEnable, but it still works without setting the value


The comment mentions "termios command" which is Unix-specific terminology. This should reference Windows-specific APIs instead. For Windows, the IOException is thrown when SetCommState or EscapeCommFunction fails for devices that don't support RTS control.

dotnet-policy-service · 2026-01-28T14:49:41Z

This pull request has been automatically marked no-recent-activity because it has not had any activity for 14 days. It will be closed if no further activity occurs within 14 more days. Any new comment (by anyone, not necessarily the author) will remove no-recent-activity.

dotnet-policy-service · 2026-02-11T15:51:31Z

This pull request will now be closed since it had been marked no-recent-activity but received no further activity in the past 14 days. It is still possible to reopen or comment on the pull request, but please note that it will be locked if it remains inactive for another 30 days.

fix: catch exception when setting dtrEnable and rtsEnable to their de…

bb4681b

…fault values Some ports do not support setting DTR or RTS. By catching the exeption when setting the value to their default value we can allow these ports to function.

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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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Benkol003 pushed a commit to Benkol003/runtime that referenced this pull request Dec 5, 2025

fix System.IO.Ports when DTR/RTS not available:

d2332f6

see dotnet#121468

Benkol003 pushed a commit to Benkol003/runtime that referenced this pull request Dec 5, 2025

fix System.IO.Ports when DTR/RTS not available:

5fda1e2

see dotnet#121468

Benkol003 pushed a commit to Benkol003/runtime that referenced this pull request Dec 9, 2025

fix System.IO.Ports when DTR/RTS not available:

f0238eb

see dotnet#121468

krwq approved these changes Dec 11, 2025

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Benkol003 pushed a commit to Benkol003/runtime that referenced this pull request Dec 11, 2025

System.IO.Ports: check device capabilities for DTR/RTS before using i…

ad426af

…n ctor/dispose (dotnet#121468)

Benkol003 pushed a commit to Benkol003/runtime that referenced this pull request Dec 11, 2025

System.IO.Ports: check device capabilities for DTR/RTS before using i…

aa866a4

…n ctor/dispose (dotnet#121468)

Benkol003 added a commit to Benkol003/runtime that referenced this pull request Dec 11, 2025

System.IO.Ports: check device capabilities for DTR/RTS before using i…

b29505d

…n ctor/dispose (dotnet#121468)

Benkol003 mentioned this pull request Dec 11, 2025

System.IO.Ports: check device capabilities for DTR/DSR in SerialStream Constructor/dispose (Windows) #122454

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Benkol003 added a commit to Benkol003/runtime that referenced this pull request Dec 11, 2025

System.IO.Ports: check device capabilities for DTR/RTS before using i…

8229338

…n ctor/dispose (dotnet#121468)

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		// An IOException can be thrown when using a port which doesn't implement
		// the required termios command for setting DtrEnable, but it still works without setting the value

	// the required termios command for setting RtsEnable, but it still works without setting the value
	// the required Windows API calls (such as SetCommState or EscapeCommFunction) for setting RtsEnable, but it still works without setting the value

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