Fix: Socket.SendFile throws when Blocking=false

[laveeshb]

Hi, I noticed this issue was unassigned and marked "good first issue" so I took a crack at it. Note this is a breaking change — happy to discuss the approach or add breaking change documentation if needed.

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Problem

Socket.SendFile does not validate that the socket is in blocking mode before attempting the operation. This leads to undefined, platform-specific behavior:

• Windows: The operation blocks regardless of the Blocking property setting
• Linux: May send only partial data with no indication of how much was actually transmitted

Solution

Added a validation check at the start of SendFile to throw InvalidOperationException when Blocking == false. This follows the same pattern used by other synchronous socket operations that require blocking mode.

Testing

• Added NonBlocking_ThrowsInvalidOperationException test covering both SendFile overloads
• All 161 existing SendFile tests continue to pass

Breaking Change

This is a breaking change. Code that previously called SendFile on a non-blocking socket will now throw an exception:

• On Linux: This surfaces existing bugs — code was silently losing data
• On Windows: Code that relied on the undocumented behavior of Windows ignoring the Blocking flag will now fail

The previous behavior was undefined and platform-inconsistent, so failing explicitly is preferable to silent misbehavior.

Fixes #47287

[laveeshb]

@dotnet-policy-service agree

[Copilot]

Pull request overview

This PR fixes issue #47287 by adding validation to ensure Socket.SendFile throws InvalidOperationException when called on a non-blocking socket, preventing undefined platform-specific behavior (Windows blocks anyway, Linux may send partial data silently).

Key changes:

• Added explicit Blocking property check in the core SendFile implementation
• Added comprehensive test coverage for both SendFile overloads with non-blocking sockets

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.cs	Added validation check to throw InvalidOperationException when Blocking == false in the core SendFile method that all overloads delegate to
src/libraries/System.Net.Sockets/tests/FunctionalTests/SendFile.cs	Added test to verify InvalidOperationException is thrown for both SendFile overloads when socket is non-blocking

[liveans]

Thanks for the PR, it's LGTM!

[liveans]

/azp list

[liveans]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[liveans]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[laveeshb]

@liveans thanks for the review. For tracking purpose, could you assign the issue #47287 to me...

[liveans]

@liveans thanks for the review. For tracking purpose, could you assign the issue #47287 to me...

Sure, can you comment on the issue for assignment? Your name doesn't appear on the list even with the filter.

[laveeshb]

@liveans thanks for the review. For tracking purpose, could you assign the issue #47287 to me...

Sure, can you comment on the issue for assignment? Your name doesn't appear on the list even with the filter.

done

[laveeshb]

@liveans I see some CI tasks failed although I don't think they are related to my change. Could you advise how to resolve these?

[liveans]

@liveans I see some CI tasks failed although I don't think they are related to my change. Could you advise how to resolve these?

Yes, they are unrelated. Since they are unrelated and referenced with necessary issues, this is not a blocker for us.

CI looks good, merging this, thanks for the contribution!