Introduce app context switch for setting MSF=true by default by apoorvdeshmukh · Pull Request #3841 · dotnet/SqlClient

apoorvdeshmukh · 2025-12-11T12:43:33Z

Description

This PR introduces new app context switch Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault to set MultiSubnetFailover=true by default in connection string.

Usage

In application

AppContext.SetSwitch("Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault", true);

Or In config file runtimeconfig.json

{
  "configProperties": {
    "Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault": true
  }
}

Or in App.Config

<runtime>
  <AppContextSwitchOverrides value="Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault=true" />
</runtime>

Issues

AB#40809

Testing

Enhanced LocalAppContextSwitchesTest.cs to test new app context switch.
Added unit tests in SqlConnectionStringTest.cs

Guidelines

Please review the contribution guidelines before submitting a pull request:

Copilot

Pull request overview

This PR introduces a new app context switch Switch.Microsoft.Data.SqlClient.EnableMSFByDefaultInConnString that allows applications to change the default value of the MultiSubnetFailover connection string property from false to true. This enables parallel IP connection attempts by default in multi-subnet environments, which can improve connection times for Always On availability groups.

Key changes:

Implements the app context switch following established patterns in the codebase
Changes DbConnectionStringDefaults.MultiSubnetFailover from a const to a property that reads the switch value
Adds comprehensive unit tests covering various scenarios including explicit overrides and FailoverPartner validation
Properly integrates with the existing test infrastructure for app context switch testing

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
`src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs`	Adds the new app context switch constant, static field, and public property with XML documentation
`src/Microsoft.Data.SqlClient/src/Microsoft/Data/Common/ConnectionString/DbConnectionStringDefaults.cs`	Changes MultiSubnetFailover from const to property that reads from the app context switch
`src/Microsoft.Data.SqlClient/tests/Common/LocalAppContextSwitchesHelper.cs`	Adds test helper infrastructure to manipulate and restore the switch state for testing
`src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/SqlConnectionStringTest.cs`	Adds comprehensive unit tests for the new switch behavior including edge cases
`src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/LocalAppContextSwitchesTest.cs`	Updates default value assertion test to include the new switch

src/Microsoft.Data.SqlClient/tests/Common/LocalAppContextSwitchesHelper.cs

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs

...Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/SqlConnectionStringTest.cs

...soft.Data.SqlClient/src/Microsoft/Data/Common/ConnectionString/DbConnectionStringDefaults.cs

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs

…by default

… by default

…by default (#3851) This commit introduces app context switch for setting MultiSubnetFailover=true by default in connection string when the app context switch is set to true. The default value of the context switch is false.

… by default (#3852) This commit introduces app context switch EnableMultiSubnetFailoverByDefault to set MultiSubnetFailover=true in connection string by default when the app context switch is set to true. The default value of this context switch is set to false.

Introduce app context switch for setting MSF=true by default

d441efa

apoorvdeshmukh requested a review from a team as a code owner December 11, 2025 12:43

Copilot AI review requested due to automatic review settings December 11, 2025 12:43

Copilot started reviewing on behalf of apoorvdeshmukh December 11, 2025 12:44 View session

Copilot AI reviewed Dec 11, 2025

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src/Microsoft.Data.SqlClient/tests/Common/LocalAppContextSwitchesHelper.cs Outdated Show resolved Hide resolved

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs Outdated Show resolved Hide resolved

Remove extra space

985e10c

paulmedynski self-assigned this Dec 11, 2025

paulmedynski requested changes Dec 11, 2025

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cheenamalhotra added the Hotfix Candidate 🚑 Issues/PRs that are candidate for backporting to earlier supported versions. label Dec 11, 2025

Address review comments

42ffb7f

Copilot AI review requested due to automatic review settings December 11, 2025 18:26

Copilot started reviewing on behalf of apoorvdeshmukh December 11, 2025 18:27 View session

Copilot AI reviewed Dec 11, 2025

View reviewed changes

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs Show resolved Hide resolved

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs Show resolved Hide resolved

apoorvdeshmukh requested a review from paulmedynski December 11, 2025 18:32

paulmedynski approved these changes Dec 11, 2025

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src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs Show resolved Hide resolved

mdaigle approved these changes Dec 11, 2025

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apoorvdeshmukh merged commit deb8c31 into main Dec 12, 2025
256 checks passed

apoorvdeshmukh deleted the dev/ad/acs-msf branch December 12, 2025 10:01

apoorvdeshmukh added a commit that referenced this pull request Dec 15, 2025

Port #3841 to 6.1: Introduce app context switch for setting MSF=true …

5572da6

…by default

apoorvdeshmukh mentioned this pull request Dec 15, 2025

Port #3841 to 6.1: Introduce app context switch for setting MSF=true #3851

Merged

apoorvdeshmukh added a commit that referenced this pull request Dec 15, 2025

Port #3841 to 6.1: Introduce app context switch for setting MSF=true …

99d6fa3

…by default

apoorvdeshmukh added a commit that referenced this pull request Dec 15, 2025

Port #3841 to 6.0 : Introduce app context switch for setting MSF=true…

a00f7fb

… by default

apoorvdeshmukh mentioned this pull request Dec 15, 2025

[6.0] Port #3841 Introduce app context switch for setting MSF=true by default #3852

Merged

apoorvdeshmukh modified the milestone: 5.1.9 Dec 16, 2025

paulmedynski added this to the 7.0.0-preview4 milestone Feb 11, 2026

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Introduce app context switch for setting MSF=true by default#3841

Introduce app context switch for setting MSF=true by default#3841
apoorvdeshmukh merged 3 commits intomainfrom
dev/ad/acs-msf

apoorvdeshmukh commented Dec 11, 2025 •

edited

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apoorvdeshmukh commented Dec 11, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Description

Usage

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Testing

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apoorvdeshmukh commented Dec 11, 2025 •

edited

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