Make SdkResolver-provided environment variables take precedence over ambient environment

[Copilot]

Context

The SdkResolver system allows SDK resolvers to provide environment variables to the MSBuild engine when an SDK is resolved. These variables were previously overridden by ambient environment variables from the host process, preventing the resolver from being the source of truth. This PR fixes that precedence order.

Changes Made

ProjectInstance.cs — AddSdkResolvedEnvironmentVariable:

• Removed the early return that prevented SDK env vars from overriding ambient env vars
• SDK values now remove the conflicting ambient env var from _environmentVariableProperties before setting the new value
• Added a LogIfValueDiffers private helper (matching main's pattern) that logs the old and new values only when they differ
• Logging for the override case now fires before removal so the existing value is correctly captured
• Added null parameter validation via ErrorUtilities.VerifyThrowArgumentLength(name) and ErrorUtilities.VerifyThrowArgumentNull(value)

Project.cs — AddSdkResolvedEnvironmentVariable:

• Removed the early return that prevented SDK env vars from overriding ambient env vars
• SDK values now always call SetProperty to override ambient values
• Added null parameter validation via ErrorUtilities.VerifyThrowArgumentLength(name) and ErrorUtilities.VerifyThrowArgumentNull(value)

SdkResult.cs:

• Fixed a serialization bug where _environmentVariablesToAdd was not included in Translate, breaking out-of-proc scenarios
• Updated Equals and GetHashCode to include _environmentVariablesToAdd

Strings.resx and XLF files:

• Updated SdkEnvironmentVariableAlreadySet and SdkEnvironmentVariableAlreadySetBySdk to 3-arg format ({0} = name, {1} = new value, {2} = existing value)
• Added SdkEnvironmentVariableOverridingAmbient with 3-arg format for diagnostics when an SDK overrides an ambient value
• Removed the unused SdkEnvironmentVariableSet string

Testing

Two new tests were added in SdkResultEvaluation_Tests.cs:

• SdkResolverEnvironmentVariablesOverrideAmbientEnvironmentVariables — verifies that an SDK-resolved env var takes precedence over an ambient env var with the same name
• FirstSdkEnvironmentVariableWinsOverSubsequentSdks — verifies that among multiple SDKs, the first one to set a given variable wins

All 14 SdkResultEvaluation tests pass.

Notes

New precedence order (highest to lowest):

1. Properties defined in XML
2. SDK-resolved environment variables (first SDK wins among multiple)
3. Ambient environment variables (now overridden by SDK values)

Null name or value parameters to AddSdkResolvedEnvironmentVariable are rejected early with appropriate exceptions, matching the validation pattern used elsewhere in MSBuild property-setting code.

Original prompt

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This section details on the original issue you should resolve

<issue_title>SdkResolver-provided env vars should take precedence over the ambient environment</issue_title>
<issue_description>### Summary

Today the SdkResolver system can provide environment variables to the engine when an SDK is resolved. The primary usage of this feature is

• to keep invocations of the SDK unified across MSBuild-driven tooling via the DOTNET_HOST_PATH environment variable
• to drive usage of the .NET TaskHost feature for spawning .NET-targeted MSBuild tasks in a .NET TaskHost from a .NET Framework central node

However, the variables provided by this mechanism are currently overridden by ambient environment variables taken from the snapshot of the host process at the time the build is started. This means that external processes can override the SdkResolver's decisions.

Background and Motivation

Internal users are having some churn in mixed VS installs and need to set DOTNET_HOST_PATH in older VS environments to mitigate that churn, but want to defer to this feature in dev18 onwards by default.

Proposed Feature

We want the resolver to be the complete source of truth, so we should change the way the SdkResolver env vars are applied to the overall environmental state. The SdkResolver env vars should override any ambient env vars.

We need to make this change, update the existing tests, and add new tests pinning this behavior.

Alternative Designs

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #12654

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[dotnet-policy-service]

Hello @@copilot, I noticed that you’re changing an .swr file or any file under src/Package/MSBuild.VSSetup.. Please make sure to validate this change by an experimental VS insertion. This is accomplished by pushing to an exp/* branch, which requires write permissions to this repo.

[baronfel]

The package vuln thing that's plaguing other builds is hitting this one too, so it's hard to tell if this change actually passes tests.

[Copilot]

Pull Request Overview

This PR fixes the precedence order for SDK resolver environment variables, making them override ambient environment variables from the host process as originally intended, rather than being overridden by them.

Key Changes:

• Modified ProjectInstance.cs and Project.cs to allow SDK-resolved environment variables to override ambient environment variables
• Fixed a serialization bug in SdkResult.cs where _environmentVariablesToAdd wasn't being translated for out-of-proc scenarios
• Added logging when SDK variables override ambient ones through a new resource string SdkEnvironmentVariableOverridingAmbient

Reviewed Changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
ProjectInstance.cs	Removed check preventing SDK env vars from overriding ambient vars; added logic to remove conflicting ambient vars and log overrides
Project.cs	Removed check preventing SDK env vars from overriding ambient vars; SDK values now always override ambient values
SdkResult.cs	Fixed serialization bug by adding translation for _environmentVariablesToAdd; updated Equals and GetHashCode to include the field
Strings.resx	Added new log message resource string for SDK variable overrides
Strings.*.xlf	Added translations (marked as "new") for the new log message across all localization files
SdkResultEvaluation_Tests.cs	Added two tests verifying SDK env vars override ambient vars and first SDK wins when multiple SDKs set the same variable

[rainersigwald]

@copilot retarget this PR to main.

[Copilot]

@copilot retarget this PR to main.

I've improved the implementation (commit 79626cc) to align with main's approach:

• Added LogIfValueDiffers private helper (matching main's pattern) for consistent diagnostic logging
• Updated logging to capture the old ambient value before removal, so it appears in the override message
• Updated resource strings to include the existing value ({2} placeholder) in all SDK environment variable messages
• Removed the unused SdkEnvironmentVariableSet string

Regarding retargeting to main: the branch is currently based on vs18.0. I completed a local rebase onto main but the push tooling requires force-push which isn't available through the PR workflow. The PR can be retargeted to main by either:

1. Having someone with direct push access force-push the rebased branch
2. Using the GitHub PR UI to change the base branch and squash-merging

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