Error and Warning Authoring in MSBuild

Error messages are MSBuild's primary user interface. They must help developers fix problems without reading source code.

For the mechanics of error code assignment, see assigning-msb-error-code.md.

Error Message Quality Rules

Every error message must answer three questions:

1. What happened? — State the problem clearly
2. Why? — Provide context (file, property, expected value)
3. What should the user do? — Give actionable guidance

<!-- BAD: What is "it"? What should I do? -->
<value>MSB4999: Invalid configuration.</value>

<!-- GOOD: States the problem, context, and fix -->
<value>MSB4999: The project "{0}" specifies TargetFramework "{1}" which is not installed. Install the SDK for "{1}" or update the TargetFramework in the project file.</value>

Severity Decision Framework

Is the condition always wrong (invalid input, impossible state)?
├── Yes → ERROR (build should fail)
└── No
    ├── Could this cause subtle build correctness issues?
    │   ├── Yes, likely → WARNING (but see WarnAsError impact below)
    │   └── Yes, maybe → MESSAGE at Normal importance
    └── Is this purely informational?
        └── Yes → MESSAGE at Low importance

The WarnAsError Constraint

New warnings are breaking changes for builds using:

• -WarnAsError (CLI)
• <TreatWarningsAsErrors>true</TreatWarningsAsErrors> (project)
• <WarningsAsErrors>MSBxxxx</WarningsAsErrors> (specific codes)

Before adding a new warning, consider:

1. Is the warning important enough to justify breaking WarnAsError builds? If yes, gate behind a ChangeWave (see changewaves skill).
2. Could this be a Message instead? Messages never break builds but still appear in binary logs.
3. Should this be an error from the start? If the condition is always wrong, skip the warning stage.

MSB Error Code Assignment

Code Ranges

Range	Area
MSB1xxx	Command-line handling
MSB3xxx	Tasks (Microsoft.Build.Tasks.Core.dll)
MSB4xxx	Engine (Microsoft.Build.dll)
MSB5xxx	Shared code across assemblies
MSB6xxx	Utilities (Microsoft.Build.Utilities)

Assignment Process

1. Open the Strings.resx file for the appropriate assembly
2. Find the "Next message code" comment at the bottom of the file
3. Search the repo to confirm the code is not already in use
4. Use the code; update the comment to the next available number

See assigning-msb-error-code.md for detailed steps.

Resource String Format

<data name="FeatureArea.DescriptiveName">
  <value>MSBxxxx: Clear description with {0} placeholders for runtime values.</value>
  <comment>{StrBegin="MSBxxxx: "}{0} is the project file path. {1} is the property name.</comment>
</data>

Rules

• Resource name uses FeatureArea.DescriptiveName convention
• Value starts with MSBxxxx: (code, colon, space)
• Comment documents the {StrBegin} marker and explains each {N} placeholder
• Comments guide translators — mention untranslatable tokens

Consuming Error Resources in Code

Use the standard formatting method that extracts and applies the error code:

// For errors
Log.LogErrorWithCodeFromResources("Copy.Error", sourceFile, destFile, ex.Message);

// For warnings
Log.LogWarningWithCodeFromResources("ResolveAssemblyReference.Conflict", assemblyName);

// For engine-level errors (not in tasks)
ProjectFileErrorUtilities.ThrowInvalidProjectFile(
    elementLocation,
    "InvalidProjectFile",
    arg1, arg2);

Never construct error messages by string concatenation — always use resource strings for localization support.

Localization Requirements

• Error codes (MSBxxxx) are never localized
• Error text is localized into all supported languages
• After adding/modifying resource strings, run a full build to generate .xlf placeholder translations
• Use <comment> elements to help translators understand context
• See Localization.md for the full localization process

Path Formatting in Error Messages

• Show paths relative to the project directory when possible
• Use the path exactly as the user specified it (don't normalize slashes)
• Include file and line number via IElementLocation when available — this enables IDE click-to-navigate
• For paths that could contain spaces, ensure they're quoted in the message

Checklist for New Errors/Warnings

• Error code assigned from correct Strings.resx range
• "Next message code" comment updated in the resx
• Message answers: what happened, why, what to do
• {StrBegin} comment and placeholder documentation added
• Severity chosen (error vs warning vs message) with WarnAsError considered
• If warning: ChangeWave gating evaluated
• Full build run to generate .xlf files
• Test verifies the error is produced with correct code and text