Maintaining Binary Log Compatibility

Binary Log Considerations

The binary log (.binlog) is MSBuild's primary diagnostic format and source of truth for build analysis. It captures the complete build event stream with full fidelity. Format changes have strict compatibility requirements.

For general binary log usage, see Binary-Log.md.

Core Principles

1. The binlog captures everything. All meaningful build events must be represented. If your change alters build behavior, it must be observable in the binlog.
2. Format changes must be backward-compatible. Older versions of MSBuild Structured Log Viewer and other tools must be able to read binlogs produced by newer MSBuild, at minimum gracefully degrading.
3. Forward compatibility matters too. Newer viewers should handle binlogs from older MSBuild without crashing, even if some events are unrecognized.

Architecture Overview

Build Engine
  → BuildEventArgs (structured events)
    → BinaryLogger (serializes to .binlog)
      → ProjectImportsCollector (embeds project/targets files)

Replay:
  .binlog file
    → BinaryLogReplayEventSource (deserializes)
      → Any ILogger (console, structured log viewer, analyzers)

Key source files:

• src/Build/Logging/BinaryLogger/BinaryLogger.cs — the logger itself
• src/Framework/BuildEventArgs.cs — base class for all build events
• src/Build/Logging/BinaryLogger/ProjectImportsCollector.cs — captures imported files

Adding New Build Event Types

When adding a new BuildEventArgs subclass:

1. Define the new event class inheriting from the appropriate base (BuildMessageEventArgs, BuildWarningEventArgs, etc.)
2. Add serialization support — implement WriteToStream and CreateFromStream methods
3. Increment the binary log version if the new event type changes the format
4. Add a new record type constant in the binary logger's record type enum
5. Handle the unknown-type case in the replay source — older readers must skip gracefully

Serialization Compatibility Rules

• Never remove fields from existing event args serialization
• New fields must be appended to the end of the serialization stream
• Use version checks when reading — if the binlog version is older, use defaults for new fields
• Nullable fields should serialize a presence flag before the value

// Pattern for backward-compatible field addition
if (logVersion >= newFieldVersion)
{
    writer.Write(newField);
}

// Reading with backward compatibility
if (logVersion >= newFieldVersion)
{
    newField = reader.ReadString();
}
else
{
    newField = defaultValue;
}

Message Importance Levels

Importance controls what appears in console output, but everything goes to binlog regardless of importance.

Rules

• Default to Normal for user-relevant information
• Use Low for information useful when debugging but noisy in normal builds
• High is reserved for important warnings/status — use sparingly
• Never skip logging because "it's too verbose" — log at Low instead

ProjectImportsCollector

The ProjectImportsCollector embeds all imported .props, .targets, and project files into the binlog. This enables the "preprocessed view" in log viewers.

Considerations

• MSBUILDLOGIMPORTS=1 (or the /bl switch) enables import collection
• Imported file content is captured at evaluation time — reflects the actual content used
• Large import chains increase binlog size — this is acceptable for diagnostic completeness
• Sensitive content in imported files will be embedded — document this for users

Changes That Affect Binlog Content

When modifying MSBuild behavior, verify binlog impact:

Testing Binlog Changes

• Round-trip test: Write a binlog, replay it, verify all events are reconstructed
• Version compatibility test: Verify older replay sources handle new events gracefully
• Content verification: Assert specific events appear in the binlog for behavioral changes
• Size regression: Monitor binlog size for unexpectedly large increases
• Use BinaryLogReplayEventSource in tests to verify binlog content programmatically

Checklist

• New event types have WriteToStream/CreateFromStream implementations
• Binary log format version incremented if format changed
• Backward compatibility: older readers skip/degrade gracefully
• Forward compatibility: newer readers handle old format
• Importance levels set correctly for new messages
• Behavioral changes produce observable binlog events
• Round-trip test passes (serialize → deserialize → verify)

Source: dotnet/msbuild — distributed by TomeVault.