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MSBuild in .NET 11 Preview 1 - Release Notes

MSBuild 18.4 includes numerous performance improvements, bug fixes, new APIs for programmatic integration, and Terminal Logger enhancements. This release spans changes from MSBuild 18.0 through 18.4.

Terminal Logger improvements

Test summary always reported

Fixed an issue where test summaries could be missing from the Terminal Logger output when the terminal node became null before receiving the finish message. Test results are now always accounted for in the final summary. (#12801)

Node status restoration after task yields

Fixed a bug where the Terminal Logger's node status display would not properly restore after an MSBuild task yields execution. (#12820)

Forwarding logger support for API-based builds

People using the MSBuild APIs to execute builds with the Terminal Logger can now benefit from forwarding logger support in multi-node builds. Forwarding loggers are a special kind of MSBuild logger that enable much more performant logging in multi-node builds. As part of this release, we've added new methods to the Terminal Logger's parsing/construction APIs that make it possible to configure the Terminal Logger as a forwarding logger. The command-line experience has always done this, but API-based users, like dotnet run or dotnet watch, were forced to use the much-less efficient approach. (#12827)

MSBuild language and evaluation fixes

Item function chaining with whitespace

Fixed a long-standing parser bug where item function chaining would fail if whitespace preceded the second or subsequent -> operator. Previously, @(I -> WithMetadataValue('M', 'T') -> WithMetadataValue('M', 'T')) (with a space before the second ->) would be treated as a literal string instead of being evaluated for its values. (#12772)

NoWarn honored when MSBuildWarningsAsMessages is set

Fixed a bug where NoWarn was silently ignored when MSBuildWarningsAsMessages was already set. The targets now always append $(NoWarn) to $(MSBuildWarningsAsMessages), ensuring both properties work together as expected. (#12828)

Whitespace-only element evaluation fix

Fixed project evaluation when an XML element contains only whitespace characters, which previously could cause incorrect evaluation behavior. (#11978)

Property function: Regex.Replace fix

Fixed incorrect invocation of Regex.Replace(...) in property function evaluation that could produce wrong results. (#12924)

New APIs and capabilities

BinaryLogger parameter parsing API

Added a new public static method BinaryLogger.ParseParameters(string) that parses binary logger parameter strings into a typed BinaryLoggerParameters object. This enables tools like dotnet-watch to parse /bl command-line options without duplicating MSBuild's internal parsing logic. (#12606)

var parsed = BinaryLogger.ParseParameters("LogFile=build.binlog;ProjectImports=None");
Console.WriteLine(parsed.LogFilePath); // "build.binlog"
Console.WriteLine(parsed.ProjectImportsCollectionMode); // None

System.Uri::EscapeDataString in property functions

System.Uri::EscapeDataString and System.Uri::UnescapeDataString are now available as MSBuild property functions. Previously, URL-encoding strings in MSBuild required workarounds with UriBuilder.Path. (#12572)

<PropertyGroup>
  <QueryParam>hello world &amp; test=123</QueryParam>
  <Encoded>$([System.Uri]::EscapeDataString($(QueryParam)))</Encoded>
  <!-- Result: hello%20world%20%26%20test%3D123 -->
</PropertyGroup>

WriteLinesToFile transactional mode

The WriteLinesToFile task now writes or appends content in a transactional way. New files are first created in a temporary location, then atomically moved to the final location. Appended-to files are atomically appended to using a FileShare.Read handle, so only one process can acquire the write lock at a time. Together, this should prevent errors during parallel builds where multiple processes may attempt to write to the same file concurrently. (#12627)

MSBUILDDEBUGONSTART=3 for task hosts

A new value 3 for the MSBUILDDEBUGONSTART environment variable allows developers to skip the debugger-attach prompt for TaskHost child processes, making it easier to debug the main MSBuild process without interruption from task host prompts. (#12679)

Performance improvements

This release includes numerous performance improvements across the build engine's internals. Many of these optimizations were driven by profiling real-world large builds and addressing specific bottlenecks, and were contributed by the Visual Studio team. We can't thank them enough for their analysis and contributions!

  • ItemDictionary: Replaced LinkedList with a more cache-friendly data structure (#12345)
  • Lookup.Scope: Reimplemented scope tables without ItemDictionary for faster lookups (#12320)
  • RAR metadata copies: Reordered output metadata copy operations for better performance (#12298)
  • WorkUnitResult: Converted from class to struct to reduce allocations (#12403)
  • ProjectMetadataInstance: Avoided unnecessary allocations (#12599)
  • LazyItemEvaluator: Reduced allocations by avoiding enumerator boxing in ProcessMetadataElements (#12908)
  • EndsWith fast path: Added optimized path for EndsWith with StringComparison parameter in WellKnownFunctions (#12401)
  • GetEnumerator optimization: Added ICollection<T> check to avoid unnecessary allocations (#12562)
  • Single capture optimization: Optimized for the common single-capture case in regex operations (#12569)
  • Lookup.ExplicitModifications: Switched to concrete dictionary type to avoid enumerator boxing (#11985)
  • ImmutableDictionary.SetItems: Used builder pattern for more efficient batch updates (#12402)
  • BuildGlobResultFromIncludeItem: Streamlined glob result construction (#12178)
  • RAR node log buffering: Buffered log events from ResolveAssemblyReference nodes to client for reduced overhead (#12558)
  • WriteLinesToFile: Internal improvements to file writing performance (#12707)
  • FileSystems abstraction: Systematically replaced direct BCL file operations with the FileSystems abstraction for consistency and potential I/O optimization (#12602)

Bug fixes

  • DistributedFileLogger: Fixed MSB1025 error when using the -dfl flag (#13040)
  • /getItem and /getTargetResult: Fixed unhandled exception when items contain illegal path characters (#12841)
  • Thread working directory: Saved thread working directory for fallback in Expander and Modifiers to prevent incorrect path resolution (#12875)
  • Assembly resolution: Fixed resolution for .exe files and legacy .NET Framework tasks (CLR35 and CLR2) (#12823)
  • slnf path separators: Fixed path separator handling to support both Unix and Windows style paths in solution filter files (#12730)
  • deps.json determinism: Fixed non-deterministic resources section ordering in some deps.json files (#12811)
  • Copy task case sensitivity: Fixed case sensitivity issue on Unix systems (#12147)
  • Inline task code file resolution: Inline task code files are now resolved relative to the project directory instead of the current working directory (#12687)
  • Preprocess crash: Fixed a crash when using the /preprocess (/pp) option (#12396)
  • Project-to-project batching: Tolerated duplication in project-to-project negotiation batches (#11878)
  • DrainPacketQueue: Fixed thread that could hang after build completes (#12561)
  • Infinite debug loops: Fixed condition that could cause infinite build loops with infinite log file creation when debug path redirected to temp folder (#12540)
  • app.config errors: Now report line numbers in app.config parse errors for easier debugging (#12535)
  • SDK environment variable logging: Environment variable messages are now only logged when values actually differ, reducing noise (#12918)
  • RunningObjectTable errors: Enhanced error handling in RunningObjectTable (#12755)
  • HostObjectException: Added catch for HostObjectException to prevent unhandled crashes (#12829)
  • SDK resolver fallback: Added fallback behavior for Microsoft.DotNet.MsBuildSdkResolver loading in external API scenarios (#12703)
  • XamlPreCompile targets: Removed obsolete GeneratedFilesOutputPath parameter (#12885)
  • Null check: Added null check for environmentVariableProperties to prevent NullReferenceException (#12594)