Imported from ojrojas/AgentsInstructions (.claude/skills/dotnet-msbuild/skills/extension-points/SKILL.md). Install upstream with npx skills add ojrojas/AgentsInstructions --skill extension-points. Copyright stays with the author (MIT).

MSBuild Extension Points

How the MSBuild pipeline provides hooks for SDKs, NuGet packages, repos, and users to inject custom logic.

CustomBefore / CustomAfter Hooks

Every major .targets file defines import hooks:

<PropertyGroup>
  <CustomBeforeMicrosoftCommonTargets Condition="'$(CustomBeforeMicrosoftCommonTargets)' == ''">
    $(MSBuildExtensionsPath)\v$(MSBuildToolsVersion)\Custom.Before.Microsoft.Common.targets
  </CustomBeforeMicrosoftCommonTargets>
</PropertyGroup>

<Import Project="$(CustomBeforeMicrosoftCommonTargets)"
    Condition="'$(CustomBeforeMicrosoftCommonTargets)' != '' and Exists('$(CustomBeforeMicrosoftCommonTargets)')"/>
<!-- ... core targets ... -->
<Import Project="$(CustomAfterMicrosoftCommonTargets)"
    Condition="'$(CustomAfterMicrosoftCommonTargets)' != '' and Exists('$(CustomAfterMicrosoftCommonTargets)')"/>

Rules

• Default path includes version (v$(MSBuildToolsVersion)) for side-by-side installations.
• Always check Exists(). The file may not be present on every machine.
• Append to the property (don't overwrite) to chain multiple hooks:

<PropertyGroup>
  <CustomBeforeMicrosoftCommonTargets>
    $(CustomBeforeMicrosoftCommonTargets);$(MSBuildThisFileDirectory)MyExtension.targets
  </CustomBeforeMicrosoftCommonTargets>
</PropertyGroup>

Wildcard Import Directories

MSBuild imports all files in extension directories, sorted alphabetically:

<Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Imports\Microsoft.Common.props\ImportBefore\*"
    Condition="'$(ImportByWildcardBeforeMicrosoftCommonProps)' == 'true'
               and Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Imports\Microsoft.Common.props\ImportBefore')" />

Key paths

Name files with numeric prefixes for ordering: 01-first.props, 02-second.props.

Import Gating — Control Properties

Every wildcard import is gated by a boolean property:

<PropertyGroup>
  <ImportByWildcardBeforeMicrosoftCommonProps
      Condition="'$(ImportByWildcardBeforeMicrosoftCommonProps)' == ''">true</ImportByWildcardBeforeMicrosoftCommonProps>
  <ImportDirectoryBuildProps
      Condition="'$(ImportDirectoryBuildProps)' == ''">true</ImportDirectoryBuildProps>
</PropertyGroup>

Available control properties

NuGet Package Build Extension Layout

NuGet packages inject build logic via build/ or buildTransitive/ folders:

MyPackage/
  build/
    MyPackage.props      ← imported via *.props wildcard
    MyPackage.targets    ← imported via *.targets wildcard
  buildTransitive/
    MyPackage.props      ← imported by transitive consumers
    MyPackage.targets

Rules

• File names must match the package ID exactly.
• build/ affects direct consumers only. buildTransitive/ affects the entire dependency chain.
• Props are imported early (before the project), targets are imported late (after the project).

Source Tree vs Packed Layout

When reviewing a NuGet build-extension package, the source layout in the repository can legitimately differ from the packed layout inside the produced .nupkg. This is a common source of false-positive "import points at a missing file" findings.

Three packaging mechanisms reshape the layout at pack time:

1. .nuspec <file src=… target=…> mappings — copy a single source file into multiple per-TFM targets:

   <!-- Source tree has ONE shared file:
          buildTransitive\common\MyAdapter.props
        Pack rewrites it to per-TFM targets inside the .nupkg:
          buildTransitive\net462\MyAdapter.props
          buildTransitive\net8.0\MyAdapter.props
          buildTransitive\net9.0\MyAdapter.props -->
   <files>
     <file src="buildTransitive\common\MyAdapter.props" target="buildTransitive\net462\MyAdapter.props" />
     <file src="buildTransitive\common\MyAdapter.props" target="buildTransitive\net8.0\MyAdapter.props" />
     <file src="buildTransitive\common\MyAdapter.props" target="buildTransitive\net9.0\MyAdapter.props" />
   </files>

   In the <file> element, a target ending in \ is treated as a folder (filename preserved from src); a target ending in a filename renames the file.

2. .csproj <PackagePath> metadata on <None Update=…> or <Content Include=…> items — same effect via SDK pack. Use one item per destination to keep the mapping unambiguous:

   <ItemGroup>
     <None Include="buildTransitive\common\MyAdapter.props" Pack="true" PackagePath="buildTransitive\net8.0\MyAdapter.props" />
     <None Include="buildTransitive\common\MyAdapter.props" Pack="true" PackagePath="buildTransitive\net9.0\MyAdapter.props" />
   </ItemGroup>

   NuGet/SDK pack also accepts a semicolon-separated list (PackagePath="buildTransitive\net8.0\;buildTransitive\net9.0\") to fan one source out to multiple destinations, but the multi-item form above is harder to misread.

3. SDK conventions — IncludeBuildOutput, BuildOutputTargetFolder, IncludeContentInPack automatically place built outputs under lib/<tfm>/ or build/<tfm>/.

Implication for reviewers

A forwarder like the following inside a packed build/net462/ folder is not a "missing-file" bug, even if the source tree has no buildTransitive/net462/ directory:

<!-- In packed build/net462/MyAdapter.props -->
<Project>
  <Import Project="$(MSBuildThisFileDirectory)..\..\buildTransitive\net462\MyAdapter.props" />
</Project>

Before flagging an unguarded <Import> inside a build/<tfm>/ or buildTransitive/<tfm>/ folder:

1. Look for *.nuspec in the project directory and its immediate parent directory (do not walk further up). Read every <file target=…> whose target matches the imported path.
2. Read the .csproj for <PackagePath> metadata on <None>/<Content> items.
3. Only flag the import if the target path is missing from both the source tree and the projected package layout.

See also msbuild-antipatterns AP-13 ("NuGet package forwarders" exception).

Import Guard Pattern

The .targets file ensures .props was imported using a guard property:

<!-- End of Microsoft.Common.props -->
<PropertyGroup>
  <MicrosoftCommonPropsHasBeenImported>true</MicrosoftCommonPropsHasBeenImported>
</PropertyGroup>

<!-- Top of Microsoft.Common.CurrentVersion.targets -->
<Import Project="Microsoft.Common.props"
    Condition="'$(MicrosoftCommonPropsHasBeenImported)' != 'true'" />

This handles projects that only import .targets.

Directory.Build Discovery

MSBuild walks up the directory tree to find the nearest Directory.Build.props:

<_DirectoryBuildPropsBasePath>
  $([MSBuild]::GetDirectoryNameOfFileAbove('$(MSBuildProjectDirectory)', 'Directory.Build.props'))
</_DirectoryBuildPropsBasePath>

Only the nearest file is discovered. Nested hierarchies must explicitly import parents:

<!-- src/Directory.Build.props -->
<PropertyGroup>
  <_ParentPropsPath>$([MSBuild]::GetPathOfFileAbove('Directory.Build.props', '$(MSBuildThisFileDirectory)../'))</_ParentPropsPath>
</PropertyGroup>
<Import Project="$(_ParentPropsPath)" Condition="'$(_ParentPropsPath)' != ''" />

Creating Your Own Extension Point

<!-- MySDK.targets -->
<Project>
  <Import Project="MySDK.props" Condition="'$(MySDKPropsImported)' != 'true'" />

  <PropertyGroup>
    <CustomBeforeMySDK Condition="'$(CustomBeforeMySDK)' == ''">$(MSBuildProjectDirectory)\MySDK.Before.targets</CustomBeforeMySDK>
    <CustomAfterMySDK Condition="'$(CustomAfterMySDK)' == ''">$(MSBuildProjectDirectory)\MySDK.After.targets</CustomAfterMySDK>
  </PropertyGroup>

  <Import Project="$(CustomBeforeMySDK)" Condition="Exists('$(CustomBeforeMySDK)')" />

  <PropertyGroup>
    <MySDKBuildDependsOn>BeforeMySDKBuild;CoreMySDKBuild;AfterMySDKBuild</MySDKBuildDependsOn>
  </PropertyGroup>
  <Target Name="MySDKBuild" DependsOnTargets="$(MySDKBuildDependsOn)" />
  <Target Name="BeforeMySDKBuild" />
  <Target Name="AfterMySDKBuild" />
  <Target Name="CoreMySDKBuild">
    <!-- implementation -->
  </Target>

  <Import Project="$(CustomAfterMySDK)" Condition="Exists('$(CustomAfterMySDK)')" />
</Project>

Common Pitfalls

• Missing Exists() on optional imports causes build failures when files are absent. Exception: imports inside published build/<tfm>/ and buildTransitive/<tfm>/ folders of a NuGet package are a package contract — the target is guaranteed by the packed layout (see "Source Tree vs Packed Layout" above). Don't guard them and don't flag them.
• Overwriting Custom properties* drops prior hooks. Append with ; separator.
• NuGet package file names not matching package ID silently skips the import.
• Nested Directory.Build.props without parent import loses repo-root settings.