Extension Points

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).

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.
• 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.