/extension-points

name: extension-points
description: "Own MSBuild import and hook discovery. USE FOR: CustomBefore/CustomAfter hooks, ordered wildcard and NuGet auto-imports, control properties, build/buildTransitive packed layout, package ID and file-name matching, per-TFM forwarders, and tracing why assets or hooks are missing, broken, or replaced. An import guard remains in scope when it is one defect in a broader hook/import flow. DO NOT USE for a focused safety verdict on whether one specific Import needs Exists or is an intentionally unguarded package contract; use msbuild-antipatterns. NEVER INVOKE when imports and hook placement already work and the request is only target Inputs/Outputs, incremental skipping, or FileWrites clean tracking; use incremental-build or target-authoring. Exclude props-versus-targets placement and non-MSBuild systems."
license: 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

| Property | Resolves to | Scope | |---|---|---| | `$(MSBuildUserExtensionsPath)` | `%APPDATA%\Microsoft\MSBuild` | Per-user | | `$(MSBuildExtensionsPath)` | MSBuild install directory | Machine-wide | | `$(MSBuildProjectExtensionsPath)` | `obj/` directory | Per-project (NuGet) |

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

| Property | What it disables | |---|---| | `ImportDirectoryBuildProps` | Directory.Build.props auto-discovery | | `ImportDirectoryBuildTargets` | Directory.Build.targets auto-discovery | | `ImportProjectExtensionProps` | NuGet-generated `*.props` in obj/ | | `ImportProjectExtensionTargets` | NuGet-generated `*.targets` in obj/ | | `ImportByWildcardBefore*` | Machine-level ImportBefore extensions | | `ImportByWildcardAfter*` | Machine-level ImportAfter extensions |

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).

Forwarding chain: `buildTransitive/` → `build/` → shared

Forward `buildTransitive/*.props` and `buildTransitive/*.targets` through their sibling `build/*.props` / `build/*.targets` files (chain `buildTransitive → build → shared`) instead of importing `buildMultiTargeting/` directly. This keeps `build/` as the single source of truth with a clear ownership chain, so transitive consumers stay in sync with direct consumers instead of the two layouts drifting apart.

When `build/` is packed **per-TFM** (`build/<tfm>/`, via `TfmSpecificPackageFile`, a per-TFM `<PackagePath>`, or SDK conventions) while `buildMultiTargeting/` is not, a `buildTransitive/<tfm>/` forwarder **must include the TFM segment** — dropping it resolves to a non-existent package-root `build/MyPackage.props` and fails transitive consumers with **`MSB4019`**. Derive the segment from the file's own folder, never `$(TargetFramework)` (NuGet nearest-match can serve a `net10.0` consumer the `net9.0` folder, so `$(TargetFramework)` may name a folder that was never restored):

<!-- buildTransitive/<tfm>/MyPackage.props -->
<Import Project="$(MSBuildThisFileDirectory)..\..\build\$([System.IO.Path]::GetFileName($([System.IO.Path]::GetDirectoryName('$(MSBuildThisFileDirectory)'))))\MyPackage.props" />

Source Tree vs Packed Layout

When reviewing a NuGet build-extension package, the **source layout** in the repository can legitimately