property-patterns — Claude Code skill: MSBuild properties

---
name: property-patterns
description: "Diagnose and fix concrete MSBuild property defects in projects and existing shared-file hierarchies. USE FOR: conditions, defaults, append versus overwrite, last-write-wins values, OS/TFM checks, portable paths, normalization, and reviews centered on those property defects. Property defects in Directory.Build.* remain in scope, including overwritten values across parent/child imports and conditions that run before TargetFramework is set, even when the fix changes import order or moves a property group to .targets. DO NOT USE FOR: placement-only or import-only requests with no concrete property defect, such as discovering shared files or choosing which file owns a target or customization (use directory-build-organization); item operations; target structure; broad reviews without a concrete property defect; non-MSBuild work."
license: MIT
---

# MSBuild Property Patterns

Canonical property definition and manipulation patterns from the MSBuild repository.

## Conditional Defaults — The Foundational Pattern

Set a property **only if not already set**, allowing callers to override:

```xml
<PropertyGroup>
  <Configuration Condition="'$(Configuration)' == ''">Debug</Configuration>
  <Platform Condition="'$(Platform)' == ''">AnyCPU</Platform>
  <BuildInParallel Condition="'$(BuildInParallel)' == ''">true</BuildInParallel>
</PropertyGroup>
```

### Rules

- Always quote both sides: `'$(Prop)' == ''`
- In `.props`: creates overridable defaults. In `.targets`: creates fallbacks.
- Properties without the condition **cannot be overridden** by earlier imports.

## Nested Conditional Groups

Group related properties under a shared condition:

```xml
<PropertyGroup Condition="$(TargetFramework.StartsWith('net4'))">
  <DefineConstants>$(DefineConstants);FEATURE_APARTMENT_STATE</DefineConstants>
  <DefineConstants>$(DefineConstants);FEATURE_APM</DefineConstants>
  <FeatureAppDomain>true</FeatureAppDomain>
</PropertyGroup>

<PropertyGroup Condition="'$([MSBuild]::GetTargetFrameworkIdentifier('$(TargetFramework)'))' == '.NETCoreApp'">
  <NetCoreBuild>true</NetCoreBuild>
  <DefineConstants>$(DefineConstants);RUNTIME_TYPE_NETCORE</DefineConstants>
</PropertyGroup>
```

Use the outer `Condition` on `PropertyGroup` to avoid repeating the same condition on every property.

> **Warning:** `$(TargetFramework)` is empty in `.props` files for single-targeting projects until the project body is evaluated. Place `TargetFramework`-conditioned property groups in `.targets` files (or the project file itself), where the value is always available.

## Composition — Semicolon Concatenation

Properties that hold lists use semicolons. Always include the existing value when appending:

```xml
<PropertyGroup>
  <DefineConstants>$(DefineConstants);MY_FEATURE</DefineConstants>
  <NoWarn>$(NoWarn);NU5131;IDE0005</NoWarn>
  <LibraryTargetFrameworks>$(FullFrameworkTFM);$(LatestDotNetCoreForMSBuild);netstandard2.0</LibraryTargetFrameworks>
</PropertyGroup>
```

## Path Normalization and Trailing Slashes

```xml
<!-- Ensure trailing slash on directories -->
<PropertyGroup>
  <OutDir Condition="'$(OutDir)' != '' and !HasTrailingSlash('$(OutDir)')">$(OutDir)\</OutDir>
</PropertyGroup>

<!-- Normalize paths for cross-platform -->
<PropertyGroup>
  <TargetRefPath>$([MSBuild]::NormalizePath('$(TargetDir)', 'ref', '$(TargetFileName)'))</TargetRefPath>
</PropertyGroup>

<!-- Make relative path absolute -->
<PropertyGroup>
  <MSBuildProjectExtensionsPath
      Condition="'$([System.IO.Path]::IsPathRooted('$(MSBuildProjectExtensionsPath)'))' == 'false'">
    $([System.IO.Path]::Combine('$(MSBuildProjectDirectory)', '$(MSBuildProjectExtensionsPath)'))
  </MSBuildProjectExtensionsPath>
</PropertyGroup>
```

### Preferred path functions

| Function | Purpose |
|---|---|
| `$([MSBuild]::NormalizePath(...))` | Combine and normalize (cross-platform) |
| `$([System.IO.Path]::Combine(...))` | Combine path segments |
| `$([System.IO.Path]::IsPathRooted(...))` | Check if absolute |
| `HasTrailingSlash(...)` | Check for trailing slash |
| `$([MSBuild]::GetDirectoryNameOfFileAbove(...))` | Walk up directory tree |
| `$(MSBuildThisFileDirectory)` | Directory of current file |

## Target Framework Detection Helpers

```xml
<!-- Get TFM identifier -->
<PropertyGroup Condition="'$([MSBuild]::GetTargetFrameworkIdentifier('$(TargetFramework)'))' == '.NETCoreApp'">
  <NetCoreBuild>true</NetCoreBuild>
</PropertyGroup>

<!-- Check TFM compatibility -->
<PropertyGroup Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net472'))">
  <UseFrozenVersions>true</UseFrozenVersions>
</PropertyGroup>

<!-- OS detection -->
<PropertyGroup Condition="$([MSBuild]::IsOSPlatform('windows'))">
  <DefineConstants>$(DefineConstants);TEST_ISWINDOWS</DefineConstants>
</PropertyGroup>
```

## Guard Properties

Mark that a file has been imported to prevent double-imports:

```xml
<!-- At the end of MySDK.props -->
<PropertyGroup>
  <MySDKPropsImported>true</MySDKPropsImported>
</PropertyGroup>

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

## Feature Gating by MSBuild Version

```xml
<PropertyGroup Condition="$([MSBuild]::AreFeaturesEnabled('17.10'))">
  <UseNewBehavior>true</UseNewBehavior>
</PropertyGroup>
```

## Fallback Chains

Set via primary source first, then fall back:

```xml
<PropertyGroup>
  <TlbExpPath>$([Microsoft.Build.Utilities.ToolLocationHelper]::GetPathToDotNetFrameworkSdkFile('tlbexp.exe'))</TlbExpPath>
  <TlbExpPath Condition="'$(TlbExpPath)' == ''">$(_NetFxToolsDir)TlbExp.exe</TlbExpPath>
</PropertyGroup>
```

## Last Write Wins — Evaluation Order

MSBuild evaluates properties top-to-bottom. The last assignment wins:

```xml
<!-- File 1 (imported first) -->
<MyProp>value1</MyProp>        <!-- set to value1 -->
<!-- File 2 (imported second) -->
<MyProp>value2</MyProp>        <!-- overwritten to value2 -->
<!-- File 3 (imported third) -->
<MyProp Condition="'$(MyProp)' == ''">value3</MyProp>  <!-- NOT set — already value2 -->
```

Properties in `.targets` (imported late) override properties in `.props` (imported early) and the project file.

## Common Pitfalls

- **Unquoted conditions** (`$(X)==true`) fail when the property is empty. Always quote both sides.
- **Overwriting DefineConstants** (`<DefineConstants>MY_CONST</DefineConstants>`) drops all prior constants. Always append with `$(DefineConstants);`.
- **Hardcoded absolute paths** break portability. Use `$(MSBuildThisFileDirectory)` or `$([MSBuild]::NormalizePath(...))`.
- **Missing `Condition` on defaults** makes properties non-overridable. Add `Condition="'$(Prop)' == ''"` for values meant to be defaults.