MSBuild Item Management Patterns

Canonical patterns for working with item groups, from Microsoft.Common.CurrentVersion.targets.

Include / Remove / Update — Three Operations

| Operation | Purpose | When to use | |---|---|---| | Include | Add new items to the group | Creating items with identity + metadata | | Remove | Remove items matching a pattern | Excluding files or clearing a group | | Update | Modify metadata on existing items | Adding/changing metadata without re-adding |

Include — Add Items

xml
<ItemGroup>
  <Compile Include="Generated\*.cs">
    <AutoGen>true</AutoGen>
  </Compile>
</ItemGroup>

Remove — Subtract Items

xml
<ItemGroup>
  <!-- Remove specific items -->
  <Reference Remove="$(AdditionalExplicitAssemblyReferences)" />

  <!-- Set subtraction: prior minus current -->
  <_CleanOrphanFileWrites Include="@(_CleanPriorFileWrites)"
      Exclude="@(_CleanCurrentFileWrites)" />

  <!-- Clear an entire group -->
  <_Temporary Remove="@(_Temporary)" />
</ItemGroup>

Update — Modify Existing Items

xml
<ItemGroup>
  <EmbeddedResource Update="@(EmbeddedResource)"
      Condition="'%(NuGetPackageId)' == 'Microsoft.CodeAnalysis.Collections'">
    <GenerateSource>true</GenerateSource>
    <ClassName>Microsoft.CodeAnalysis.Collections.SR</ClassName>
  </EmbeddedResource>
</ItemGroup>

Update does not add items — it only modifies items already in the group.

Item Batching — %(Metadata)

When %(Metadata) appears in target attributes or task parameters, MSBuild batches execution per unique metadata value.

Target-level batching (Outputs)

xml
<Target Name="GenerateSatelliteAssemblies"
    Inputs="$(MSBuildAllProjects);@(_SatelliteAssemblyResourceInputs)"
    Outputs="$(IntermediateOutputPath)%(Culture)\$(TargetName).resources.dll">
  <!-- Runs once per unique Culture value -->
</Target>

Task-level batching

xml
<Copy SourceFiles="@(_SourceItems)"
    DestinationFiles="@(_SourceItems->'$(OutDir)%(TargetPath)')">
</Copy>

Per-item filtering with Condition

xml
<ItemGroup>
  <_ResxOutput Include="@(EmbeddedResource->'%(OutputResource)')"
      Condition="'%(EmbeddedResource.WithCulture)' == 'false'" />
</ItemGroup>

Batching rules

• %(Metadata) in Condition or Outputs → target batches per unique value.
• %(Metadata) in task parameters → task batches per unique value.
• Do not mix %() from different item groups in the same expression — this causes a cross-product (see Common Pitfalls).

Item Transforms — @(Item->'expression')

Transforms create new item lists by applying an expression to each item:

xml
<!-- Transform file paths to destinations -->
<Copy SourceFiles="@(IntermediateAssembly)"
    DestinationFiles="@(IntermediateAssembly->'$(OutDir)%(Filename)%(Extension)')"/>

<!-- Transform with separator for display -->
<Message Text="Files: @(Compile->'%(Filename)', ', ')" />

Exclude Pattern — Set Subtraction on Include

xml
<ItemGroup>
  <Compile Include="**\*.cs" Exclude="Generated\**;Tests\**" />
</ItemGroup>

Exclude only works on Include — it cannot be used with Update or Remove.

Conditional Item Inclusion

xml
<!-- Condition on ItemGroup — all or nothing -->
<ItemGroup Condition="'$(NetCoreBuild)' == 'true'">
  <PackageReference Include="System.IO.Pipelines" />
</ItemGroup>

<!-- Condition on individual items -->
<ItemGroup>
  <PackageReference Include="System.IO.Pipelines"
      Condition="'$(NetCoreBuild)' == 'true'" />
</ItemGroup>

PrivateAssets on Tool/Analyzer Packages

xml
<ItemGroup>
  <PackageReference Include="Microsoft.CodeAnalysis.NetAnalyzers" PrivateAssets="all" />
  <PackageReference Include="StyleCop.Analyzers" PrivateAssets="all" />
</ItemGroup>

Common Pitfalls

Cross-product batching

Referencing %(Metadata) from two different item groups creates O(N×M) executions:

xml
<!-- BAD: Cross-product of @(Source) × @(Config) -->
<Exec Command="process %(Source.Identity) with %(Config.Identity)" />

<!-- GOOD: Reference one group via batching, the other via property -->
<Exec Command="process %(Source.Identity) with $(ConfigFile)" />

Generated files in source tree

Write to $(IntermediateOutputPath) (obj/), not the source directory. Source-tree generation pollutes version control and can cause duplicate compilation via globs.

Missing FileWrites

Every file created during a target must be added to @(FileWrites) for dotnet clean support.