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copy-to-output-directory

Choosing an MSBuild CopyToOutputDirectory / CopyToPublishDirectory mode: Never, PreserveNewest, Always, and IfDifferent (MSBuild 17.13+), plus $(SkipUnchangedFilesOnCopyAlways). USE FOR: removing the per-build Always copy perf hit; resetting output files mutated between builds. DO NOT USE FOR: general incremental-build diagnosis (use incremental-build); non-MSBuild build systems.

plugins/dotnet-msbuild/skills/copy-to-output-directory/SKILL.md

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# Choosing a CopyToOutputDirectory Mode

## Overview

The `CopyToOutputDirectory` metadata (and its publish counterpart `CopyToPublishDirectory`) controls whether an item — `Content`, `None`, `EmbeddedResource`, or `Compile` — is copied next to your build output, and under what conditions the copy happens. Picking the wrong mode causes either stale files in `bin/` or an unnecessary per-build performance hit.

As of **MSBuild 17.13 / .NET SDK 9.0.2xx** there are four values:

| Mode | Copies when… | Incremental cost | Typical use |
| --- | --- | --- | --- |
| `Never` (default) | Never | None | Files not needed at runtime |
| `PreserveNewest` | Source is **newer** than destination (or destination missing) | Cheap (timestamp check) | The common case — source files you edit |
| `Always` | **Every build**, unconditionally | Expensive — copies on every build even in no-op builds | Legacy workaround; avoid (see below) |
| `IfDifferent` | Source **differs** from destination in either direction (newer **or** older, or size differs, or destination missing) | Cheap (timestamp + size check) | Destination may be mutated between builds |