building-projects

name	building-projects
description	Build tool selection and orchestration for .NET projects during modernization upgrades. Use this skill whenever the agent modifies code files, project files, packages, or configuration and needs to validate the build — which is virtually every upgrade task. Covers: choosing between `dotnet build` and `msbuild.exe`, restore strategy, targeted vs solution builds, resolving MSBuild errors (MSB3086, MSB4019, NETSDK1005, etc.), handling multi-targeting scenarios, resource generation failures with .resx files, WPF/WinForms/XAML compilation issues, SDK-style vs legacy csproj differences, and recovering when a build fails. Also use when diagnosing why a project compiles under one tool but not another, or when deciding build scope (single project vs full solution) during iterative migration work.
metadata	{"traits":".NET|CSharp|VisualBasic|DotNetCore","discovery":"lazy","autoMatch":true}

Building Projects

Overview

During .NET modernization, the choice of build tool is not trivial. dotnet build and msbuild.exe are not interchangeable — they have different capabilities, different SDK resolution paths, and different behavior with legacy project features. This skill encodes the expert decision logic so the agent picks the right tool on the first attempt and recovers intelligently when a build fails.

Related Skills

Action	Skill
Convert legacy project files to SDK-style format	converting-to-sdk-style
Add/update/remove target frameworks	managing-target-frameworks
Add/update package references or resolve NuGet version conflicts	managing-package-references
Modify project properties	modifying-project-properties

Defer to these skills for project format conversion, TFM changes, package management, and property edits.

Repo-Level Build Skills Take Priority

If the repository contains a custom build skill (e.g., in .github/skills/, .copilot/skills/, or a project-level BUILD.md with build instructions), defer to that skill for project-specific build decisions. This skill provides general .NET modernization build logic and should serve as a fallback when no repo-specific guidance exists.

IDE Build Tools Take Preference When Available

If the agent is running inside an IDE (Visual Studio, VS Code, Rider) that exposes build-related tools — such as build_project, build_solution or similar MCP/Copilot tools — prefer those over shelling out to the command line. IDE build tools respect the user's local configuration and surface diagnostics where the user can click-to-navigate.

Fall back to this skill's CLI patterns when the IDE doesn't expose a build tool, the IDE build tool fails with insufficient error output, the agent needs fine-grained control (targeting a specific TFM, adjusting verbosity, choosing between dotnet build and msbuild.exe), or the agent needs to capture structured build output for automated validation.

Workflow

Track progress using this checklist:

Task Progress:
- [ ] Step 1: Determine the right build tool
- [ ] Step 2: Execute the build
- [ ] Step 3: Diagnose and recover from failures (if any)
- [ ] Step 4: Validate the build output

Step 1: Determine the Right Build Tool

Default: Start with dotnet build

For SDK-style projects targeting modern .NET only, dotnet build is the correct default. It ships with the .NET SDK, handles NuGet restore implicitly, and works cross-platform.

When to switch to msbuild.exe

Escalate to the full Visual Studio MSBuild (msbuild.exe) when ANY of these conditions apply:

1. The project has .resx files containing images, icons, or binary resources. dotnet build uses a portable resource generator that cannot process embedded images. Symptom: MSB3086, MSB3552, or LC.exe errors.

2. The project is WPF or targets net*-windows with XAML. WPF's markup compiler requires the full Windows SDK targets that only ship with VS MSBuild. Symptom: MC1000, XDG0008, or missing Microsoft.WinFX.targets.

3. The project is WinForms with designer-generated resources. The ResGen.exe that ships with the .NET SDK may lack support for System.Drawing types referenced in .resx files.

4. The project multi-targets and includes a net472 (or any netXXX <= net48) TFM. Building classic .NET Framework TFMs requires reference assemblies and the full MSBuild toolset. dotnet build can handle this IF Microsoft.NETFramework.ReferenceAssemblies is in the project or Directory.Build.props, but frequently fails with NETSDK1005.

5. The project uses COM references, VSIX packaging, or T4 templates. These rely on VS-specific targets/tasks that the .NET SDK MSBuild doesn't include.

6. The solution contains C++/CLI (vcxproj) projects. The .NET CLI cannot build these at all.

7. Legacy (non-SDK-style) project file that hasn't been converted yet. If the project still uses <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets"> rather than <Project Sdk="Microsoft.NET.Sdk">, only msbuild.exe will work.

Decision flowchart

Is the project file SDK-style?
├── NO → Use msbuild.exe (full VS). Stop.
└── YES
    ├── Does it target any net4xx TFM?
    │   ├── YES → Is Microsoft.NETFramework.ReferenceAssemblies present?
    │   │   ├── YES → Try dotnet build first, fall back to msbuild.exe on failure
    │   │   └── NO → Use msbuild.exe (or add the ReferenceAssemblies package first)
    │   └── NO
    │       ├── Does it contain .resx with embedded images/icons?
    │       │   ├── YES → Use msbuild.exe
    │       │   └── NO
    │       │       ├── Is it WPF / has XAML pages?
    │       │       │   ├── YES → Use msbuild.exe
    │       │       │   └── NO → Use dotnet build ✓
    │       └── COM refs / VSIX / T4 / vcxproj?
    │           ├── YES → Use msbuild.exe
    │           └── NO → Use dotnet build ✓

Step 2: Execute the Build

Always Restore During Migration

During modernization, NuGet packages change constantly — versions get bumped, packages get swapped, new ones added, old ones removed. A stale obj/project.assets.json is one of the most common causes of phantom build failures.

Default behavior: always let restore run.

• For dotnet build: restore runs by default — do not add --no-restore during migration. Package references change frequently, and skipping restore causes phantom build failures.
• For msbuild.exe: use msbuild /restore (or /r) to run NuGet restore before building. This is especially important when already using msbuild.exe for the build — mixing dotnet restore as a separate step with msbuild /build can cause resolver mismatches.

When restore can be skipped: Only when the current migration step modified nothing but .cs files — no *proj, Directory.Build.props, Directory.Packages.props, or nuget.config was touched. If in doubt, restore. The cost is a few seconds; the cost of a stale restore is a misleading build result.

Build Scope: Targeted vs Full Solution

Building the entire solution on every change is too slow for iterative migration work. Use two tiers:

Targeted project build (default during iterative work): When the agent modifies a project or its files, build only that specific project:

dotnet build <specific-project.csproj> -r
msbuild.exe <specific-project.csproj> /restore /t:Build /p:Configuration=Release

This gives fast feedback (seconds, not minutes) on whether the migration change compiled. The agent is iterating — it needs signal on its change, not the entire dependency graph.

Full solution build (final validation only): Build the entire solution when:

• The agent is about to mark a task as complete
• A batch of related migration tasks has finished
• The migration step changed something that could ripple across projects (shared interfaces, public API changes, package updates consumed by multiple projects)

dotnet build <solution.sln> -r
msbuild.exe <solution.sln> /restore /t:Build /p:Configuration=Release

Do NOT default to solution builds for every change. Reserve them as a gate check.

Build Patterns

Simple SDK build (happy path):

dotnet build <project.csproj> -r

MSBuild build:

msbuild.exe <project.csproj> /restore /t:Build /p:Configuration=Release /v:minimal

Clean build (when incremental build is suspect):

dotnet build <project.csproj> --no-incremental

Or for MSBuild:

msbuild.exe <project.csproj> /restore /t:Clean;Build /p:Configuration=Release

Debugging restore failures in isolation: When restore itself needs diagnosing, run it separately:

dotnet restore <project.csproj> --verbosity detailed

Then build without restore to isolate the compilation phase:

dotnet build <project.csproj> --no-restore

This is a diagnostic pattern only — not the default workflow.

Caching Build Tool Decisions

After determining the right build tool for a project, save the decision to scenario-instructions.md so it doesn't need to be re-derived on every build.

Add or update a ## Build Tool Decisions section:

## Build Tool Decisions
- **MyWebApp.csproj**: msbuild.exe (non-SDK-style, System.Web references)
- **Common.csproj**: dotnet build (SDK-style, no special requirements)
- **Tests.csproj**: dotnet build (SDK-style after conversion)

On subsequent builds, check this section first:

• If the project is listed → use the cached decision
• If not listed or the project changed → re-read this skill and determine the right tool

Step 3: Diagnose and Recover from Failures

When a build fails, analyze the error codes and take targeted action rather than blindly retrying.

Load references/error-codes.md for the full error catalog with specific recovery actions per error code.

Key recovery principles:

• Capture full build output (use -v:detailed or /v:diag on retry for more info)
• Parse for specific error codes, not just "build failed"
• Distinguish between compilation errors (CS*), MSBuild errors (MSB*), SDK errors (NETSDK*), and NuGet errors (NU*)
• Report which project within a solution failed (not just "the solution failed")

Do NOT:

• Retry the same command more than once without changing something
• Switch tools without understanding why the first tool failed
• Suppress warnings with /nowarn without the user's explicit approval
• Downgrade TreatWarningsAsErrors without flagging it

Step 4: Validate the Build Output

After a successful build during migration, verify:

1. All warnings fixed — fix all warnings in the projects being built, not just new ones introduced by this step
2. Output assembly exists in the expected bin/ path
3. Target framework is correct — the output folder should match the target TFM (e.g., net10.0/), not the old TFM
4. No implicit fallbacks — if the project was supposed to drop a TFM, confirm it's gone

Locating MSBuild

When the decision is msbuild.exe, the agent must locate it reliably.

On Windows

Priority 1 — VSINSTALLDIR environment variable: If the agent is running inside Visual Studio or was launched from a VS Developer Command Prompt, the VSINSTALLDIR environment variable is already set and points to the correct VS installation. Use it directly:

"%VSINSTALLDIR%\MSBuild\Current\Bin\MSBuild.exe"

This is the preferred approach — it matches the user's active VS context and avoids picking a different installation than the one they're working with.

Priority 2 — vswhere.exe: If VSINSTALLDIR is not set (agent running outside VS), use vswhere.exe:

"C:\Program Files (x86)\Microsoft Visual Studio\Installer\vswhere.exe" ^
  -latest -requires Microsoft.Component.MSBuild ^
  -find MSBuild\**\Bin\MSBuild.exe

Priority 3 — well-known paths: If neither is available, fall back to well-known paths under Program Files.

On CI / Linux / macOS

Full msbuild.exe is generally not available outside Windows+VS. If the project requires it, flag it as a Windows-only build requirement or suggest restructuring to remove the dependency (e.g., convert .resx to use LogicalName with precompiled resources, or extract the WPF layer).

Multi-Targeting Considerations

Projects that multi-target (e.g., <TargetFrameworks>net472;net10.0</TargetFrameworks>) during a transitional migration phase need special handling:

• dotnet build builds ALL TFMs by default. To build just one:

  dotnet build -f net10.0
  

• Build the NEW target first to validate migration changes, then optionally build the old target to confirm backward compatibility.

• If the old TFM build is no longer needed (migration complete), recommend removing it from <TargetFrameworks> — defer to the managing-target-frameworks skill for the actual edit.

Anti-Patterns

• ❌ Retrying the same build command without changing anything — wastes time and produces identical errors
• ❌ Switching from dotnet build to msbuild.exe (or vice versa) without understanding the root cause of the failure
• ❌ Suppressing warnings with /nowarn or downgrading TreatWarningsAsErrors without explicit user approval
• ❌ Batching all build validation at the end of a migration — invoke this skill after EVERY discrete change that needs build validation; catching failures early keeps the error surface small
• ❌ Running a full solution build when only one project changed — use targeted project builds for faster feedback
• ❌ Ignoring which project within a solution failed — always report the specific project name

Success Criteria

• Correct build tool selected based on project characteristics (SDK-style, TFMs, resource types)
• Build executed with appropriate configuration and verbosity
• Build errors diagnosed by error code category (CS*, MSB*, NETSDK*, NU*)
• Recovery actions targeted to root cause, not blind retries
• Output assembly exists in the correct TFM-specific bin/ path
• All warnings fixed in touched projects — not just new ones, all of them
• Multi-target builds validated per-TFM when applicable

Resources

• dotnet build command
• MSBuild command-line reference
• SDK-style project overview
• Multi-targeting