# Dotnet Build Analysis > Interprets MSBuild output, NuGet errors, analyzer warnings. Error codes, CI drift. - Skill: `rudironsoni/dotnet-build-analysis` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add rudironsoni/dotnet-build-analysis` - Raw SKILL.md: https://api.skillmd.com/api/skills/rudironsoni/dotnet-build-analysis/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools, AI & ML - License: MIT - Author: rudironsoni (https://skillmd.com/u/rudironsoni) - Updated: 2026-09-10 - Page: https://skillmd.com/skills/rudironsoni/dotnet-build-analysis --- # dotnet-build-analysis Help agents interpret and act on MSBuild build output. Covers error code prefixes, NuGet restore failures, analyzer warning interpretation, multi-targeting build differences, and "works locally, fails in CI" diagnosis patterns. Each subsection includes example output, diagnosis steps, and a fix pattern. ## Scope - MSBuild error code prefix interpretation (CS, MSB, NU, CA, IDE, NETSDK) - NuGet restore failure diagnosis and resolution - Analyzer warning triage and suppression guidance - CI vs local build drift diagnosis ## Out of scope - Writing or modifying .csproj files -- see [skill:dotnet-csproj-reading] - Project structure decisions -- see [skill:dotnet-project-structure] - Common agent code mistakes -- see [skill:dotnet-agent-gotchas] ## Prerequisites .NET 8.0+ SDK. MSBuild (included with .NET SDK). Understanding of SDK-style project format. Cross-references: [skill:dotnet-agent-gotchas] for common code mistakes that cause build errors, [skill:dotnet-csproj-reading] for project file structure and modification, [skill:dotnet-project-structure] for project organization and SDK selection. --- ## Error Code Prefixes MSBuild output uses standardized prefixes to indicate the error source. Understanding the prefix tells you which system produced the error and where to look for fixes. ### CS -- C# Compiler Errors and Warnings Produced by the Roslyn C# compiler. These are language-level issues in source code. **Example output:** ````csharp src/MyApp.Api/Services/OrderService.cs(42,17): error CS0246: The type or namespace name 'OrderDto' could not be found (are you missing a using directive or an assembly reference?) src/MyApp.Api/Models/User.cs(15,9): warning CS8618: Non-nullable property 'Name' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable. ```csharp **Diagnosis:** 1. Parse the file path and line number from the error -- `src/MyApp.Api/Services/OrderService.cs` line 42, column 17. 2. CS0246 means a type is missing. Check: is the type defined? Is the namespace imported? Is the project referencing the assembly that contains it? 3. CS8618 is a nullable reference type warning. The property needs a `required` modifier, nullable annotation (`string?`), or constructor initialization. **Fix pattern:** - CS0xxx (syntax/type errors): Fix source code at the indicated location. Add `using` directives, fix type names, add missing references. - CS8xxx (nullable warnings): Add null annotations, null checks, or `required` modifiers. Do NOT suppress with `#pragma` or `!` operator. ### MSB -- MSBuild Engine Errors Produced by the MSBuild build engine itself. These indicate project file problems, target failures, or build system misconfiguration. **Example output:** ```text error MSB4019: The imported project "C:\Program Files\dotnet\sdk\9.0.100\Microsoft\VisualStudio\v17.0\WebApplications\Microsoft.WebApplication.targets" was not found. Confirm that the expression in the Import declaration "..." is correct. error MSB3644: The reference assemblies for .NETFramework,Version=v4.8 were not found. You might need to install the developer pack for this framework version. ```text **Diagnosis:** 1. MSB4019: An MSBuild `.targets` file is missing. This usually means wrong SDK type, missing workload, or corrupt SDK installation. 2. MSB3644: Targeting a framework version whose targeting pack is not installed. Common when a project targets .NET Framework but only .NET (Core) SDK is installed. **Fix pattern:** - MSB4019: Verify `` is correct (e.g., `Microsoft.NET.Sdk.Web` for ASP.NET Core). Run `dotnet workload list` and install missing workloads. - MSB3xxx: Check `` value. Ensure the required SDK or targeting pack is installed. ### NU -- NuGet Errors and Warnings Produced by the NuGet package manager during restore or pack operations. **Example output:** ```text error NU1101: Unable to find package Newtonsoft.Json.Extensions. No packages exist with this id in source(s): nuget.org warning NU1603: Microsoft.EntityFrameworkCore 9.0.0 depends on Microsoft.Extensions.Caching.Memory (>= 9.0.0) but version Microsoft.Extensions.Caching.Memory 8.0.1 was resolved. Approve the package to suppress this warning. error NU1605: Detected package downgrade: Microsoft.Extensions.Logging from 9.0.0 to 8.0.1. Reference the package directly from the project to select a different version. ```json **Diagnosis:** 1. NU1101: Package ID does not exist. Check spelling, verify the package source is configured, check if the package was renamed or deprecated. 2. NU1603: Transitive dependency version conflict. A package wants a newer version than what is resolved. 3. NU1605: Explicit downgrade detected. Two packages require different versions of the same dependency. **Fix pattern:** - NU1101: Fix the package name. Search [nuget.org](https://www.nuget.org/) for the correct ID. - NU1603/NU1605: Add a direct `` for the conflicting package at a compatible version, or use central package management to pin versions. ### IDE -- IDE/Roslyn Analyzer Code Style Diagnostics Produced by Roslyn IDE analyzers for code style enforcement. These are usually warnings, not errors (unless `.editorconfig` escalates them). **Example output:** ```text src/MyApp.Api/Program.cs(1,1): warning IDE0005: Using directive is unnecessary. src/MyApp.Api/Models/Order.cs(8,12): warning IDE0044: Make field readonly src/MyApp.Api/Services/Report.cs(22,5): warning IDE0058: Expression value is never used ```csharp **Diagnosis:** 1. IDE0005: Unused `using` directive. Safe to remove. 2. IDE0044: Field can be `readonly` because it is only assigned in the constructor. 3. IDE0058: A method return value is discarded. Either assign it or use `_ = ...` to explicitly discard. **Fix pattern:** - IDE analyzers enforce code style. Fix them by applying the suggested change. Configure severity in `.editorconfig` to promote warnings to errors for CI enforcement. ### CA -- .NET Code Analysis (FxCop/Microsoft.CodeAnalysis.NetAnalyzers) Produced by the .NET code analysis SDK analyzers for API design, performance, reliability, and security rules. **Example output:** ```text src/MyApp.Api/Services/CacheService.cs(34,9): warning CA1848: Use the LoggerMessage delegates instead of calling 'LoggerExtensions.LogInformation(ILogger, string?, params object?[])'. Using LoggerMessage delegates provides better performance. src/MyApp.Api/Controllers/UserController.cs(12,5): warning CA2007: Consider calling ConfigureAwait on the awaited task src/MyApp.Api/Crypto/HashService.cs(8,9): warning CA5351: Do Not Use Broken Cryptographic Algorithms (MD5) ```csharp **Diagnosis:** 1. CA1848: High-performance logging. Use `[LoggerMessage]` source generator attributes instead of string interpolation in log calls. 2. CA2007: `ConfigureAwait(false)` guidance for library code. Not applicable to ASP.NET Core app code (no `SynchronizationContext`). 3. CA5351: Security-critical. MD5 is broken for cryptographic purposes. Switch to SHA-256 or SHA-512. **Fix pattern:** - CA1xxx (design): Apply suggested API changes. These improve API consistency. - CA2xxx (reliability/performance): Fix per suggestion. CA2007 can be suppressed in ASP.NET Core apps via `.editorconfig`. - CA5xxx (security): Always fix. These flag real security vulnerabilities. --- ## NuGet Restore Failures NuGet restore is the first build step. When it fails, no compilation occurs. These are the most common restore failure patterns. ### Pattern: Package Not Found **Example output:** ```text Determining projects to restore... Writing assets file to disk. Path: /src/MyApp.Api/obj/project.assets.json /src/MyApp.Api/MyApp.Api.csproj : error NU1101: Unable to find package MyCompany.Shared.Models. No packages exist with this id in source(s): nuget.org Failed to restore /src/MyApp.Api/MyApp.Api.csproj (in 2.14 sec). ```csharp **Diagnosis:** 1. Is the package ID spelled correctly? NuGet IDs are case-insensitive but must be exact. 2. Is the package from a private feed? Check `nuget.config` for feed configuration. NuGet searches feeds hierarchically upward from the project directory. 3. Is `packageSourceMapping` configured? If so, the package must be mapped to a source that contains it. `MyCompany.*` patterns take precedence over `*` wildcard. **Fix pattern:** ```bash # Check configured sources dotnet nuget list source # Check if nuget.config exists (searches upward from project dir) ls nuget.config ../nuget.config ../../nuget.config 2>/dev/null ```text ```xml ```text ### Pattern: Version Conflict **Example output:** ```text error NU1107: Version conflict detected for Microsoft.Extensions.DependencyInjection.Abstractions. MyApp.Api -> Microsoft.EntityFrameworkCore 9.0.0 -> Microsoft.Extensions.DependencyInjection.Abstractions (>= 9.0.0) MyApp.Api -> Microsoft.Extensions.Hosting 8.0.1 -> Microsoft.Extensions.DependencyInjection.Abstractions (>= 8.0.1) ```text **Diagnosis:** 1. Two dependency chains require different major versions of the same package. 2. Trace each chain to find which top-level package is pinned at an older version. 3. The fix is usually upgrading the older top-level package. **Fix pattern:** ```xml ```text ### Pattern: Authentication Failure on Private Feed **Example output:** ```text Retrying 'FindPackagesByIdAsyncCore' for source 'https://pkgs.dev.azure.com/myorg/_packaging/myfeed/nuget/v3/index.json'. Response status code does not indicate success: 401 (Unauthorized). error NU1301: Unable to load the service index for source https://pkgs.dev.azure.com/myorg/_packaging/myfeed/nuget/v3/index.json. ```json **Diagnosis:** 1. Credentials are missing or expired for the private feed. 2. In CI, check that the credential provider or PAT is configured. 3. Locally, run `dotnet nuget update source` with credentials or use Azure Artifacts Credential Provider. **Fix pattern:** ```bash # Install Azure Artifacts Credential Provider (see official docs for platform-specific steps): # https://github.com/microsoft/artifacts-credprovider#setup # Windows: iex "& { $(irm https://aka.ms/install-artifacts-credprovider.ps1) }" # macOS/Linux: sh -c "$(curl -fsSL https://aka.ms/install-artifacts-credprovider.sh)" # Or add credentials explicitly to a specific source dotnet nuget update source MyCompany --username az --password $PAT --store-password-in-clear-text ```text --- ## Analyzer Warning Interpretation Analyzer warnings are produced by Roslyn analyzers bundled with the SDK or added via NuGet. Understanding when to fix vs. when to configure severity is critical. ### Example Output ```text src/MyApp.Api/Controllers/OrdersController.cs(27,5): warning CA2007: Consider calling ConfigureAwait on the awaited task [/src/MyApp.Api/MyApp.Api.csproj] src/MyApp.Api/Services/OrderService.cs(15,16): warning CA1062: In externally visible method 'OrderService.Process(string)', validate parameter 'input' is non-null before using it [/src/MyApp.Api/MyApp.Api.csproj] src/MyApp.Api/Models/UserDto.cs(8,12): warning IDE0032: Use auto-implemented property [/src/MyApp.Api/MyApp.Api.csproj] ```csharp **Diagnosis:** 1. Identify the prefix: `CA` = Code Analysis (.NET analyzers), `IDE` = IDE code style analyzers. 2. Check severity: warnings don't break builds unless `true` is set. 3. Determine if the rule applies to your project type (e.g., CA2007 is irrelevant in ASP.NET Core — no SynchronizationContext). 4. Decide: fix the code, configure severity in `.editorconfig`, or suppress with documented justification. **Fix pattern:** - **Fix the code** when the analyzer identifies a real issue (CA1062 — add null validation or use `ArgumentNullException.ThrowIfNull`). - **Configure severity** in `.editorconfig` when the rule doesn't apply project-wide (see below). - **Suppress inline** only with documented justification (see "When Suppression Is Acceptable" below). ### Severity Levels | Severity | Build Impact | Action | | -------------- | ----------------------------------------------------------------------------- | ----------------------------------- | | **Error** | Build fails | Must fix before build succeeds | | **Warning** | Build succeeds (unless `true`) | Fix or configure in `.editorconfig` | | **Suggestion** | Build succeeds; shown in IDE | Fix when practical | | **Hidden** | Not shown; available via code fix | Ignore unless actively refactoring | ### Configuring Severity Use `.editorconfig` to control analyzer behavior across the project: ```ini # .editorconfig (place at solution root) [*.cs] # Promote nullable warnings to errors (recommended) dotnet_diagnostic.CS8600.severity = error dotnet_diagnostic.CS8602.severity = error dotnet_diagnostic.CS8603.severity = error # Suppress ConfigureAwait warning in ASP.NET Core apps (no SynchronizationContext) dotnet_diagnostic.CA2007.severity = none # Promote security warnings to errors dotnet_diagnostic.CA5350.severity = error dotnet_diagnostic.CA5351.severity = error ```text ### When Suppression Is Acceptable Suppression is acceptable ONLY when: 1. The analyzer cannot understand the code's safety guarantee (e.g., a custom guard clause that ensures non-null). 2. The rule does not apply to the project type (e.g., CA2007 in ASP.NET Core apps). 3. A documented justification is provided. ```csharp // ACCEPTABLE: documented justification [SuppressMessage("Reliability", "CA2007:ConfigureAwait", Justification = "ASP.NET Core has no SynchronizationContext")] // TODO: Audit suppression - add justification or remove if not applicable public async Task GetOrderAsync(int id, CancellationToken ct) { return await _repo.GetByIdAsync(id, ct); } // NOT ACCEPTABLE: no justification, hides a real issue #pragma warning disable CA1062 ## Detailed Examples See [references/detailed-examples.md](references/detailed-examples.md) for complete code samples and advanced patterns.