Dotnet CLI Architecture

dotnet-cli-architecture

Layered CLI application architecture for .NET: command/handler/service separation following clig.dev principles, configuration precedence (appsettings → environment variables → CLI arguments), structured logging in CLI context, exit code conventions, stdin/stdout/stderr patterns, and testing CLI applications via in-process invocation with output capture.

Version assumptions: .NET 8.0+ baseline. Patterns apply to CLI tools built with System.CommandLine 2.0 and generic host.

Out of scope: System.CommandLine API details (RootCommand, Option, middleware, hosting setup) -- see [skill:dotnet-system-commandline]. Native AOT compilation and publish pipeline -- see [skill:dotnet-native-aot]. CLI distribution, packaging, and release automation -- see [skill:dotnet-cli-distribution] and [skill:dotnet-cli-packaging]. General CI/CD patterns -- see [skill:dotnet-gha-patterns] and [skill:dotnet-ado-patterns]. DI container internals -- see [skill:dotnet-csharp-dependency-injection]. General testing strategies -- see [skill:dotnet-testing-strategy].

Cross-references: [skill:dotnet-system-commandline] for System.CommandLine 2.0 API, [skill:dotnet-native-aot] for AOT publishing CLI tools, [skill:dotnet-csharp-dependency-injection] for DI patterns, [skill:dotnet-csharp-configuration] for configuration integration, [skill:dotnet-testing-strategy] for general testing patterns.

clig.dev Principles for .NET CLI Tools

The Command Line Interface Guidelines provide language-agnostic principles for well-behaved CLI tools. These translate directly to .NET patterns.

Core Principles

Stdout vs Stderr in .NET

// Data output -- goes to stdout (can be piped)
Console.Out.WriteLine(JsonSerializer.Serialize(result, jsonContext.Options));

// Status/diagnostic output -- goes to stderr (user sees it, pipe ignores it)
Console.Error.WriteLine("Processing 42 files...");

// With ILogger (when using hosting)
// ILogger writes to stderr via console provider by default
logger.LogInformation("Connected to {Endpoint}", endpoint);

Layered Command → Handler → Service Architecture

Separate CLI concerns into three layers:

┌─────────────────────────────────────┐
│  Commands (System.CommandLine)      │  Parse args, wire options
│  ─ RootCommand, Command, Option<T>  │
├─────────────────────────────────────┤
│  Handlers (orchestration)           │  Coordinate services, format output
│  ─ ICommandHandler implementations  │
├─────────────────────────────────────┤
│  Services (business logic)          │  Pure logic, no CLI concerns
│  ─ Interfaces + implementations     │
└─────────────────────────────────────┘

Why Three Layers

• Commands know about CLI syntax (options, arguments, subcommands) but not business logic
• Handlers bridge CLI inputs to service calls and format results for output
• Services contain domain logic and are reusable outside the CLI (tests, libraries, APIs)

Example Structure

src/
  MyCli/
    MyCli.csproj
    Program.cs                    # RootCommand + CommandLineBuilder
    Commands/
      SyncCommandDefinition.cs    # Command, options, arguments
    Handlers/
      SyncHandler.cs              # ICommandHandler, orchestrates services
    Services/
      ISyncService.cs             # Business logic interface
      SyncService.cs              # Implementation (no CLI awareness)
    Output/
      ConsoleFormatter.cs         # Table/JSON output formatting

Command Definition Layer

// Commands/SyncCommandDefinition.cs
public static class SyncCommandDefinition
{
    public static readonly Option<Uri> SourceOption = new(
        "--source", "Source endpoint URL") { IsRequired = true };

    public static readonly Option<bool> DryRunOption = new(
        "--dry-run", "Preview changes without applying");

    public static Command Create()
    {
        var command = new Command("sync", "Synchronize data from source");
        command.AddOption(SourceOption);
        command.AddOption(DryRunOption);
        return command;
    }
}

Handler Layer

// Handlers/SyncHandler.cs
public class SyncHandler : ICommandHandler
{
    private readonly ISyncService _syncService;
    private readonly ILogger<SyncHandler> _logger;

    public SyncHandler(ISyncService syncService, ILogger<SyncHandler> logger)
    {
        _syncService = syncService;
        _logger = logger;
    }

    // Bound by naming convention from options
    public Uri Source { get; set; } = null!;
    public bool DryRun { get; set; }

    public int Invoke(InvocationContext context) =>
        InvokeAsync(context).GetAwaiter().GetResult();

    public async Task<int> InvokeAsync(InvocationContext context)
    {
        var ct = context.GetCancellationToken();

        _logger.LogInformation("Syncing from {Source}", Source);

        var result = await _syncService.SyncAsync(Source, DryRun, ct);

        if (result.HasErrors)
        {
            context.Console.Error.Write($"Sync failed: {result.ErrorMessage}\n");
            return ExitCodes.SyncFailed;
        }

        context.Console.Out.Write($"Synced {result.ItemCount} items.\n");
        return ExitCodes.Success;
    }
}

Service Layer

// Services/ISyncService.cs -- no CLI dependency
public interface ISyncService
{
    Task<SyncResult> SyncAsync(Uri source, bool dryRun, CancellationToken ct);
}

// Services/SyncService.cs
public class SyncService : ISyncService
{
    private readonly HttpClient _httpClient;

    public SyncService(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

    public async Task<SyncResult> SyncAsync(
        Uri source, bool dryRun, CancellationToken ct)
    {
        // Pure business logic -- testable without CLI infrastructure
        var data = await _httpClient.GetFromJsonAsync<SyncData>(source, ct);
        // ...
        return new SyncResult(ItemCount: data.Items.Length);
    }
}

Configuration Precedence

CLI tools use a specific configuration precedence (lowest to highest priority):

1. Compiled defaults -- hardcoded fallback values
2. appsettings.json -- shipped with the tool
3. appsettings.{Environment}.json -- environment-specific overrides
4. Environment variables -- set by shell or CI
5. CLI arguments -- explicit user input (highest priority)

Implementation with Generic Host

var builder = new CommandLineBuilder(rootCommand)
    .UseHost(_ => Host.CreateDefaultBuilder(args), host =>
    {
        host.ConfigureAppConfiguration((ctx, config) =>
        {
            // Layers 2-3 handled by CreateDefaultBuilder:
            //   appsettings.json, appsettings.{env}.json, env vars

            // Layer 4: User-specific config file
            var configPath = Path.Combine(
                Environment.GetFolderPath(Environment.SpecialFolder.UserProfile),
                ".mycli", "config.json");
            if (File.Exists(configPath))
            {
                config.AddJsonFile(configPath, optional: true);
            }
        });

        // Layer 5: CLI args override everything
        // System.CommandLine options take precedence via handler binding
    })
    .UseDefaults()
    .Build();

User-Level Configuration

Many CLI tools support user-level config (e.g., ~/.mycli/config.json, ~/.config/mycli/config.yaml). Follow platform conventions:

Structured Logging in CLI Context

Configuring Logging for CLI

CLI tools need different logging than web apps: logs go to stderr, and verbosity is controlled by flags.

host.ConfigureLogging((ctx, logging) =>
{
    logging.ClearProviders();
    logging.AddConsole(options =>
    {
        // Write to stderr, not stdout
        options.LogToStandardErrorThreshold = LogLevel.Trace;
    });
});

Verbosity Mapping

Map --verbose/--quiet flags to log levels:

public static class VerbosityMapping
{
    public static LogLevel ToLogLevel(bool verbose, bool quiet) => (verbose, quiet) switch
    {
        (true, _) => LogLevel.Debug,
        (_, true) => LogLevel.Warning,
        _ => LogLevel.Information  // default
    };
}

// In host configuration
host.ConfigureLogging((ctx, logging) =>
{
    var level = VerbosityMapping.ToLogLevel(verbose, quiet);
    logging.SetMinimumLevel(level);
});

Exit Code Conventions

Standard Exit Codes

public static class ExitCodes
{
    public const int Success = 0;
    public const int GeneralError = 1;
    public const int InvalidUsage = 2;    // Bad arguments or options
    public const int IoError = 3;         // File not found, permission denied
    public const int NetworkError = 4;    // Connection failed, timeout
    public const int AuthError = 5;       // Authentication/authorization failure

    // Tool-specific codes start at 10+
    public const int SyncFailed = 10;
    public const int ValidationFailed = 11;
}

Guidelines

• 0 = success (always)
• 1 = general/unspecified error
• 2 = invalid usage (bad arguments) -- System.CommandLine returns this for parse errors automatically
• 3-9 = reserved for common categories
• 10+ = tool-specific error codes
• Never use exit codes > 125 (reserved by shells; 126 = not executable, 127 = not found, 128+N = killed by signal N)

Propagating Exit Codes

public async Task<int> InvokeAsync(InvocationContext context)
{
    try
    {
        await _service.ProcessAsync(context.GetCancellationToken());
        return ExitCodes.Success;
    }
    catch (HttpRequestException ex)
    {
        _logger.LogError(ex, "Network error");
        context.Console.Error.Write($"Error: {ex.Message}\n");
        return ExitCodes.NetworkError;
    }
    catch (UnauthorizedAccessException ex)
    {
        context.Console.Error.Write($"Permission denied: {ex.Message}\n");
        return ExitCodes.IoError;
    }
}

Stdin/Stdout/Stderr Patterns

Reading from Stdin

Support piped input as an alternative to file arguments:

public async Task<int> InvokeAsync(InvocationContext context)
{
    string input;

    if (InputFile is not null)
    {
        input = await File.ReadAllTextAsync(InputFile.FullName);
    }
    else if (Console.IsInputRedirected)
    {
        // Read from stdin: echo '{"data":1}' | mycli process
        input = await Console.In.ReadToEndAsync();
    }
    else
    {
        context.Console.Error.Write("Error: Provide input via --file or stdin.\n");
        return ExitCodes.InvalidUsage;
    }

    var result = _processor.Process(input);
    context.Console.Out.Write(JsonSerializer.Serialize(result));
    return ExitCodes.Success;
}

Machine-Readable Output

// Global --json option for machine-readable output
var jsonOption = new Option<bool>("--json", "Output as JSON");
rootCommand.AddGlobalOption(jsonOption);

// In handler
if (useJson)
{
    Console.Out.WriteLine(JsonSerializer.Serialize(result, jsonContext.Options));
}
else
{
    // Human-friendly table format
    ConsoleFormatter.WriteTable(result, context.Console);
}

Progress to Stderr

// Progress reporting goes to stderr (does not pollute piped stdout)
await foreach (var item in _service.StreamAsync(ct))
{
    Console.Error.Write($"\rProcessing {item.Index}/{total}...");
    Console.Out.WriteLine(item.ToJson());
}
Console.Error.WriteLine();  // Clear progress line

Testing CLI Applications

In-Process Invocation with CommandLineBuilder

Test the full CLI pipeline without spawning a child process:

public class CliTestHarness
{
    private readonly RootCommand _rootCommand;
    private readonly Action<IServiceCollection>? _configureServices;

    public CliTestHarness(Action<IServiceCollection>? configureServices = null)
    {
        _rootCommand = Program.BuildRootCommand();
        _configureServices = configureServices;
    }

    public async Task<(int ExitCode, string Stdout, string Stderr)> InvokeAsync(
        string commandLine)
    {
        var console = new TestConsole();

        var builder = new CommandLineBuilder(_rootCommand)
            .UseHost(_ => Host.CreateDefaultBuilder(), host =>
            {
                if (_configureServices is not null)
                {
                    host.ConfigureServices(_configureServices);
                }
            })
            .UseDefaults()
            .Build();

        var exitCode = await builder.InvokeAsync(commandLine, console);

        return (exitCode, console.Out.ToString()!, console.Error.ToString()!);
    }
}

Testing with Service Mocks

[Fact]
public async Task Sync_WithValidSource_ReturnsZero()
{
    var fakeSyncService = new FakeSyncService(
        new SyncResult(ItemCount: 5));

    var harness = new CliTestHarness(services =>
    {
        services.AddSingleton<ISyncService>(fakeSyncService);
    });

    var (exitCode, stdout, stderr) = await harness.InvokeAsync(
        "sync --source https://api.example.com");

    Assert.Equal(0, exitCode);
    Assert.Contains("Synced 5 items", stdout);
}

[Fact]
public async Task Sync_WithMissingSource_ReturnsNonZero()
{
    var harness = new CliTestHarness();

    var (exitCode, _, stderr) = await harness.InvokeAsync("sync");

    Assert.NotEqual(0, exitCode);
    Assert.Contains("--source", stderr);  // Parse error mentions missing option
}

Exit Code Assertion

[Theory]
[InlineData("sync --source https://valid.example.com", 0)]
[InlineData("sync", 2)]  // Missing required option
[InlineData("invalid-command", 1)]
public async Task ExitCode_MatchesExpected(string args, int expectedExitCode)
{
    var harness = new CliTestHarness();
    var (exitCode, _, _) = await harness.InvokeAsync(args);
    Assert.Equal(expectedExitCode, exitCode);
}

Testing Output Format

[Fact]
public async Task List_WithJsonFlag_OutputsValidJson()
{
    var harness = new CliTestHarness(services =>
    {
        services.AddSingleton<IItemRepository>(
            new FakeItemRepository([new Item(1, "Widget")]));
    });

    var (exitCode, stdout, _) = await harness.InvokeAsync("list --json");

    Assert.Equal(0, exitCode);
    var items = JsonSerializer.Deserialize<Item[]>(stdout);
    Assert.NotNull(items);
    Assert.Single(items);
}

[Fact]
public async Task List_StderrContainsLogs_StdoutContainsDataOnly()
{
    var harness = new CliTestHarness();
    var (_, stdout, stderr) = await harness.InvokeAsync("list --json --verbose");

    // Stdout must be valid JSON (no log noise)
    // xUnit: just call it -- if it throws, the test fails
    var doc = JsonDocument.Parse(stdout);
    Assert.NotNull(doc);

    // Stderr contains diagnostic output
    Assert.Contains("Connected to", stderr);
}

Agent Gotchas

1. Do not write diagnostic output to stdout. Logs, progress, and errors go to stderr. Stdout is reserved for data output that can be piped. A CLI tool that mixes logs into stdout breaks shell pipelines.
2. Do not hardcode exit code 1 for all errors. Use distinct exit codes for different failure categories (I/O, network, auth, validation). Callers and scripts rely on exit codes to determine what went wrong.
3. Do not put business logic in command handlers. Handlers should orchestrate calls to injected services and format output. Business logic in handlers cannot be reused or unit-tested independently.
4. Do not test CLI tools only via process spawning. Use in-process invocation with CommandLineBuilder and TestConsole for fast, reliable tests. Reserve process-level tests for smoke testing the published binary.
5. Do not ignore Console.IsInputRedirected when accepting stdin. Without checking, the tool may hang waiting for input when invoked without piped data.
6. Do not use exit codes above 125. Codes 126-255 have special meanings in Unix shells (126 = not executable, 127 = not found, 128+N = killed by signal N). Tool-specific codes should be in the 1-125 range.

References

• Command Line Interface Guidelines (clig.dev)
• System.CommandLine overview
• 12 Factor CLI Apps
• Generic Host in .NET
• Console logging in .NET