dotnet-cli-architecture

name	dotnet-cli-architecture
description	Structures CLI app layers. Command/handler/service separation, clig.dev principles, exit codes.
allowed-tools	["Read","Grep","Glob","Bash","Write","Edit"]

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.

Scope

• Layered command/handler/service architecture for CLI apps
• clig.dev principles for .NET (stdout/stderr, exit codes, NO_COLOR)
• Configuration precedence (appsettings, env vars, CLI args)
• Structured logging in CLI context
• Stdin/stdout/stderr patterns and machine-readable output
• Testing CLI applications via in-process invocation

Out of scope

• System.CommandLine API details (RootCommand, Option, SetAction) -- see [skill:dotnet-system-commandline]
• Native AOT compilation and publish pipeline -- see [skill:dotnet-native-aot]
• CLI distribution and packaging -- 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

Principle	Implementation
Human-first output by default	Use Console.Out for data, Console.Error for diagnostics
Machine-readable output with --json	Add a --json global option that switches output format
Stderr for status/diagnostics	Logging, progress bars, and prompts go to stderr
Stdout for data only	Piped output (mycli list | jq .) must not contain log noise
Non-zero exit on failure	Return specific exit codes (see conventions below)
Fail early, fail loudly	Validate inputs before doing work
Respect NO_COLOR	Check Environment.GetEnvironmentVariable("NO_COLOR")
Support --verbose and --quiet	Global options controlling output verbosity

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);

```text

---

## Layered Command → Handler → Service Architecture

Separate CLI concerns into three layers:

```bash

┌─────────────────────────────────────┐
│  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     │
└─────────────────────────────────────┘

```text

### 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

```text

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

```csharp

### Command Definition Layer

```csharp

// Commands/SyncCommandDefinition.cs
public static class SyncCommandDefinition
{
       Option<Uri> SourceOption = (
        , ) { IsRequired =  };

       Option<> DryRunOption = (
        , );

    
    {
         command =  Command(, );
        command.AddOption(SourceOption);
        command.AddOption(DryRunOption);
         command;
    }
}

```bash



```csharp


   : 
{
      ISyncService _syncService;
      ILogger<SyncHandler> _logger;

    
    {
        _syncService = syncService;
        _logger = logger;
    }

    
     Uri Source { ; ; } = !;
      DryRun { ; ; }

     =>
        InvokeAsync(context).GetAwaiter().GetResult();

    
    {
         ct = context.GetCancellationToken();

        _logger.LogInformation(, Source);

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

         (result.HasErrors)
        {
            context.Console.Error.Write();
             ExitCodes.SyncFailed;
        }

        context.Console.Out.Write();
         ExitCodes.Success;
    }
}

```text



```csharp


  
{
    ;
}


   : 
{
      HttpClient _httpClient;

    
    {
        _httpClient = httpClient;
    }

    
    {
        
         data =  _httpClient.GetFromJsonAsync<SyncData>(source, ct);
        
          SyncResult(ItemCount: data.Items.Length);
    }
}

```text

---



{Environment}.json** -- environment-specific overrides
 **Environment variables** --   shell  CI
 **CLI arguments** --  =  CommandLineBuilder(rootCommand)
    .UseHost(_ => Host.CreateDefaultBuilder(), host =>
    {
        host.ConfigureAppConfiguration((ctx, config) =>
        {
            
            

            
             configPath = Path.Combine(
                Environment.GetFolderPath(Environment.SpecialFolder.UserProfile),
                , );
             (File.Exists(configPath))
            {
                config.AddJsonFile(configPath, optional: );
            }
        });

        
        
    })
    .UseDefaults()
    .Build();

```bash



Many CLI tools support user- =>
{
    logging.ClearProviders();
    logging.AddConsole(options =>
    {
        
        options.LogToStandardErrorThreshold = LogLevel.Trace;
    });
});

```text



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

```csharp

   
{
     => (verbose, quiet) 
    {
        (, _) => LogLevel.Debug,
        (_, ) => LogLevel.Warning,
        _ => LogLevel.Information  
    };
}


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

```text

---





```csharp

   
{
       Success = ;
       GeneralError = ;
       InvalidUsage = ;    
       IoError = ;         
       NetworkError = ;    
       AuthError = ;       

    
       SyncFailed = ;
       ValidationFailed = ;
}

```text



- **** = success (always)
- **** = general/unspecified error
- **** =  = reserved  common categories
- **+** = tool-specific error codes
- Never use exit codes >  (reserved  shells;  =  executable,  =  found, +N = killed  signal N)



```
{
    
    {
         _service.ProcessAsync(context.GetCancellationToken());
         ExitCodes.Success;
    }
     (HttpRequestException ex)
    {
        _logger.LogError(ex, );
        context.Console.Error.Write();
         ExitCodes.NetworkError;
    }
     (UnauthorizedAccessException ex)
    {
        context.Console.Error.Write();
         ExitCodes.IoError;
    }
}

```text

---





Support piped input  an alternative to  arguments:

```
{
     input;

     (InputFile   )
    {
        input =  File.ReadAllTextAsync(InputFile.FullName);
    }
      (Console.IsInputRedirected)
    {
        
        input =  Console.In.ReadToEndAsync();
    }
    
    {
        context.Console.Error.Write();
         ExitCodes.InvalidUsage;
    }

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

```json



```csharp


 jsonOption =  Option<>(, );
rootCommand.AddGlobalOption(jsonOption);


 (useJson)
{
    Console.Out.WriteLine(JsonSerializer.Serialize(result, jsonContext.Options));
}

{
    
    ConsoleFormatter.WriteTable(result, context.Console);
}

```text



```csharp


  ( item  _service.StreamAsync(ct))
{
    Console.Error.Write();
    Console.Out.WriteLine(item.ToJson());
}
Console.Error.WriteLine();  

```json

---





Test the full CLI pipeline without spawning a child process:

```csharp

  
{
      RootCommand _rootCommand;
      Action<IServiceCollection>? _configureServices;

    
    {
        _rootCommand = Program.BuildRootCommand();
        _configureServices = configureServices;
    }

      Task<( ExitCode,  Stdout,  Stderr)> InvokeAsync(
         commandLine)
    {
         console =  TestConsole();

         builder =  CommandLineBuilder(_rootCommand)
            .UseHost(_ => Host.CreateDefaultBuilder(), host =>
            {
                 (_configureServices   )
                {
                    host.ConfigureServices(_configureServices);
                }
            })
            .UseDefaults()
            .Build();

         exitCode =  builder.InvokeAsync(commandLine, console);

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

```text



```csharp

[]

{
     fakeSyncService =  FakeSyncService(
         SyncResult(ItemCount: ));

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

     (exitCode, stdout, stderr) =  harness.InvokeAsync(
        );

    Assert.Equal(, exitCode);
    Assert.Contains(, stdout);
}

[]

{
     harness =  CliTestHarness();

     (exitCode, _, stderr) =  harness.InvokeAsync();

    Assert.NotEqual(, exitCode);
    Assert.Contains(, stderr);  
}

```text



```csharp

[]
[]
[]  
[]

{
     harness =  CliTestHarness();
     (exitCode, _, _) =  harness.InvokeAsync();
    Assert.Equal(expectedExitCode, exitCode);
}

```text



```csharp

[]

{
     harness =  CliTestHarness(services =>
    {
        services.AddSingleton<IItemRepository>(
             FakeItemRepository([ Item(, )]));
    });

     (exitCode, stdout, _) =  harness.InvokeAsync();

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

[]

{
     harness =  CliTestHarness();
     (_, stdout, stderr) =  harness.InvokeAsync();

    
    
     doc = JsonDocument.Parse(stdout);
    Assert.NotNull(doc);

    
    Assert.Contains(, stderr);
}

```text

---



 **Do  write diagnostic output to stdout.** Logs, progress,  errors go to stderr. Stdout  reserved  data
   output that can be piped. A CLI tool that mixes logs  stdout breaks shell pipelines.
 **Do  hardcode exit code   all errors.**