dotnet-file-based-apps

Imported from wshaddix/dotnet-skills (skills/dotnet-file-based-apps/SKILL.md). Install upstream with npx skills add wshaddix/dotnet-skills --skill dotnet-file-based-apps. Copyright stays with the author.

dotnet-file-based-apps

.NET 10 SDK file-based apps let you build, run, and publish C# applications from a single .cs file without creating a .csproj project file. The SDK auto-generates project configuration from #: directives embedded in the source file. This feature targets scripts, utilities, and small applications where traditional project scaffolding is unnecessary.

This is NOT file I/O. For FileStream, RandomAccess, FileSystemWatcher, and path handling, see [skill:dotnet-file-io].

Prerequisites: Requires .NET 10 SDK or later. Run [skill:dotnet-version-detection] to confirm SDK version.

Cross-references: [skill:dotnet-version-detection] for SDK version gating, [skill:dotnet-project-analysis] for project-based analysis (file-based apps have no .csproj), [skill:dotnet-scaffold-project] for csproj-based project scaffolding.

Directives Overview

File-based apps use #: directives to configure the build. Directives are SDK-level instructions, not C# syntax. They must appear at the top of the .cs file, before any C# code.

Four directive types are supported:

#:package Directive

Adds a NuGet package reference. Specify the package name, optionally followed by @version.

#:package Newtonsoft.Json
#:package Serilog@3.1.1
#:package Spectre.Console@*

Version behavior:

• @version -- pins to a specific version
• @* -- uses the latest stable version (NuGet floating version)
• No version -- only works when Central Package Management (CPM) is configured with a Directory.Packages.props file; otherwise, specify a version explicitly or use @*

#:sdk Directive

Specifies which SDK to use. Defaults to Microsoft.NET.Sdk if omitted.

#:sdk Microsoft.NET.Sdk.Web

#:sdk Aspire.AppHost.Sdk@9.2.0

Use this directive to access SDK-specific features. For example, Microsoft.NET.Sdk.Web enables ASP.NET Core features and automatically includes *.json configuration files in the build.

#:property Directive

Sets an MSBuild property value. Use this to customize build behavior.

#:property TargetFramework=net10.0
#:property PublishAot=false

Conditional Property Values

Property directives support MSBuild property functions and expressions for conditional configuration.

Environment variables with defaults:

#:property LogLevel=$([MSBuild]::ValueOrDefault('$(LOG_LEVEL)', 'Information'))

Conditional expressions:

#:property EnableLogging=$([System.Convert]::ToBoolean($([MSBuild]::ValueOrDefault('$(ENABLE_LOGGING)', 'true'))))

#:project Directive

References another project file or directory containing a project file. Use this to share code between a file-based app and a traditional project.

#:project ../SharedLibrary/SharedLibrary.csproj

The referenced project is built and linked as a project reference, just like <ProjectReference> in a .csproj.

CLI Commands

The .NET CLI supports file-based apps through familiar commands.

Run

# Preferred: pass file directly
dotnet run app.cs

# Explicit --file option
dotnet run --file app.cs

# Shorthand (no 'run' subcommand)
dotnet app.cs

# Pass arguments after --
dotnet run app.cs -- arg1 arg2

When a .csproj exists in the current directory, dotnet run app.cs (without --file) runs the project and passes app.cs as an argument to preserve backward compatibility. Use dotnet run --file app.cs to force file-based execution.

Pipe from stdin

echo 'Console.WriteLine("hello");' | dotnet run -

The - argument reads C# code from standard input. Useful for quick testing and shell script integration.

Build

dotnet build app.cs

Build output goes to a cached location under the system temp directory by default. Override with --output or #:property OutputPath=./output.

Clean

# Clean build artifacts for a specific file
dotnet clean app.cs

# Clean all file-based app caches in the current directory
dotnet clean file-based-apps

# Clean caches unused for N days (default: 30)
dotnet clean file-based-apps --days 7

Publish

dotnet publish app.cs

File-based apps enable native AOT by default. The output goes to an artifacts directory next to the .cs file. Disable AOT with #:property PublishAot=false.

Pack as .NET Tool

dotnet pack app.cs

File-based apps set PackAsTool=true by default. Disable with #:property PackAsTool=false.

Restore

dotnet restore app.cs

Restore runs implicitly on build/run. Pass --no-restore to dotnet build or dotnet run to skip it.

Shell Execution (Unix)

Enable direct execution on Unix-like systems with a shebang line.

#!/usr/bin/env dotnet
#:package Spectre.Console

using Spectre.Console;

AnsiConsole.MarkupLine("[green]Hello, World![/]");

chmod +x app.cs
./app.cs

The file must use LF line endings (not CRLF) and must not include a BOM.

Launch Profiles

File-based apps support launch profiles via a flat [AppName].run.json file in the same directory as the source file. For app.cs, create app.run.json:

{
  "profiles": {
    "https": {
      "commandName": "Project",
      "launchBrowser": true,
      "applicationUrl": "https://localhost:5001;http://localhost:5000",
      "environmentVariables": {
        "ASPNETCORE_ENVIRONMENT": "Development"
      }
    }
  }
}

Select a profile with --launch-profile:

dotnet run app.cs --launch-profile https

If both app.run.json and Properties/launchSettings.json exist, the traditional location takes priority.

User Secrets

File-based apps generate a stable user secrets ID from the file's full path.

dotnet user-secrets set "ApiKey" "your-secret-value" --file app.cs
dotnet user-secrets list --file app.cs

Implicit Build Files

File-based apps respect MSBuild and NuGet configuration files in the same or parent directories:

• Directory.Build.props -- inherited MSBuild properties
• Directory.Build.targets -- inherited MSBuild targets
• Directory.Packages.props -- Central Package Management versions
• nuget.config -- NuGet package source configuration
• global.json -- SDK version pinning

Be mindful of these files when placing file-based apps in a repository that also contains traditional projects. Inherited properties may cause unexpected build behavior.

Build Caching

The SDK caches build outputs based on source content, directives, SDK version, and implicit build files. Caching improves repeated dotnet run performance.

Known caching pitfalls:

• Changes to implicit build files (Directory.Build.props, etc.) may not trigger rebuilds
• Moving files to different directories does not invalidate the cache
• Concurrent execution of the same file-based app can cause build contention errors -- build first with dotnet build app.cs, then run multiple instances with dotnet run app.cs --no-build

Clear the cache with dotnet clean app.cs or dotnet clean file-based-apps.

Folder Layout

Do not place file-based apps inside a .csproj project's directory tree. The project's implicit build configuration will interfere.

# Recommended layout
repo/
  src/
    MyProject/
      MyProject.csproj
      Program.cs
  scripts/           # Separate directory for file-based apps
    utility.cs
    tool.cs

Migration: File-Based to Project-Based

When a file-based app outgrows a single file, convert to a traditional project.

Automatic Conversion

dotnet project convert app.cs

This creates a new directory named after the app, containing:

• A .csproj with equivalent SDK, properties, and package references derived from the #: directives
• A copy of the .cs file with #: directives removed

The original .cs file is left untouched.

When to Convert

Convert to a project-based app when:

• Multiple source files are needed
• Complex MSBuild customization is required beyond what #:property supports
• The app needs dotnet test with a test framework
• The app needs integration with CI/CD workflows that expect a .csproj
• Team members need IDE project support (Solution Explorer, etc.)

Default Behaviors

File-based apps differ from project-based apps in several default settings:

Agent Gotchas

1. Do not confuse file-based apps with file I/O -- dotnet-file-based-apps covers running C# without a project file (.NET 10 SDK feature). For FileStream, RandomAccess, and path handling, use [skill:dotnet-file-io].
2. Do not use #: directives after C# code -- all directives must appear at the top of the file, before any C# statements, using directives, or namespace declarations. The SDK ignores directives placed later in the file.
3. Do not omit package versions without CPM -- #:package SomePackage without a version only works when Central Package Management is configured via Directory.Packages.props. Without CPM, use #:package SomePackage@1.0.0 or #:package SomePackage@*.
4. Do not assume dotnet build and dotnet test work the same -- dotnet build app.cs compiles via a virtual project, but dotnet test does not apply to file-based apps. Convert to a project for test framework support.
5. Do not place file-based apps inside a .csproj project directory -- the project's implicit build files (Directory.Build.props, etc.) will affect the file-based app, causing unexpected behavior. Use a separate directory.
6. Do not run concurrent instances without pre-building -- parallel execution of the same file-based app causes build output contention. Build first with dotnet build app.cs, then run instances with dotnet run app.cs --no-build.
7. Do not forget backward compatibility -- when a .csproj exists in the current directory, dotnet run app.cs passes app.cs as an argument to the project rather than running it as a file-based app. Use dotnet run --file app.cs to force file-based execution.
8. Do not use CRLF line endings with shebang -- Unix shebang execution requires LF line endings and no BOM. Files with CRLF will fail with /usr/bin/env: 'dotnet\r': No such file or directory.

References

• File-based apps - .NET
• Native AOT deployment
• Central Package Management
• MSBuild property functions