C# File-based apps skill

SKILL.md

name	description
csharp-scripts	Run single-file C# programs as scripts for quick experimentation, prototyping, and concept testing. Use when the user wants to write and execute a small C# program without creating a full project.

C# Scripts

When to Use

• Testing a C# concept, API, or language feature with a quick one-file program
• Prototyping logic before integrating it into a larger project

When Not to Use

• The user needs a full project with multiple files or project references
• The user is working inside an existing .NET solution and wants to add code there
• The program is too large or complex for a single file

Inputs

Input	Required	Description
C# code or intent	Yes	The code to run, or a description of what the script should do

Workflow

Step 1: Check the .NET SDK version

Run dotnet --version to verify the SDK is installed and note the major version number. File-based apps require .NET 10 or later. If the version is below 10, follow the fallback for older SDKs instead.

Step 2: Write the script file

Create a single .cs file using top-level statements. Place it outside any existing project directory to avoid conflicts with .csproj files.

// hello.cs
Console.WriteLine("Hello from a C# script!");

var numbers = new[] { 1, 2, 3, 4, 5 };
Console.WriteLine($"Sum: {numbers.Sum()}");

Guidelines:

• Use top-level statements (no Main method, class, or namespace boilerplate)
• Place using directives at the top of the file
• Place type declarations (classes, records, enums) after all top-level statements

Step 3: Run the script

dotnet hello.cs

Builds and runs the file automatically. Cached so subsequent runs are fast. Pass arguments after --:

dotnet hello.cs -- arg1 arg2 "multi word arg"

Step 4: Add NuGet packages (if needed)

Use the #:package directive at the top of the file to reference NuGet packages. Always specify a version:

#:package Humanizer@2.14.1

using Humanizer;

Console.WriteLine("hello world".Titleize());

Step 5: Clean up

Remove the script file when the user is done. To clear cached build artifacts:

dotnet clean hello.cs

Unix shebang support

On Unix platforms, make a .cs file directly executable:

1. Add a shebang as the first line of the file:

   #!/usr/bin/env dotnet
   Console.WriteLine("I'm executable!");

2. Set execute permissions:

   chmod +x hello.cs

3. Run directly:

   ./hello.cs

Use LF line endings (not CRLF) when adding a shebang. This directive is ignored on Windows.

Source-generated JSON

File-based apps enable native AOT by default. Reflection-based APIs like JsonSerializer.Serialize<T>(value) fail at runtime under AOT. Use source-generated serialization instead:

using System.Text.Json;
using System.Text.Json.Serialization;

var person = new Person("Alice", 30);
var json = JsonSerializer.Serialize(person, AppJsonContext.Default.Person);
Console.WriteLine(json);

var deserialized = JsonSerializer.Deserialize(json, AppJsonContext.Default.Person);
Console.WriteLine($"Name: {deserialized!.Name}, Age: {deserialized.Age}");

record Person(string Name, int Age);

[JsonSerializable(typeof(Person))]
partial class AppJsonContext : JsonSerializerContext;

Converting to a project

When a script outgrows a single file, convert it to a full project:

dotnet project convert hello.cs

Fallback for .NET 9 and earlier

If the .NET SDK version is below 10, file-based apps are not available. Use a temporary console project instead:

mkdir -p /tmp/csharp-script && cd /tmp/csharp-script
dotnet new console -o . --force

Replace the generated Program.cs with the script content and run with dotnet run. Add NuGet packages with dotnet add package <name>. Remove the directory when done.

Validation

• dotnet --version reports 10.0 or later (or fallback path is used)
• The script compiles without errors (can be checked explicitly with dotnet build <file>.cs)
• dotnet <file>.cs produces the expected output
• Script file and cached artifacts are cleaned up after the session

Common Pitfalls

Pitfall	Solution
.cs file is inside a directory with a .csproj	Move the script outside the project directory, or use dotnet run --file file.cs
#:package without a version	Specify a version: #:package PackageName@1.2.3 or @* for latest
Reflection-based JSON serialization fails	Use source-generated JSON with JsonSerializerContext (see Source-generated JSON)
Unexpected build behavior or version errors	File-based apps inherit global.json, Directory.Build.props, Directory.Build.targets, and nuget.config from parent directories. Move the script to an isolated directory if the inherited settings conflict

More info

See https://learn.microsoft.com/en-us/dotnet/core/sdk/file-based-apps for a full reference on file-based apps.