csharp-scripts

File-Based C# Apps

When to Use

• Testing a C# concept, API, or language feature with a quick file-based app
• Prototyping logic before integrating it into a larger project
• Building a small utility from one entry-point file and a few helper .cs files

When Not to Use

• The user asks for a language-agnostic quick script, throwaway computation, or shell/Python/PowerShell-style automation
• The user needs a full project, solution integration, or project references in an existing app
• The user is working inside an existing .NET solution and wants to add code there
• The app is large enough that project structure, build customization, tests, or publish configuration should live in a .csproj

Inputs

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

Workflow

Step 1: Check the .NET SDK version

Run dotnet --version to verify the SDK is installed and note the full version, including the feature band. File-based apps require .NET 10 or later. #:include, #:exclude, and transitive directive processing require SDK 10.0.300 or later; SDK 10.0.100/10.0.200 builds can run single-file apps but do not support those multi-file directives. If the version is below 10, follow the fallback for older SDKs instead.

Step 2: Write the app file

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

#!/usr/bin/env dotnet
// hello.cs
Console.WriteLine("Hello from a file-based app!");

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 (after the #! line and any #: directives if present)
• Place type declarations (classes, records, enums) after all top-level statements