Csharp Scripts Agent Skill

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

Step 3: Run the app

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 directives (if needed)

Place directives at the top of the file (immediately after an optional shebang line), before any using directives or other C# code. All directives start with #:.

#:package — NuGet package references

Specify a version unless the app intentionally uses central package management. Use @* when the latest available package is acceptable (or @*-* for pre-release):

#:package [email protected]

using Humanizer;

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

#:property — MSBuild properties

Set any MSBuild property inline. Syntax: #:property PropertyName=Value

#:property AllowUnsafeBlocks=true
#:property PublishAot=false
#:property NoWarn=CS0162

MSBuild expressions and property functions are supported:

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

Common properties:

| Property | Purpose |

|----------|---------|

| AllowUnsafeBlocks=true | Enable unsafe code |

| PublishAot=false | Disable native AOT (enabled by default) |

| NoWarn=CS0162;CS0219 | Suppress specific warnings |

| LangVersion=preview | Enable preview language features |

| InvariantGlobalization=false | Enable culture-specific globalization |

#:project — Project references

Reference another project by relative path:

#:project ../MyLibrary/MyLibrary.csproj

#:ref — File-based app references

Reference another .cs file as a separate file-based app project when it should compile into a separate assembly instead of being included in the same compilation. Use #:include for ordinary helper files that should share the same assembly as the entry point; use #:ref when you want project-reference-like boundaries.

#:property ExperimentalFileBasedProgramEnableRefDirective=true
#:ref ../Shared/Formatter.cs

Console.WriteLine(Formatter.Title("hello world"));

Guidelines:

• The referenced file is compiled as its own virtual project and added as a project reference.
• If the referenced file is a library without top-level statements, put #:property OutputType=Library in that referenced file.
• Members that must be consumed by the referencing app should be public; internal members are not visible across the assembly boundary.
• #:ref is transitive: a referenced file can contain its own #:ref and other #: directives.
• Relative paths are resolved relative to the file containing the directive.
• Some SDK builds require #:property ExperimentalFileBasedProgramEnableRefDirective=true; remove that property if the SDK accepts #:ref without it.

#:sdk — SDK selection

Override the default SDK (Microsoft.NET.Sdk):

#:sdk Microsoft.NET.Sdk.Web

#:include and #:exclude — Multi-file apps

In .NET SDK 10.0.300 and later, file-based apps can include additional files in the same virtual project. Check the full dotnet --version output before using these directives; a 10.0.100 or 10.0.200 SDK is still .NET 10 but does not support them. Use #:include for helper source files and supported assets, and #:exclude to remove files from an include pattern or default item set.

#!/usr/bin/env dotnet
#:include Helpers.cs
#:include Models/*.cs
#:exclude Models/Generated/*.cs

Console.WriteLine(Formatter.Title("hello world"));

Guidelines:

• Treat the file passed to dotnet as the entry point; put top-level statements there.
• Put declarations such as classes, records, and enums in included .cs files.
• Prefer explicit globs such as Helpers.cs or Models/*.cs over broad recursive globs.
• Paths are resolved relative to the file containing the directive.
• Include directives from non-entry-point C# files are processed too, so a helper file can declare its own #:package, #:property, #:sdk, #:project, #:ref, #:include, or #:exclude directives.
• Avoid duplicate directives across included files unless the directive kind explicitly supports duplicates; duplicate #:package, #:property, #:sdk, #:include, and #:exclude entries can fail.
• When an app uses #:include, add a shebang (#!/usr/bin/env dotnet) to the entry-point file on Unix-like systems to make the entry point clear to tools. Use LF line endings and no BOM for shebang files.

Example layout:

scratch/
    hello.cs
    Helpers.cs
    Models/
        Person.cs

#!/usr/bin/env dotnet
// hello.cs
#:include Helpers.cs
#:include Models/*.cs

var person = new Person("Ada");
Console.WriteLine(Formatter.Title(person.Name));

// Helpers.cs
static class Formatter
{
    public static string Title(string value) => value.ToUpperInvariant();
}

// Models/Person.cs
record Person(string Name);

Step 5: Clean up

Remove the app files 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:

• Add a shebang as the first line of the file:

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

• Set execute permissions:

    chmod +x hello.cs

• 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 file-based app outgrows this workflow, 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-file-based-app && cd /tmp/csharp-file-based-app
dotnet new console -o . --force

Replace the generated Program.cs with the app 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)
• [ ] If the app uses #:include, #:exclude, or transitive directives from included files, dotnet --version reports SDK 10.0.300 or later
• [ ] The app compiles without errors (can be checked explicitly with dotnet build <file>.cs)
• [ ] dotnet <file>.cs produces the expected output
• [ ] Multi-file apps include every required helper file and exclude unintended matches
• [ ] App files and cached artifacts are cleaned up after the session

Common Pitfalls

| Pitfall | Solution |

|---------|----------|

| .cs file is inside a directory with a .csproj | Move the app outside the project directory, or use dotnet run --file file.cs |

| #:package without a version | Specify a version: #:package [email protected] or @* for latest |

| #:property with wrong syntax | Use PropertyName=Value with no spaces around = and no quotes: #:property AllowUnsafeBlocks=true |

| Directives placed after C# code | All #: directives must appear immediately after an optional shebang line (if present) and before any using directives or other C# statements |

| Helper file is not compiled | Add #:include Helper.cs or an appropriate glob to the entry-point file |

| Shared file needs an assembly boundary | Use #:ref Shared.cs instead of #:include Shared.cs, and set #:property OutputType=Library in the referenced file if it has no entry point |

| Broad include pulls in unrelated files | Prefer narrow include patterns and use #:exclude for generated, backup, or experimental files |

| Duplicate directives in included files | Keep package, property, SDK, include, and exclude directives unique across the entry point and included C# files |

| 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 app 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.