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C#: file

Handbook C# · Practical Guide

Limits a type’s visibility to its source file. It modifies a top-level type, rather than a class member.

Simple example (Example.cs)

Save this example separately as Example.cs. Run it independently of the other examples on this page.

Example.cs:

// PROJECT-EXAMPLE
System.Console.WriteLine(Helper.Double(3));
file static class Helper { public static int Double(int x) => x * 2; }

Result:

6

Syntax

file class Helper { ... }

Example 1: simple form

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        Console.WriteLine(Helper.Double(3));
    }
}
file static class Helper { public static int Double(int value) => value * 2; }

Example 2: another practical form

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        Token value = new(7);
        Console.WriteLine(value.Value);
    }
}
file readonly record struct Token(int Value);

Official source: Microsoft Learn.