Create a file only when it does not exist.

C#: FileMode.CreateNew prevents accidental replacement of existing content.

using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.Json;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.Globalization;
using System.Security.Cryptography;
using System.Threading;
using System.Threading.Tasks;

try
{
    using var file = new FileStream("notes.txt", FileMode.CreateNew, FileAccess.Write);
}
catch (IOException) when (File.Exists("notes.txt"))
{
    Console.WriteLine("Already exists");
}
Console.WriteLine(File.ReadAllText("notes.txt"));

Expected output EXAMPLE

Already exists
original

Details

Use Program.cs in a .NET 8 console project and run dotnet run. Uses only the .NET standard libraries. FileMode.CreateNew prevents accidental replacement of existing content.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:06:12 UTC · 8.0.425 · libc6 2.39-0ubuntu8.9

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Unprivileged snippet process, external networking disabled, read-only root filesystem, and a fresh temporary working directory. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.

Architecture / host kernel: x86_64 / 6.18.54.1-microsoft-standard-WSL2
Snippet identity / locale: uid=65534, gid=65534 (unprivileged) / C.UTF-8
Interpreter invocation: dotnet build --no-restore && dotnet out/csharp.dll
Container image ID: sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd
Code SHA-256: 92acbd9a25914632fee6976a7186bd84b69ff881798d53d5e4bb6c3579a20d9d
Dependencies: libc6 2.39-0ubuntu8.9

Test input setup

These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.

printf original > notes.txt

Observed standard output

Already exists
original

Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.

Additional assertions that passed

Checks recorded by the test harness. Where Python assertions are shown, browser cases use Playwright and simulator cases may inspect pin transitions: result.stdout is captured output, work is the temporary directory, and Path is pathlib.Path. The harness also imports json and csv.

assert Path('notes.txt').read_text() == 'original'