Add a dictionary entry only if absent.

C#: TryAdd reports a duplicate key without replacing its existing value.

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;

var values = new Dictionary<string, int> { ["count"] = 1 };
Console.WriteLine(values.TryAdd("count", 2));
Console.WriteLine(values["count"]);

Expected output EXAMPLE

False
1

Details

Use Program.cs in a .NET 8 console project and run dotnet run. Uses only the .NET standard libraries. TryAdd reports a duplicate key without replacing its existing value.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:05:35 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: bb497ee759fc6bb4a2ed41e4f036e2bdc61f40011e188e7daf0fd08e3c27ea48
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.

# No input files required.

Observed standard output

False
1

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