using System;

using System.Collections.Generic;

using System.Runtime.InteropServices;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        Dictionary<int, string> d = new();

        ref string s = ref d.GetValueRefOrAddDefault(42, out var exists);

        s = exists ? "Still the answer" : "The answer";

        Console.WriteLine(JsonSerializer.Serialize(d));

    }

}

​

public static class Extensions

{

    public static ref TValue GetValueRefOrAddDefault<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, TKey key) 

        where TKey : notnull

        => ref CollectionsMarshal.GetValueRefOrAddDefault(dictionary, key, out _);

}