using System;

using System.Collections.Generic;

using System.Runtime.InteropServices;

using System.Linq;

​

public static class Program

{

    public static void Main()

    {

        Dictionary<string, List<int>> dictionary = new();

​

        List<int> list;

        list = dictionary.GetOrAdd("A", key => new List<int>());

        list.Add(13);

        list = dictionary.GetOrAdd("A", key => new List<int>());

        list.Add(42);

        list = dictionary.GetOrAdd("B", (key, arg) => new List<int>(arg), factoryArgument: 10);

        list.Add(120);

        Console.WriteLine($"dictionary: {String.Join(", ", dictionary.Select(e => $"[{e.Key}: [{String.Join(", ", e.Value)}]]"))}");

    }

​

    /// <summary>

    /// Adds a key/value pair to the dictionary by using the specified function

    /// if the key does not already exist. Returns the new value, or the

    /// existing value if the key exists.

    /// </summary>

    public static TValue GetOrAdd<TKey, TValue>(

        this Dictionary<TKey, TValue> dictionary,

        TKey key,

        Func<TKey, TValue> valueFactory)

    {

        ArgumentNullException.ThrowIfNull(dictionary);

        ArgumentNullException.ThrowIfNull(valueFactory);

​

        ref TValue value = ref CollectionsMarshal

            .GetValueRefOrAddDefault(dictionary, key, out bool exists);

        if (!exists)

        {

            try { value = valueFactory(key); }

            catch { dictionary.Remove(key); throw; }

        }

        return value;

    }

​

    /// <summary>

    /// Adds a key/value pair to the dictionary by using the specified function

    /// and an argument if the key does not already exist. Returns the new value,

    /// or the existing value if the key exists.

    /// </summary>

    public static TValue GetOrAdd<TKey, TValue, TArg>(

        this Dictionary<TKey, TValue> dictionary,

        TKey key,

        Func<TKey, TArg, TValue> valueFactory,

        TArg factoryArgument)

    {

        ArgumentNullException.ThrowIfNull(dictionary);

        ArgumentNullException.ThrowIfNull(valueFactory);

​

        ref TValue value = ref CollectionsMarshal

            .GetValueRefOrAddDefault(dictionary, key, out bool exists);

        if (!exists)

        {

            try { value = valueFactory(key, factoryArgument); }

            catch { dictionary.Remove(key); throw; }

        }

        return value;

    }

}

​