using System;

using System.Collections;

using System.Collections.Generic;

using System.Text.Json;

using System.Text.Json.Serialization;

public class Program

{

    public static void Main()

    {

        var obj = JsonSerializer.Deserialize<SimpleClassWitExtData>(@"{""key"": ""value""}");

        Console.WriteLine("value" == obj.ExtensionData["key"].GetString());

    }

    private class SimpleClassWitExtData

    {

        [JsonExtensionData]

        public GenericIDictionaryWrapper<string, JsonElement> ExtensionData { get; set; }

    }

    public class GenericIDictionaryWrapper<TKey, TValue> : IDictionary<TKey, TValue>

    {

        private Dictionary<TKey, TValue> _dict = new Dictionary<TKey, TValue>();

​

        public TValue this[TKey key] { get => ((IDictionary<TKey, TValue>)_dict)[key]; set => ((IDictionary<TKey, TValue>)_dict)[key] = value; }

​

        public ICollection<TKey> Keys => ((IDictionary<TKey, TValue>)_dict).Keys;

​

        public ICollection<TValue> Values => ((IDictionary<TKey, TValue>)_dict).Values;

​

        public int Count => ((IDictionary<TKey, TValue>)_dict).Count;

​

        public bool IsReadOnly => ((IDictionary<TKey, TValue>)_dict).IsReadOnly;

​

        public void Add(TKey key, TValue value)

        {

            ((IDictionary<TKey, TValue>)_dict).Add(key, value);

        }

​

        public void Add(KeyValuePair<TKey, TValue> item)

        {

            ((IDictionary<TKey, TValue>)_dict).Add(item);

        }

​

        public void Clear()

        {

            ((IDictionary<TKey, TValue>)_dict).Clear();

        }

​

        public bool Contains(KeyValuePair<TKey, TValue> item)

        {

            return ((IDictionary<TKey, TValue>)_dict).Contains(item);

        }

​

        public bool ContainsKey(TKey key)

        {

            return ((IDictionary<TKey, TValue>)_dict).ContainsKey(key);

        }

​

        public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)

        {

            ((IDictionary<TKey, TValue>)_dict).CopyTo(array, arrayIndex);

        }

​

        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()

        {

            return ((IDictionary<TKey, TValue>)_dict).GetEnumerator();

        }

​

        public bool Remove(TKey key)

        {

            return ((IDictionary<TKey, TValue>)_dict).Remove(key);

        }

​

        public bool Remove(KeyValuePair<TKey, TValue> item)

        {

            return ((IDictionary<TKey, TValue>)_dict).Remove(item);

        }

​

        public bool TryGetValue(TKey key, out TValue value)

        {

            return ((IDictionary<TKey, TValue>)_dict).TryGetValue(key, out value);

        }

​

        IEnumerator IEnumerable.GetEnumerator()

        {

            return ((IDictionary<TKey, TValue>)_dict).GetEnumerator();

        }

    }

}