using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        List<Item> items = new List<Item>

        {

            new Item { Name = "A", Value = "Value A" },

            new Item { Name = "B", Value = "Value B" }

        };

        Dictionary<string, string> dictionary = items.ToDictionary(x => x.Name, x => x.Value);

        dictionary.Add("Date", $"{DateTime.Now:yyyy-MM-dd}");

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

    }

}

​

public class Item

{

    public string Name { get; set; }

    public string Value { get; set; }

}