Simple json serializer written in C#

Json.cs

using System;
using System.Collections.Generic;
using System.Text;
using System.Linq;

public static class Json
{
    /// <summary>
    /// Implement this interface if you want custom
    /// json serialization for your class
    /// </summary>
    public interface IJSONSerializable
    {
        public string ToJson();
    }

    private static string Dictionary2Json(Dictionary<string, object> dictionary)
    {
        return "{" + string.Join(",", dictionary.Select(x => $"\"{x.Key}\":{ Object2Json(x.Value) }") ) + "}";
    }

    private static string List2Json(List<object> list)
    {
        return "[" + string.Join(",", list.Select(Object2Json) ) + "]";
    }

    private static string Array2Json(object[] array)
    {
        return "[" + string.Join(",", array.Select(Object2Json) ) + "]";
    }

    /// <summary>
    /// Serialize an object into JSON string
    /// </summary>
    /// <param name="obj">Object to serialize</param>
    public static string Object2Json(object obj)
    {
        return obj switch
        {
            // Nested object
            Dictionary<string, object> dict => Dictionary2Json(dict),
            // Object list
            List<object> list               => List2Json(list),
            // Object array
            object[] array                  => Array2Json(array),
            // User-defined json serialization
            IJSONSerializable objValue      => objValue.ToJson(),
            // String value, wrap with quotation marks
            string strValue                 => $"\"{strValue}\"",
            // Boolean value, should be lowercase 'true' or 'false'
            bool boolValue                  => boolValue ? "true" : "false",
            // Null value, should be lowercase 'null'
            null                            => "null",
            // Other types, just convert to string
            _                               => $"\"{obj}\""
        };
    }
}