using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Runtime.Serialization.Formatters;

using System.IO;

using System.Data;

using System.ComponentModel.DataAnnotations;

using System.Globalization;

using System.Reflection;

using System.Collections.ObjectModel;

​

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

using Newtonsoft.Json.Converters;

using Newtonsoft.Json.Serialization;    

​

using NUnit.Framework; // Throws NUnit.Framework.AssertionException

​

//https://stackoverflow.com/questions/72153012/json-dictionary-payload-for-net-core-api

​

    public class NoDuplicateKeysDictionaryConverter<TValue> : NoDuplicateKeysDictionaryConverter<Dictionary<string, TValue>, TValue> 

    {

    }

​

    public class NoDuplicateKeysDictionaryConverter<TDictionary, TValue> : JsonConverter<TDictionary> where TDictionary : IDictionary<string, TValue>

    {

        public override TDictionary ReadJson(JsonReader reader, Type objectType, TDictionary existingValue, bool hasExistingValue, JsonSerializer serializer)

        {

            if (reader.MoveToContentAndAssert().TokenType == JsonToken.Null)

                return typeof(TDictionary).IsValueType && Nullable.GetUnderlyingType(typeof(TDictionary)) == null ? throw new JsonSerializationException("null value") : default;

            reader.AssertTokenType(JsonToken.StartObject);

            var dictionary = existingValue ?? (TDictionary)serializer.ContractResolver.ResolveContract(typeof(TDictionary)).DefaultCreator();

            // Todo: decide whether you want to clear the incoming dictionary.

            while (reader.ReadToContentAndAssert().TokenType != JsonToken.EndObject)

            {

                var key = (string)reader.AssertTokenType(JsonToken.PropertyName).Value;

                var value = serializer.Deserialize<TValue>(reader.ReadToContentAndAssert());

                // https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.idictionary-2.add#exceptions

                // Add() will throw an ArgumentException when an element with the same key already exists in the IDictionary<TKey,TValue>.

                dictionary.Add(key, value);

            }

            return dictionary;

        }

​

        public override bool CanWrite => false;

        public override void WriteJson(JsonWriter writer, TDictionary value, JsonSerializer serializer) => throw new NotImplementedException();

    }

    public static partial class JsonExtensions

    {

        public static JsonReader AssertTokenType(this JsonReader reader, JsonToken tokenType) => 

            reader.TokenType == tokenType ? reader : throw new JsonSerializationException(string.Format("Unexpected token {0}, expected {1}", reader.TokenType, tokenType));

        public static JsonReader ReadToContentAndAssert(this JsonReader reader) =>

            reader.ReadAndAssert().MoveToContentAndAssert();

​

        public static JsonReader MoveToContentAndAssert(this JsonReader reader)

        {

            if (reader == null)

                throw new ArgumentNullException();

            if (reader.TokenType == JsonToken.None)       // Skip past beginning of stream.

                reader.ReadAndAssert();

            while (reader.TokenType == JsonToken.Comment) // Skip past comments.

                reader.ReadAndAssert();

            return reader;

        }

​

        public static JsonReader ReadAndAssert(this JsonReader reader)

        {

            if (reader == null)

                throw new ArgumentNullException();

            if (!reader.Read())

                throw new JsonReaderException("Unexpected end of JSON stream.");

            return reader;

        }

    }

    public class Model

    {

        [Newtonsoft.Json.JsonConverter(typeof(NoDuplicateKeysDictionaryConverter<string>))]

        public Dictionary<string, string> Attributes { get; set; }

    }

​

    class TestClass

    {

        public static void Test()

        {

            var json = @"{""Attributes"":{""a"":""1"",""a"":""2""}}";

            var ex = Assert.Throws<ArgumentException>(() => JsonConvert.DeserializeObject<Model>(json));

            Console.WriteLine("Exception successfully thrown: \"{0}: {1}\"", ex.GetType().FullName, ex.Message);

​

            var json2 = @"{""Attributes"":{""a"":""1"",""b"":""2""}}";

            var model = JsonConvert.DeserializeObject<Model>(json2);

            Assert.AreEqual(2, model.Attributes.Count);

            var json2back = JsonConvert.SerializeObject(model);

            Assert.IsTrue(JToken.DeepEquals(JToken.Parse(json2), JToken.Parse(json2back)));

        }   

        static string GetJson() => @"

";

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1})", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , Environment.Version);

            Console.WriteLine("{0} version: {1}", typeof(JsonSerializer).Assembly.GetName().Name,  typeof(JsonSerializer).Assembly.FullName);

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

                Console.WriteLine("Failed with unhandled exception: ");

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​