using System;

using System.Text.Json;

using System.Text.Json.Serialization;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var emptyJson = 

            """

{

    "values": []

}

""";

        var nonEmptyJson = 

            """

{

    "values": {

        "key1": "value1",

        "key2": "value2"

    }

}

""";

        var emptyResult = JsonSerializer.Deserialize<Model>(emptyJson);

        var nonEmptyResult = JsonSerializer.Deserialize<Model>(nonEmptyJson);

        emptyResult.Dump();

        nonEmptyResult.Dump();

    }

}

​

public class Model

{

    [JsonPropertyName("values")]

    [JsonConverter(typeof(MySpecialConverter))]

    public Dictionary<string,string> Values {get;set;} = new();

}

​

public class MySpecialConverter : JsonConverter<Dictionary<string,string>>

{

    public override void Write(Utf8JsonWriter writer,

            Dictionary<string,string> dictionaryValue,

            JsonSerializerOptions options) => throw new NotSupportedException();

    public override Dictionary<string,string> Read(ref Utf8JsonReader reader,

            Type typeToConvert,

            JsonSerializerOptions options)

            {

                if (reader.TokenType == JsonTokenType.StartArray)

                {

                  reader.Read();

                  return new Dictionary<string,string>();

                }

                var converter = (JsonConverter<Dictionary<string,string>>)options.GetConverter(typeof(Dictionary<string,string>));

                return converter.Read(ref reader, typeToConvert, options);

            }

}