JSON.NET Deserialize null string to string.Empty.

SampleCodeForm.cs

using JsonConvertOptionForm.Entity;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Diagnostics;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace JsonConvertOptionForm
{
    public partial class SampleCodeForm : Form
    {
        public SampleCodeForm()
        {
            InitializeComponent();
        }

        // Add two buttons in the designer.


        private void writeTestJsonButton_Click(object sender, EventArgs e)
        {
            var x = new SomeEntity
            {
                Number = 10,
                // Text = "TextProp" + new string('X', 1024 * 1024 * 30), // 30MB // 30MBでもexistingValue 引数はセットされなかった。
                                                                                  // The existingValue argument was not set even at 30 MB.
                Text = "TextProp",
                TextNull = null,
                Products = new List<string> { "l1", "l2", null },
                Dic = new Dictionary<string, int> { { "abc", 100 }, { "efg", 100 } },
                Nested = new List<List<string>>{ 
                    new List<string> { "1-1", "1-2" },
                    new List<string> { "2-1", null, "2-3" },
                },
                UpdateDate = DateTime.Now,
                UpdateDates = new List<DateTime> { DateTime.Now, DateTime.Now, }
            };

            var json = Newtonsoft.Json.JsonConvert.SerializeObject(x, Newtonsoft.Json.Formatting.Indented);
            File.WriteAllText("sample.json", json, new UTF8Encoding(false));
        }

        private void readJsonTestButton_Click(object sender, EventArgs e)
        {

            var json =File.ReadAllText("sample.json", new UTF8Encoding(false));

            {
                var defaultDeserialize = Newtonsoft.Json.JsonConvert.DeserializeObject<SomeEntity>(json);

                Debug.Print("-- with default converter ------------------------");
                Debug.Print(defaultDeserialize.Text.Length.ToString());
                Debug.Print(defaultDeserialize.TextNull ?? "<null>");
                Debug.Print(defaultDeserialize.Number.ToString() + " " + defaultDeserialize.Number.GetType());
                Debug.Print(defaultDeserialize.Products[2] ?? "<null>");
                Debug.Print(defaultDeserialize.Nested[1][1] ?? "<null>");
                Debug.Print($"'{defaultDeserialize.Products[2] ?? "<null>"}' is null {defaultDeserialize.Products[2] == null}  : JSON上のnullアイテム が null かどうか。expect true");
                Debug.Print($"'{defaultDeserialize.Nested[1][1] ?? "<null>"}' is null {defaultDeserialize.Nested[1][1] == null}  : JSON上のnullアイテム が null かどうか。expect true");
                Debug.Print($"'{defaultDeserialize.Products[1]}' is NOT null {defaultDeserialize.Products[1] != null}  : JSON上の非nullアイテムもちゃんと取れていることの確認");
                Debug.Print($"'{defaultDeserialize.Nested[1][2]}' is NOT null {defaultDeserialize.Nested[1][2] != null}  : JSON上の非nullアイテムもちゃんと取れていることの確認");
            }

            {
                var desNumText = JsonConvert.DeserializeObject<SomeEntityWithNumberAsText>(json, new NullToEmptyForStringTypeDeserializeConverter());

                Debug.Print("-- with my StringConverter ------------------------");
                Debug.Print(desNumText.Text.Length.ToString());
                Debug.Print(desNumText.TextNull ?? "<null>");
                Debug.Print((desNumText.Number ?? "<null>") + " " + desNumText.Number.GetType() + ":  シリアル化（JSON）時点では数値オブジェクト、デシリアライズで文字列プロパティで受け付けたとき。");
                Debug.Print(desNumText.Products[2] ?? "<null>");
                Debug.Print(desNumText.Nested[1][1] ?? "<null>");
                Debug.Print($"'{desNumText.Products[2] ?? "<null>"}' is null {desNumText.Products[2] == null}  : JSON上のnullアイテム が null かどうか。expect false");
                Debug.Print($"'{desNumText.Nested[1][1] ?? "<null>"}' is null {desNumText.Nested[1][1] == null}  : JSON上のnullアイテム が null かどうか。expect false");
                Debug.Print($"'{desNumText.Products[1]}' is not null {desNumText.Products[1] != null}  : JSON上の非nullアイテムもちゃんと取れていることの確認");
                Debug.Print($"'{desNumText.Nested[1][2]}' is not null {desNumText.Nested[1][2] != null}  : JSON上の非nullアイテムもちゃんと取れていることの確認");
                Debug.Print("finish desealize");

            }

            // if you want to use settings. you can specify Converter like following.
            var settings = new JsonSerializerSettings
            {
                Converters = new List<JsonConverter> { new NullToEmptyForStringTypeDeserializeConverter(), new NullToEmptyForStringTypeDeserializeConverter() }
            };
        }

        public class NullToEmptyForStringTypeDeserializeConverter : JsonConverter<String>
        {

            // when CanWrite == false, use this only for deserialize.
            public override bool CanWrite => false;

            public override void WriteJson(JsonWriter writer, String value, JsonSerializer serializer)
            {
                throw new NotImplementedException("Not Support for Serialize");
            }

            public override String ReadJson(JsonReader reader, Type objectType, String existingValue, bool hasExistingValue, JsonSerializer serializer)
            {
                if (hasExistingValue) throw new JsonSerializationException($"{nameof(NullToEmptyForStringTypeDeserializeConverter)} does not support desealize with existingValue.");

                if (reader.Value != null && reader.ValueType != typeof(String)) {
                    // TODO: さらに複雑には別のConverterを呼んだ方が良いのかもしれないが、未検証。（間違えるとConverter同士が呼びあい無限ループの可能性もあるのでハンドル済みCOnverterが持てるオーバーロード作るとか工夫しないとだめかも）
                    // TODO: For more complexity, it may be better to call another Converter, but this has not been tested. (If you make a mistake, there is a possibility that converters will call each other, resulting in an infinite loop, so you may need to create an overload that allows handled COnverters to have their own handles.)
                    // 
                    // if (serializer.Converters.Count > 0)
                    // {
                    //     foreach (var conv in serializer.Converters)
                    //     {
                    //         if (!ReferenceEquals(this, conv) && conv.CanRead && conv.CanConvert(objectType))
                    //         {
                    //             return conv.ReadJson(reader, objectType, existingValue, serializer) as string; 
                    //         }
                    //     }
                    // }

                    // 他の型である場合は、JObjectに任せる。
                    var token = JObject.ReadFrom(reader);
                    return token.ToObject<String>();
                }

                string s = (string)reader.Value ?? "";
                return s;
            }
        }
    }
}

SomeEntity.cs

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

namespace JsonConvertOptionForm.Entity
{
    public class SomeEntity
    {
        public int Number { get; set; }

        public string Text { get; set; }
        public string TextNull { get; set; }

        public List<string> Products { get; set; }


        public Dictionary<string, int> Dic { get; set; }


        public List<List<string>> Nested { get; set; }


        public DateTime UpdateDate;

        public List<DateTime> UpdateDates;
    }

    public class SomeEntityWithNumberAsText
    {
        public string Number { get; set; }

        public string Text { get; set; }
        public string TextNull { get; set; }

        public List<string> Products { get; set; }


        public Dictionary<string, int> Dic { get; set; }


        public List<List<string>> Nested { get; set; }


        public DateTime UpdateDate;

        public List<DateTime> UpdateDates;
    }
}