###### tags: `C#`,`2022`

# System.Text.Json

System.Text.Json是.NET Core 3.0新的命名空間，取代掉原本對Newtonsoft Json的依賴。

內建的System.Text.Json 效能更強大(利用Span<T>)，也降低記憶體使用量，以下介紹一些常用的功能。

## Serialize 序列化
物件解析成字串

參數之後可利用多載的方法，透過 JsonSerializerOptions 指定序列化的參數設定

string strJson = JsonSerializer.Serialize(object);

## Deserialize 反序列化

字串解析成物件

var jsonString = @"{""PId"":1,""Name"":""Ashley""}";
 
var emp = JsonSerializer.Deserialize<Employee>(jsonString);

## 屬性標示

如果希望有忽略或是變更名稱的屬性，可以加上以下標籤

    public class Employee
    {
        [JsonIgnore]
        public int PId { get; set; }
        public string Name { get; set; }

        public Employee Manager { get; set; }

        [JsonPropertyName("DirectReports List")]
        public List<Employee> DirectReports { get; set; }
    }

## 重複使用 JsonSerializerOptions

如果每次都new 一個新的實例，會導致效能下降

如果真的要改變JsonSerializerOptions,利用"複製"的方式來實作。

JsonSerializerOptions optionsCopy = new(options);


上圖可以看到 options 的時間 和 每次都新增一個實力的時間，差非常多。

完整程式碼:

  
            Forecast forecast = new(DateTime.Now, 40, "Hot");
            
            // WriteIndented: true 顯示容易閱讀的 JSON 格式
           // 預設是 false 顯示最小化的JSON格式
            JsonSerializerOptions options = new() { WriteIndented = true };
            int iterations = 100000;

            var watch = Stopwatch.StartNew();
            for (int i = 0; i < iterations; i++)
            {
                Serialize(forecast, options);
            }

            watch.Stop();
            Console.WriteLine($"using one options instance: {watch.ElapsedMilliseconds}");

            watch = Stopwatch.StartNew();
            for (int i = 0; i < iterations; i++)
            {
                Serialize(forecast);
            }
            watch.Stop();
            Console.WriteLine($"creating new options instances: {watch.ElapsedMilliseconds}");

            JsonSerializerOptions optionsCopy = new(options);


           
            Console.WriteLine($"forecast Serialize : {JsonSerializer.Serialize(forecast, options)}");

            // WriteIndented: true 顯示容易閱讀的 JSON 格式
            // 預設是 false 顯示最小化的JSON格式
            optionsCopy.WriteIndented = false;
            string forecastJson =
                JsonSerializer.Serialize<Forecast>(forecast, optionsCopy);
            Console.WriteLine($"optionsCopy Output JSON:\n{forecastJson}");
        }

        private static void Serialize(Forecast forecast, 
        JsonSerializerOptions? options = null)
        {
            _ = JsonSerializer.Serialize<Forecast>(
                forecast,
                options ?? new JsonSerializerOptions() { WriteIndented = true });
        

## JsonDocument

Json讀取Sample


     var json = @"[
                            {
                                ""PID"":1,
                                ""Name"":""Ashley""
                            },
                            {
                                ""PID"":2,
                                ""Name"":""Alison""
                            }
                            ]";

            using (JsonDocument document = JsonDocument.Parse(json))
            {
                var sumPid = 0;

                foreach (var element in document.RootElement.EnumerateArray())
                {
                    var tempCount = element.GetProperty("PID").GetInt16();

                    sumPid += tempCount;

                    Console.WriteLine($"Name : {element.GetProperty("Name")}");
                }

                Console.WriteLine($"sumPid : {sumPid}");
            }

## Utf8JsonWriter

Json寫入Sample


            var options = new JsonWriterOptions
            {
                Indented = true
            };

            using (var stream = new MemoryStream())
            {
                using (var writer = new Utf8JsonWriter(stream, options))
                {
                    writer.WriteStartObject();

                    writer.WriteNumber("PID", 1);
                    writer.WriteString("Name", "Ashley");
    
                    writer.WriteStartArray("ExtraNumberArray");
                    for (var i = 0; i < 5; i++)
                    {
                        writer.WriteNumberValue(i);
                    }
                    writer.WriteEndArray();

                    writer.WriteStartArray("ExtraArray");
                    writer.WriteStringValue("Ash");
                    writer.WriteStringValue(DateTime.Now);
                    writer.WriteStringValue("425-000-1213");
                    writer.WriteNullValue();
                    writer.WriteEndArray();
                   
                    writer.WriteStartObject("Address");
                    writer.WriteString("street", "street");
                    writer.WriteString("city", "Taipei");
                    writer.WriteNumber("zip", 110);
                    writer.WriteString("option", string.Empty);
                    writer.WriteStartArray("people");
                    writer.WriteStringValue("Ash");
                    writer.WriteStringValue("Ash2");
                    writer.WriteEndArray();
                    writer.WriteEndObject();

                    writer.WriteEndObject();
                }

                string json = Encoding.UTF8.GetString(stream.ToArray());
                Console.WriteLine(json);
            }

## Reference
[Microsoft Doc](https://docs.microsoft.com/zh-tw/dotnet/standard/serialization/system-text-json-configure-options?pivots=dotnet-6-0#reuse-jsonserializeroptions-instances)

[黑暗執行緒](https://blog.darkthread.net/blog/system-text-json/)

[遷移 Newtonsoft.Json 至 System.Text.Json](https://docs.microsoft.com/zh-tw/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to?pivots=dotnet-6-0)

{%hackmd BJrTq20hE %}