JSON and Serialization

Q: What if property names have mismatched case during deserialization?

Set PropertyNameCaseInsensitive = true in JsonSerializerOptions to ignore case differences.

Q: How do I exclude a property from serialization?

Add the [JsonIgnore] attribute to that property to skip it.

Q: How do I control the date format in JSON?

Create a custom JsonConverter<DateTime> and override the Write method to specify the desired date format string.

JSON Format Basics

JSON (JavaScript Object Notation) is a lightweight data interchange format that stores and transmits structured data as plain text.

Object: Wrapped in {}, contains key-value pairs; keys must be strings
Array: Wrapped in [], an ordered list of values
Data types: String, number, boolean, null, object, array

TEXT

{
  "name": "Zhang San",
  "age": 28,
  "isStudent": false,
  "hobbies": ["programming", "reading"],
  "address": {
    "city": "Beijing",
    "zip": "100000"
  }
}

System.Text.Json Overview

System.Text.Json is a high-performance JSON library built into .NET Core 3.0+, requiring no third-party packages. It is recommended over Newtonsoft.Json.

CSHARP

using System.Text.Json;

Serialization: Object to JSON

JsonSerializer.Serialize() converts an object to a JSON string.

Example

CSHARP

using System;
using System.Text.Json;

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
    public bool IsStudent { get; set; }
}

class Program
{
    static void Main()
    {
        var person = new Person
        {
            Name = "Zhang San",
            Age = 28,
            IsStudent = false
        };
        string json = JsonSerializer.Serialize(person);
        Console.WriteLine(json);
    }
}

▶ Try it Yourself

TEXT

{"Name":"Zhang San","Age":28,"IsStudent":false}

Deserialization: JSON to Object

JsonSerializer.Deserialize<T>() converts a JSON string back to an object of the specified type.

Example

CSHARP

using System;
using System.Text.Json;

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
    public bool IsStudent { get; set; }
}

class Program
{
    static void Main()
    {
        string json = @"{""Name"":""Zhang San"",""Age"":28,""IsStudent"":false}";
        Person person = JsonSerializer.Deserialize<Person>(json);
        Console.WriteLine($"Name: {person.Name}, Age: {person.Age}");
    }
}

▶ Try it Yourself

TEXT

Name: Zhang San, Age: 28

JsonSerializerOptions Configuration

Use JsonSerializerOptions to control serialization behavior, such as naming policy, indentation, and ignoring null values.

Option	Description
`PropertyNamingPolicy`	Property naming policy, e.g. `JsonNamingPolicy.CamelCase`
`WriteIndented`	Whether to indent output for readability
`IgnoreNullValues`	Whether to ignore null-valued properties
`PropertyNameCaseInsensitive`	Ignore property name case during deserialization

Example

CSHARP

using System;
using System.Text.Json;

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
    public string Email { get; set; }
}

class Program
{
    static void Main()
    {
        var person = new Person
        {
            Name = "Zhang San",
            Age = 28,
            Email = null
        };

        var options = new JsonSerializerOptions
        {
            PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
            WriteIndented = true,
            IgnoreNullValues = true
        };

        string json = JsonSerializer.Serialize(person, options);
        Console.WriteLine(json);
    }
}

▶ Try it Yourself

TEXT

{
  "name": "Zhang San",
  "age": 28
}

Serializing Collections and Dictionaries

JsonSerializer can directly serialize collection types such as lists and dictionaries.

Example

CSHARP

using System;
using System.Collections.Generic;
using System.Text.Json;

public class Student
{
    public string Name { get; set; }
    public int Score { get; set; }
}

class Program
{
    static void Main()
    {
        var students = new List<Student>
        {
            new Student { Name = "Zhang San", Score = 90 },
            new Student { Name = "Li Si", Score = 85 },
            new Student { Name = "Wang Wu", Score = 92 }
        };

        var options = new JsonSerializerOptions { WriteIndented = true };
        string json = JsonSerializer.Serialize(students, options);
        Console.WriteLine(json);

        var scores = new Dictionary<string, int>
        {
            ["Chinese"] = 90,
            ["Math"] = 95,
            ["English"] = 88
        };
        string dictJson = JsonSerializer.Serialize(scores, options);
        Console.WriteLine(dictJson);
    }
}

▶ Try it Yourself

TEXT

[
  {
    "Name": "Zhang San",
    "Score": 90
  },
  {
    "Name": "Li Si",
    "Score": 85
  },
  {
    "Name": "Wang Wu",
    "Score": 92
  }
]
{
  "Chinese": 90,
  "Math": 95,
  "English": 88
}

Dynamic JSON: JsonDocument and JsonElement

When the JSON structure is uncertain, you can use JsonDocument and JsonElement for dynamic parsing without predefined classes.

Example

CSHARP

using System;
using System.Text.Json;

class Program
{
    static void Main()
    {
        string json = @"{""name"":""Zhang San"",""age"":28,""hobbies"":[""programming"",""reading""]}";

        using JsonDocument doc = JsonDocument.Parse(json);
        JsonElement root = doc.RootElement;

        string name = root.GetProperty("name").GetString();
        int age = root.GetProperty("age").GetInt32();
        JsonElement hobbies = root.GetProperty("hobbies");

        Console.WriteLine($"Name: {name}, Age: {age}");
        foreach (JsonElement hobby in hobbies.EnumerateArray())
        {
            Console.WriteLine($"Hobby: {hobby.GetString()}");
        }
    }
}

▶ Try it Yourself

TEXT

Name: Zhang San, Age: 28
Hobby: programming
Hobby: reading

Custom Serialization: JsonConverter

Inherit from JsonConverter<T> and override the Read and Write methods to implement custom serialization/deserialization logic.

Example

CSHARP

using System;
using System.Text.Json;
using System.Text.Json.Serialization;

public class DateOnlyConverter : JsonConverter<DateTime>
{
    public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        return DateTime.ParseExact(reader.GetString(), "yyyy-MM-dd", null);
    }

    public override void Write(Utf8JsonWriter writer, DateTime value, JsonSerializerOptions options)
    {
        writer.WriteStringValue(value.ToString("yyyy-MM-dd"));
    }
}

public class Event
{
    public string Title { get; set; }

    [JsonConverter(typeof(DateOnlyConverter))]
    public DateTime Date { get; set; }
}

class Program
{
    static void Main()
    {
        var evt = new Event
        {
            Title = "Product Launch",
            Date = new DateTime(2025, 6, 15)
        };

        string json = JsonSerializer.Serialize(evt);
        Console.WriteLine(json);

        Event deserialized = JsonSerializer.Deserialize<Event>(json);
        Console.WriteLine($"Title: {deserialized.Title}, Date: {deserialized.Date:yyyy-MM-dd}");
    }
}

▶ Try it Yourself

TEXT

{"Title":"Product Launch","Date":"2025-06-15"}
Title: Product Launch, Date: 2025-06-15

Combining JSON with LINQ

After deserializing JSON into a collection, you can use LINQ for querying, filtering, and aggregation.

Example

CSHARP

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

public class Student
{
    public string Name { get; set; }
    public int Score { get; set; }
    public string City { get; set; }
}

class Program
{
    static void Main()
    {
        string json = @"[
            {""Name"":""Zhang San"",""Score"":90,""City"":""Beijing""},
            {""Name"":""Li Si"",""Score"":85,""City"":""Shanghai""},
            {""Name"":""Wang Wu"",""Score"":92,""City"":""Beijing""},
            {""Name"":""Zhao Liu"",""Score"":78,""City"":""Shanghai""},
            {""Name"":""Qian Qi"",""Score"":95,""City"":""Beijing""}
        ]";

        List<Student> students = JsonSerializer.Deserialize<List<Student>>(json);

        var beijingTop = students
            .Where(s => s.City == "Beijing")
            .OrderByDescending(s => s.Score)
            .ToList();

        Console.WriteLine("Beijing students (sorted by score, descending):");
        foreach (var s in beijingTop)
        {
            Console.WriteLine($"  {s.Name}: {s.Score}");
        }

        var cityAvg = students
            .GroupBy(s => s.City)
            .Select(g => new { City = g.Key, Avg = g.Average(s => s.Score) });

        Console.WriteLine("\nAverage score by city:");
        foreach (var item in cityAvg)
        {
            Console.WriteLine($"  {item.City}: {item.Avg:F1}");
        }
    }
}

▶ Try it Yourself

TEXT

Beijing students (sorted by score, descending):
  Qian Qi: 95
  Wang Wu: 92
  Zhang San: 90

Average score by city:
  Beijing: 92.3
  Shanghai: 81.5

CSV Read/Write Introduction

CSV (Comma-Separated Values) is a text format that separates fields with commas, commonly used for data export and exchange. The C# standard library has no built-in CSV support; you can parse it manually or use a third-party library like CsvHelper.

Example

CSHARP

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;

class Program
{
    static void Main()
    {
        var records = new List<string[]>
        {
            new[] { "Name", "Age", "City" },
            new[] { "Zhang San", "28", "Beijing" },
            new[] { "Li Si", "25", "Shanghai" }
        };

        string csvPath = "data.csv";
        File.WriteAllLines(csvPath, records.Select(r => string.Join(",", r)));

        string[] lines = File.ReadAllLines(csvPath);
        foreach (string line in lines)
        {
            string[] fields = line.Split(',');
            Console.WriteLine($"{fields[0],-10} {fields[1],-4} {fields[2]}");
        }
    }
}

▶ Try it Yourself

TEXT

Name       Age   City
Zhang San  28    Beijing
Li Si      25    Shanghai

💡 Tip: For production environments, it is recommended to use CsvHelper (NuGet package), which correctly handles fields containing commas, quotes, and newlines, avoiding common pitfalls of manual parsing.

❓ FAQ

Q What is the difference between System.Text.Json and Newtonsoft.Json?

A System.Text.Json is a high-performance library built into .NET, while Newtonsoft.Json is a third-party library with richer features. New projects should prefer the built-in library.

Q What if property names have mismatched case during deserialization?

A Set PropertyNameCaseInsensitive = true in JsonSerializerOptions to ignore case differences.

Q How do I exclude a property from serialization?

A Add the [JsonIgnore] attribute to that property to skip it.

Q How do I control the date format in JSON?

A Create a custom JsonConverter<DateTime> and override the Write method to specify the desired date format string.

📖 Summary

JSON uses {} for objects and [] for arrays; it is a universal data interchange format
System.Text.Json is the built-in .NET JSON library, using Serialize/Deserialize for conversion
JsonSerializerOptions configures naming policy, indentation, null-value handling, and more
JsonDocument/JsonElement are suitable for parsing JSON with uncertain structure
Custom JsonConverter<T> controls serialization logic for specific types
JSON deserialized into collections can be combined with LINQ for querying and aggregation
CSV can be parsed manually; for production use, the CsvHelper library is recommended

📝 Exercises

Define a Book class (Title, Author, Price, IsPublished), serialize it to an indented JSON string and output it, then deserialize it back to an object
Create a List<Book> with at least 3 books, serialize to JSON, then use LINQ to filter books with a price greater than 50
Write a custom JsonConverter that serializes DateTime in "yyyy/MM/dd" format, and verify that deserialization correctly restores it
Serialize a Dictionary<string, string> to JSON and save it to a file, then read from the file and deserialize it back
Write manual CSV read/write logic that handles fields containing commas (wrap them in double quotes)

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JSON and Serialization

JSON Format Basics

System.Text.Json Overview

Serialization: Object to JSON

Example

Deserialization: JSON to Object

Example

JsonSerializerOptions Configuration

Example

Serializing Collections and Dictionaries

Example

Dynamic JSON: JsonDocument and JsonElement

Example

Custom Serialization: JsonConverter

Example

Combining JSON with LINQ

Example

CSV Read/Write Introduction

Example

❓ FAQ

📖 Summary

📝 Exercises

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