1. System.Text.Json vs Newtonsoft.Json Comparison

`System.Text.Json` was built from the ground up to leverage modern C# features like `Span<T>`, `ReadOnlySpan<byte>`, and `Utf8JsonReader`. By processing UTF-8 bytes directly without converting them to intermediate C# strings, it eliminates heap memory allocations.

High-Speed Deserialization in .NET 8 
using System.Text.Json;
using System.Text.Json.Serialization;

public class UserDto
{
    [JsonPropertyName("user_id")]
    public int Id { get; set; }

    [JsonPropertyName("full_name")]
    public string Name { get; set; } = string.Empty;
}

// High-performance deserialization
string jsonString = "{\"user_id\": 101, \"full_name\": \"Sarah Conner\"}";
UserDto? user = JsonSerializer.Deserialize<UserDto>(jsonString);
Explanation: System.Text.Json relies on UTF-8 byte spans for high-throughput microservice pipelines.

2. Configuring Global JsonSerializerOptions

By default, `System.Text.Json` enforces strict case-sensitive property matching and uses PascalCase output. Configure global `JsonSerializerOptions` to handle camelCase naming conventions and ignore null values.

Global Serialization Options in ASP.NET Core 
var options = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
    WriteIndented = true // Pretty-print JSON output
};

string prettyJson = JsonSerializer.Serialize(user, options);
Explanation: WriteIndented formats output with 2-space indentation for human readability.

3. Implementing a Custom JsonConverter<T>

For non-standard data types or legacy API date string formats, override `JsonConverter<T>` to control byte-level reading and writing.

Custom DateTime ISO-8601 JsonConverter Example 
public class DateTimeCustomConverter : JsonConverter<DateTime>
{
    public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        return DateTime.Parse(reader.GetString()!);
    }

    public override void Write(Utf8JsonWriter writer, DateTime value, JsonSerializerOptions options)
    {
        writer.WriteStringValue(value.ToString("yyyy-MM-dd HH:mm:ss"));
    }
}
Explanation: Utf8JsonWriter writes custom string representations directly to the UTF-8 output stream.