Optimize your API performance with the System.Text.Json source generator

.NET 6 ships with a System.Text.Json source generator as a way to improve your API performance. By default the System.Text.Json serializer is using a lot of reflection behind the scenes. Of course this has a negative impact on startup performance, memory usage and is a problem for assembly trimming.

With the introduction of the System.Text.Json source generator you get a compile-time alternative that can give your API performance a boost. It introduces the following benefits:

• Increased serialization throughput
• Reduced start-up time
• Reduced private memory usage
• Removed runtime use of System.Reflection and System.Reflection.Emit
• Trim-compatible serialization which reduces application size

Let me walk you through the steps to configure this for your application.

Configure the System.Text.Json source generator

Source generators are a little bit magical. So we have to take some steps to get it working.

Remark: The source generator is part of the 6.0 release of the System.Text.Json NuGet package.

First we need to create an (internal) partial class which derives from JsonSerializerContext.:

	internal partial class JsonContext : JsonSerializerContext
	{
	}

view raw JsonContext.cs hosted with ❤ by GitHub

For every type we want to serialize through the source generator, we need to add a JsonSerializableAttribute on top of this class:

	[JsonSerializable(typeof(OrderDto))]
	[JsonSerializable(typeof(ProductDto))]
	internal partial class JsonContext : JsonSerializerContext
	{
	}

view raw JsonContext.cs hosted with ❤ by GitHub

We can further control the serialization process through the JsonSerializerOptionsAttribute:

	[JsonSourceGenerationOptions(PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
	[JsonSerializable(typeof(OrderDto))]
	[JsonSerializable(typeof(ProductDto))]
	internal partial class JsonContext : JsonSerializerContext
	{
	}

view raw JsonContext.cs hosted with ❤ by GitHub

Let’s have a look what is generated by the source generator. You can use a decompiler for that or the MSBuild trick mentioned here.

Here are our data contracts:

public record OrderDto(int Id, DateTime OrderDate, DateTime? ShippedDate);

view raw OrderDto.cs hosted with ❤ by GitHub

	public record ProductDto
	{
	public ProductDto()
	{
	}
	public int ProductId { get; init; }
	public string ProductName { get; init; }
	}

view raw ProductDto.cs hosted with ❤ by GitHub

And here is a part of the generated code:

	internal partial class JsonContext
	{
	private global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::eShopExample.Application.Products.Dto.ProductDto>? _ProductDto;
	public global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::eShopExample.Application.Products.Dto.ProductDto> ProductDto
	{
	get
	{
	if (_ProductDto == null)
	{
	global::System.Text.Json.Serialization.JsonConverter? customConverter;
	if (Options.Converters.Count > 0 && (customConverter = GetRuntimeProvidedCustomConverter(typeof(global::eShopExample.Application.Products.Dto.ProductDto))) != null)
	{
	_ProductDto = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreateValueInfo<global::eShopExample.Application.Products.Dto.ProductDto>(Options, customConverter);
	}
	else
	{
	global::System.Text.Json.Serialization.Metadata.JsonObjectInfoValues<global::eShopExample.Application.Products.Dto.ProductDto> objectInfo = new global::System.Text.Json.Serialization.Metadata.JsonObjectInfoValues<global::eShopExample.Application.Products.Dto.ProductDto>()
	{
	ObjectCreator = static () => new global::eShopExample.Application.Products.Dto.ProductDto(),
	ObjectWithParameterizedConstructorCreator = null,
	PropertyMetadataInitializer = ProductDtoPropInit,
	ConstructorParameterMetadataInitializer = null,
	NumberHandling = default,
	SerializeHandler = ProductDtoSerializeHandler
	};
	_ProductDto = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreateObjectInfo<global::eShopExample.Application.Products.Dto.ProductDto>(Options, objectInfo);
	}
	}
	return _ProductDto;
	}
	}
	private static global::System.Text.Json.Serialization.Metadata.JsonPropertyInfo[] ProductDtoPropInit(global::System.Text.Json.Serialization.JsonSerializerContext context)
	{
	global::eShopExample.Web.JsonContext jsonContext = (global::VeShopExample.Web.JsonContext)context;
	global::System.Text.Json.JsonSerializerOptions options = context.Options;
	global::System.Text.Json.Serialization.Metadata.JsonPropertyInfo[] properties = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfo[3];
	global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<global::System.Type> info0 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<global::System.Type>()
	{
	IsProperty = true,
	IsPublic = false,
	IsVirtual = true,
	DeclaringType = typeof(global::eShopExample.Application.Products.Dto.ProductDto),
	PropertyTypeInfo = jsonContext.Type,
	Converter = null,
	Getter = null,
	Setter = null,
	IgnoreCondition = null,
	HasJsonInclude = false,
	IsExtensionData = false,
	NumberHandling = default,
	PropertyName = "EqualityContract",
	JsonPropertyName = null
	};
	properties[0] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<global::System.Type>(options, info0);
	global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<global::System.Int32> info1 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<global::System.Int32>()
	{
	IsProperty = true,
	IsPublic = true,
	IsVirtual = false,
	DeclaringType = typeof(global::eShopExample.Application.Products.Dto.ProductDto),
	PropertyTypeInfo = jsonContext.Int32,
	Converter = null,
	Getter = static (obj) => ((global::eShopExample.Application.Products.Dto.ProductDto)obj).ProductId,
	Setter = static (obj, value) => throw new global::System.InvalidOperationException("Deserialization of init-only properties is currently not supported in source generation mode."),
	IgnoreCondition = null,
	HasJsonInclude = false,
	IsExtensionData = false,
	NumberHandling = default,
	PropertyName = "ProductId",
	JsonPropertyName = null
	};
	properties[1] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<global::System.Int32>(options, info1);
	global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<global::System.String> info2 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<global::System.String>()
	{
	IsProperty = true,
	IsPublic = true,
	IsVirtual = false,
	DeclaringType = typeof(global::eShopExample.Application.Products.Dto.ProductDto),
	PropertyTypeInfo = jsonContext.String,
	Converter = null,
	Getter = static (obj) => ((global::eShopExample.Application.Products.Dto.ProductDto)obj).ProductName!,
	Setter = static (obj, value) => throw new global::System.InvalidOperationException("Deserialization of init-only properties is currently not supported in source generation mode."),
	IgnoreCondition = null,
	HasJsonInclude = false,
	IsExtensionData = false,
	NumberHandling = default,
	PropertyName = "ProductName",
	JsonPropertyName = null
	};
	properties[2] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<global::System.String>(options, info2);
	return properties;
	}
	private static void ProductDtoSerializeHandler(global::System.Text.Json.Utf8JsonWriter writer, global::eShopExample.Application.Products.Dto.ProductDto? value)
	{
	if (value == null)
	{
	writer.WriteNullValue();
	return;
	}
	writer.WriteStartObject();
	writer.WriteNumber(PropName_productId, ((global::eShopExample.Application.Products.Dto.ProductDto)value).ProductId);
	writer.WriteString(PropName_productName, ((global::eShopExample.Application.Products.Dto.ProductDto)value).ProductName);
	writer.WriteEndObject();
	}
	}

view raw JsonContext.g.cs hosted with ❤ by GitHub

Whow! That is a lot of code...

Use the generated code with the JsonSerializer

The JsonSerializer introduces some new overloads that allow you to use the generated code:

	OrderDto order = new("1", DateTime.Now,DateTime.Now);
	byte[] utf8Json = JsonSerializer.SerializeToUtf8Bytes(order, MyJsonContext.Default.OrderDto);
	order = JsonSerializer.Deserialize(utf8Json, MyJsonContext.Default.OrderDto):

view raw JsonSerializer.cs hosted with ❤ by GitHub

Or:

string json=JsonSerializer.Serialize(order, MyJsonContext.Default.OrderDto);

view raw JsonSerializer.cs hosted with ❤ by GitHub

Integrate it in your ASP.NET Core application

Of course in your ASP.NET Core application you typically don’t invoke the JsonSerializer directly and typically just return a model:

	namespace eShopExample.Web.Controllers
	{
	[Route("api/[controller]")]
	[ApiController]
	public class OrdersController : ControllerBase
	{
	private readonly IOrderService _orderService;
	public OrdersController(IOrderService orderService)
	{
	_orderService = orderService;
	}
	// GET api/orders
	[HttpGet]
	public async Task<ActionResult<IEnumerable<OrderDto>>> Get()
	{
	var orders = await _orderService.GetOrders();
	return Ok(orders);
	}
	}
	}

view raw OrdersController.cs hosted with ❤ by GitHub

In that case how can we use this source generator? We need to call the AddJsonOptions() on the IServiceCollection and specify our JsonContext class on the JsonSerializerOptions:

	public void ConfigureServices(IServiceCollection services)
	{
	services
	.AddControllersWithViews()
	.AddJsonOptions(options => options.JsonSerializerOptions.AddContext<JsonContext>())
	.AddControllersAsServices();
	}

view raw Startup.cs hosted with ❤ by GitHub

So that should be enough to get started with the System.Text.Json source generator.

If you want to learn more, have a look here: https://devblogs.microsoft.com/dotnet/try-the-new-system-text-json-source-generator/