Dotnet Source Gen JSON

Configure System.Text.Json source generation for AOT-compatible, reflection-free JSON serialization with compile-time type metadata.

When to Use

• Preparing application for Native AOT compilation
• Eliminating reflection-based serialization overhead
• Improving startup performance by generating serialization code at compile time
• Ensuring JSON serialization errors surface at compile time

Requirements

• .NET 8 or higher for full options support
• .NET 6+ for basic source generation

Steps

1. Invoke polymorphic skill first:

   • Run dotnet-json-polymorphic skill to ensure polymorphic types are configured
   • This ensures [JsonDerivedType] attributes are in place before generating context

2. Ask scope:

   • Ask user: "Apply to entire solution or specific project?"
   • Filter to ASP.NET Core projects (those with web SDK or API endpoints)

3. Scan for API endpoint types:

   • Search for Minimal API patterns: MapGet, MapPost, MapPut, MapDelete, MapPatch
   • Extract request/response types from lambda parameters and return types

4. Scan for controller types:

   • Search for controller attributes: [HttpGet], [HttpPost], [HttpPut], [HttpDelete], [HttpPatch]
   • Extract parameter types and return types from action methods

5. Find direct serialization calls:

   • Search for JsonSerializer.Serialize and JsonSerializer.Deserialize calls
   • Extract type arguments from these calls

6. Check for existing JsonSerializerContext:

   • Search for classes inheriting from JsonSerializerContext
   • Note existing [JsonSerializable] types to avoid duplicates

7. Ask about enum serialization:

   • Ask user: "Enable enum-as-string converter? (Recommended: Yes)"

8. Ask about property naming:

   • Ask user: "Select property naming policy:"
     • CamelCase (Recommended)
     • SnakeCaseLower
     • SnakeCaseUpper
     • KebabCaseLower
     • KebabCaseUpper
     • None (preserve original casing)

9. Ask about null value handling:

   • Ask user: "Serialize null values? (Recommended: No)"
     • No - Omit null properties from output (smaller payloads)
     • Yes - Include null properties in output
   • If No: Add DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull

10. Create JsonSerializerContext:

    • File: {ProjectName}JsonContext.cs in project root (same level as Program.cs)
    • Class name: {ProjectName}JsonContext

11. Add source generation options:

    [JsonSourceGenerationOptions(
        PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase,
        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull)]
    [JsonSerializable(typeof(WeatherForecast))]
    [JsonSerializable(typeof(List<WeatherForecast>))]
    public partial class MyApiJsonContext : JsonSerializerContext
    {
    }
    

12. Include collection types:

    • For each type T, also add [JsonSerializable(typeof(List<T>))]
    • Add [JsonSerializable(typeof(T[]))] if arrays are used

13. Configure ASP.NET Core:

    • Add to Program.cs:

    builder.Services.ConfigureHttpJsonOptions(options =>
    {
        options.SerializerOptions.TypeInfoResolver = MyApiJsonContext.Default;
    });
    

14. Verify with build:

    dotnet build
    

15. Report results:

    • List the JsonSerializerContext file created
    • List all types added to [JsonSerializable]
    • Confirm ASP.NET Core integration added
    • Confirm build status

Example Context

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

namespace MyApi;

[JsonSourceGenerationOptions(
    PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase,
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
    Converters = [typeof(JsonStringEnumConverter<Status>)])]
[JsonSerializable(typeof(WeatherForecast))]
[JsonSerializable(typeof(List<WeatherForecast>))]
[JsonSerializable(typeof(CreateOrderRequest))]
[JsonSerializable(typeof(OrderResponse))]
[JsonSerializable(typeof(List<OrderResponse>))]
[JsonSerializable(typeof(ErrorResponse))]
public partial class MyApiJsonContext : JsonSerializerContext
{
}

ASP.NET Core Integration

Program.cs:

var builder = WebApplication.CreateBuilder(args);

builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.TypeInfoResolver = MyApiJsonContext.Default;
});

var app = builder.Build();

app.MapGet("/weather", () => new WeatherForecast("Seattle", 72));
app.MapPost("/orders", (CreateOrderRequest request) => Results.Ok(new OrderResponse()));

app.Run();

AOT Configuration

For full AOT compatibility, add to the Directory.Build.props file (preferred), or project file:

<PropertyGroup>
  <JsonSerializerIsReflectionEnabledByDefault>false</JsonSerializerIsReflectionEnabledByDefault>
</PropertyGroup>

This ensures compile-time errors if any type is missing from the context.

Notes

• Context naming: Use {ProjectName}JsonContext for consistency
• File location: Place in project root alongside Program.cs
• Collection types: Always include List<T> and array types if used in APIs
• Polymorphic types: Run dotnet-json-polymorphic first; polymorphic types use metadata-based generation only
• Enum handling: JsonStringEnumConverter<TEnum> requires .NET 8+ for AOT support
• Multiple contexts: Can have multiple contexts; use JsonTypeInfoResolver.Combine() to merge them

Documentation

• System.Text.Json Source Generation