Amazon.Lambda.Serialization.SystemTextJson

This package contains a custom Amazon.Lambda.Core.ILambdaSerializer implementation which uses System.Text.Json to serialize/deserialize .NET types in Lambda functions. This serializer targets .NET Core 3.1 so can not be used with the .NET Core 2.1 Lambda runtime.

If targeting .NET Core 3.1 this serializer is highly recommend over Amazon.Lambda.Serialization.Json and can significantly reduce cold start performance in Lambda.

This serializer can be present on the assembly or on the handler method. If you specify both, the method attribute takes priority.

Here is an example of setting this attribute on the assembly.

[assembly: Amazon.Lambda.Core.LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

And this is how the method can be applied to the handler method.

[Amazon.Lambda.Core.LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]
public Response CustomSerializerMethod(Request input)
{
    ...
}

Using C# source generator for serialization

C# 9 provides source generators, which allow code generation during compilation. This can reduce the use of reflection APIs and improve application startup time. .NET 6 updated the native JSON library System.Text.Json to use source generators, allowing JSON parsing without requiring reflection APIs.

To use the source generator for JSON serializing in Lambda, you must define a new empty class in your project that derives from System.Text.Json.Serialization.JsonSerializerContext. This class must be a partial class because the source generator adds code to this class to handle serialization. On the empty partial class, add the JsonSerializable attribute for each .NET type the source generator must generate the serialization code for.

Here is an example called HttpApiJsonSerializerContext that registers the Amazon API Gateway HTTP API event and response types to have the serialization code generated:

[JsonSerializable(typeof(APIGatewayHttpApiV2ProxyRequest))]
[JsonSerializable(typeof(APIGatewayHttpApiV2ProxyResponse))]
public partial class HttpApiJsonSerializerContext : JsonSerializerContext
{
}

To register the source generator for serialization use the LambdaSerializer attribute passing in the SourceGeneratorLambdaJsonSerializer type along with the custom context type (i.e. HttpApiJsonSerializerContext) as the generic parameter.

[assembly: LambdaSerializer(
        typeof(SourceGeneratorLambdaJsonSerializer
                    <APIGatewayExampleImage.HttpApiJsonSerializerContext>))]

Customizing serialization options

Both DefaultLambdaJsonSerializer and SourceGeneratorLambdaJsonSerializer construct an instance of JsonSerializerOptions that is used to customize the serialization and deserialization of the Lambda JSON events. For example adding special converters and naming policies.

To further customize the JsonSerializerOptions create a new type extending from extend either DefaultLambdaJsonSerializer or SourceGeneratorLambdaJsonSerializer and pass in an Action customizer to the base constructor. Then register the new type as the serializer using the LambdaSerializer attribute. Below is an example of a custom serializer.

public class CustomLambdaSerializer : DefaultLambdaJsonSerializer
{
    public CustomLambdaSerializer()
        : base(CreateCustomizer())
    { }

    private static Action<JsonSerializerOptions> CreateCustomizer()
    {
        return (JsonSerializerOptions options) =>
        {
            // Customize options
        };
    }
}