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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.:

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

We can further control the serialization process through the JsonSerializerOptionsAttribute:

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:

And here is a part of the generated code:

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:

Or:

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:

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:

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/

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