How can I preserve specific built-in types for System.Text.Json AOT?

Question

I have a object structure like List<Dictionary<string, int>> and I serialize it directly with JsonSerializer. Obviously I can't use it as it is for AOT or I get:

JSON serialization and deserialization might require types that cannot be statically analyzed. Use the overload that takes a JsonTypeInfo or JsonSerializerContext, or make sure all of the required types are preserved.

Now I know how it is typically done when using your own custom types, but what are my options for built-in?

Answer 1

You set up JSON source generation for system types TSystem in exactly the same way you do for custom types: define some JsonSerializerContext with [JsonSerializable(typeof(TSystem))] applied:

[JsonSourceGenerationOptions(WriteIndented = true)] // Add whatever options you want here.
[JsonSerializable(typeof(List<Dictionary<string, int>>))]
// Add additional [JsonSerializable(typeof(T))] attributes as required.
public partial class MySerializationContext : JsonSerializerContext;

The only question is, what JsonTypeInfo<TSystem> property name will be generated for MySerializationContext for a generic system type like List<Dictionary<string, int>>? As it turns out, it's formed by concatenating all the type names in order, i.e. ListDictionaryStringInt32:

List<Dictionary<string, int>> list = new()
{
    new() { {"foo", 10}, {"bar", 20}, }
};

var json = JsonSerializer.Serialize(
    list, 
    MySerializationContext.Default.ListDictionaryStringInt32);

If you're not sure, you can instead call JsonSerializerContext.GetTypeInfo(Type) to get the required type info in runtime:

var typeInfo = (JsonTypeInfo<List<Dictionary<string, int>>>?)MySerializationContext.Default.GetTypeInfo(list.GetType());
if (typeInfo == null)
    throw new JsonException($"TypeInfo not found for {list.GetType()}");
var json = JsonSerializer.Serialize(list, typeInfo);

You can also determine the property name by using reflection in the debugger:

Console.WriteLine(string.Join(",", MySerializationContext.Default.GetType().GetProperties().Select(p => p.Name)));