Aaru.Server/Aaru.Server.Api/Program.cs

using System.Text.Json.Serialization;

var builder = WebApplication.CreateSlimBuilder(args);

builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.TypeInfoResolverChain.Insert(0,
                                                           AppJsonSerializerContext.Default);
});

var app = builder.Build();

var sampleTodos = new Todo[]
{
    new(1, "Walk the dog"), new(2, "Do the dishes", DateOnly.FromDateTime(DateTime.Now)),
    new(3, "Do the laundry", DateOnly.FromDateTime(DateTime.Now.AddDays(1))), new(4, "Clean the bathroom"),
    new(5, "Clean the car", DateOnly.FromDateTime(DateTime.Now.AddDays(2)))
};

var todosApi = app.MapGroup("/todos");
todosApi.MapGet("/", () => sampleTodos);

todosApi.MapGet("/{id}",
                (int id) => sampleTodos.FirstOrDefault(a => a.Id == id) is {} todo
                                ? Results.Ok(todo)
                                : Results.NotFound());

app.Run();

public record Todo(int Id, string? Title, DateOnly? DueBy = null, bool IsComplete = false);

[JsonSerializable(typeof(Todo[]))]
internal partial class AppJsonSerializerContext : JsonSerializerContext {}