Program.cs

using System.Text.Json.Serialization;

using TodoAPI;

var b = WebApplication.CreateBuilder(args);
var appold = b.Build();

var builder = WebApplication.CreateSlimBuilder(args);
builder.Logging.AddConsole();

builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.AddContext<AppJsonSerializerContext>();
});

var app = builder.Build();

var sampleTodos = TodoGenerator.GenerateTodos().ToArray();

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();

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