List<TaskEntity> tasks = new()
{
    new TaskEntity() { Uuid= Guid.NewGuid(), Subject = "Сдать третью лабу", IsCompleted = false },
};

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

// Массив сущностей.
app.MapGet("/", () =>
{
    return tasks.Select(t => new TaskEntityDto()
    {
        Uuid = t.Uuid,
        Subject = t.Subject,
        IsCompleted = t.IsCompleted,
    });
});

// Подробности сущности.
app.MapGet("/{uuid}", (Guid uuid) =>
{
    var task = tasks.FirstOrDefault(t => t.Uuid == uuid);
    if (task == null)
        return Results.NotFound();
    return Results.Json(new TaskEntityDto()
    {
        Uuid = task.Uuid,
        Subject = task.Subject,
        IsCompleted = task.IsCompleted,
    });
});

// Создание сущности.
app.MapPost("/", () => { throw new NotImplementedException(); });

// Изменение сущности.
app.MapPut("/{uuid}", () => { throw new NotImplementedException(); });

// Удаление сущности.
app.MapPost("/", () => { throw new NotImplementedException(); });

app.Run();

/// <summary>
/// Сама сущность.
/// </summary>
public class TaskEntity
{
    public Guid Uuid { get; set; }
    public string Subject { get; set; } = string.Empty;
    public bool IsCompleted { get; set; }
}

/// <summary>
/// DTO-класс для описания сущности в API.
/// </summary>
public class TaskEntityDto : TaskEntity { }

/// <summary>
/// Запрос на создание сущности.
/// </summary>
public class TaskCreateRequest
{
    public string Subject { get; set; } = string.Empty;
    public bool IsCompleted { get; set; }
}

/// <summary>
/// Запрос на изменение сущности.
/// </summary>
public class TaskEditRequest
{
    public string Subject { get; set; } = string.Empty;
    public bool IsCompleted { get; set; }
}